How are atoms structured? What particles are they
made up of?
What are their size, mass and electric charges?
What particles is a nucleus made up of?
What is the structure of the atomic shell?
The relation between the atom structure and the
properties of an element.
The nuclear reaction at the
Nuclear Rearch Institute, Da Lat
City (Lam Dong Province)
COMPOSITION OF ATOMS
What are the size, mass and structural composition of
What are the size, mass and electric charge of the
particles forming atoms?
Figure 1.1 A bust of Democritus
Figure 1.2. The sliver coin in
Around 440 BC, the philosopher Democritus said that a sliver coin cuold infinitely be
divided into tiny pieces and finally it would become an “indivisible” particle called atom
(from the Greek word atomos, which mean “indivisible”).
Nowadays, scientists can divide the sliver atoms but the pieces received do not retain the
properties of sliver anymore.
It was not until the mid XIX century that is was believed that matter was made up of tiny
indivisible particles called atoms. The researches done in the late nineteenth crntury and the
early twentieth century proved that atom was real and had a complex structure.
I STRUCTURAL COMPOSITION OF ATOMS
a) The discovery of electrons
In 1897, the British scientist J.J Thomson studied the electrical discharge between two
electrodes with the voltage of 15 kV, placed in an almost empty tube (that pressure was about
0.001 mmHg) and observed that the fluorescent screen in the tube glowed because the rays
called the cathode were amitted from the negative electrode.
The eletrodes that changed the route of the
A cathode ray has the following
- If a light windmill is put on the route
of the ray, then it will spin. This shows a
cathode ray is a beam of particles, which
has mass and moves at high speeds.
- Without the deflection of electric and
magnetic fields, cathode rays will travel
in a straight line.
Figure 1.3. Experiment diagram of
Thomson to discover the cathode rays
- When passing through two electrodes with opposite charges, a cathode ray shifts towards
the positive electrode. This proves that the cathode ray is a beam of negatively charged
particles (figure 1.3)
The particles creating the cathode rays are called electrons, denoted by e.
b) Mass and charge of electrons
The mass and charge of electrons have been determined in the experiments
Mass: m e 9.1094 �10 kg .
Charge: q e 1.602 �10 C (Coulomb).
No charge of less than 1.602 �10 C has been discovered, so it is used as a unit of electrical
charge, denoted by e0. Therefore, the charge of electron is denoted by e0 and it is 1 for
2. The discovery of the atomic nucleus
In 1911. E.Rutherford, an English physics, and his assistants bombarded a beam of a alpha
α(1) particles on a thin gold foil and used the fluorescent screen behind the foil to observe the
route of the α particles. The result of the experiment showed the most of the α particles
passed through the gold foil, but a few particles deviated from its initial direction and very
few particles even bounced back on contact with the gold foil (figure 1.4a, b).
The particle with a +2 charge and a mass four times larger than that of a hygrogen atom.
(replaced by a high-resolution image)
Figure 1.4. Diagram of the experiment discovering the atom nucles
Therefore, atoms must contain a positively charged particle with a large mass to be able to
deflect the α particles in the collision. However, the size of this positively charged particle
must be much smaller than that of the atom so that most of the α particles can pass through
the space between the positively charged particles of the gold atoms without any deflection.
This proves that an atom is hollow space and the positively charged particle is nucleus
Around the nucleus there are electrons forming the atomic shell. For atoms to be electrically
neutral, the number of the positive charges of the nucleus have to be equal to that of
electrons orbiting it.
Because of the very small mass of electrons, the atomic mass is mostly concentrated in the
3. Structure of the atomic nucleus
a) The discovery of protons
In 1918, while bombarding nitrogen atomic nucleus with α particles, Ruttherford discovered
oxygen atomic nucleus and a particle with a mass of 1.6726 �10 kg and a unit of positive
charge (symbol eo; equal 1+ by convention). He called them protons, symbol p.
Proton is one of the constituents of the atomic nucleus.
b) The discovery of neutrons
In 1932, J.Chadwick (Ruthford’s assistant), while bombarding beryllium atomic nucleus with
α partiles, discovered a new particle with a similar mass to protons, but with no charge. He
named it neutrons (symbol n).
So, neutron is also one of the constituents of the atomic nucleus.
c) The structure of atomic nucleus
After the experiments above, it was concluded that:
Atomic nucleus is made up of protons and neutrons. Since neutrons have no charge, the
number of protons in the nucleus must be equal to that of the positively charged untis of the
nucleus and the number of electrons orbiting the nucleus.
II SIZE AND MASS OF ATOMS
Nowadays, scientists have determined the size and mass of the particles making up the
Atoms of different elements have different sizes and masses.
a) If we imagine an atom as a sphere, in wich electrons move verey fast around the nucleus,
then the atom has a diameter of about 10-10 m.
Nanometre unit (acronym: nm) or Angstrom unit (acronym: A ) is used to express the size
of an atom.
1 nm = 10 m, 1 A = 10
m, 1 nm = 10 A .
A hydrogen atom is the smallest atom with a radius around 0,053 nm.
b) The diameter of the atomic nucleus is even smaller, about 10-5 nm.
Therefore, the diameter of an atom is about 10 000 times greater than that of the nucleus .
If we imagine nucleus as a sphere with a diameter of 10 cm, then an atom is a sphere with a
diameter of 1000 m = 1 km.
c) The diameters of protons and electrons are much smaller (about 10 -8 nm), electrons move
around nucleus in the empty space of atom.
It is hard to imagine that every 1 g of an arbitrary substance contain a billion, billion, billion
Example: 1g of carbon contains 5.1022 (50 000.109.109) carbon atoms (means fifty
thousand billion billion carbon atoms).
Therefore, the mass of atoms, molecules and protons, neutrons, electrons is measured in
aomic mass units, symbol u, u is also called đvC (Carbon unit)
1 u is equal to of mass of an atom of carbon -12.
This carbon atom has a mass of 19,9265.10-27 kg.
The mass of a hydrogen atom is 1,008u 1u.
The mass of a carbon atom is = 12u.
The mass, electric charge of particles making up an atom are stated in table 1.
Table 1. The mass and electric charge of particles making up atoms
Electric charge q
qe = -1,602.1019
C = -eo = 1-
qp = 1,602.10-19C
= eo = 1+
qn = 0
me = 9,1094.10-31 mp = 1,6726.10-27 mn = 1,6748.10-27
In some international reference, u is called a.m.u (atom mass unit).
1. The particles that make up the nucleus of most of atoms are
A.Electrons and protons.
B. Protons and neutrons.
C. Neutrons and electrons.
D.Electrons, protons and neutrons.
Choose the correct answer.
2. The particles that make up most of the atoms are
A.Protons and electrons.
B. Neutrons and electrons.
C. Neutrons and protons.
D.Neutrons, protons and electrons.
Choose the correct answer.
3. The diameter of an atom is about 10 000 times bigger than that of the nucleus. If we
enlarge the nucleus into a ball with a diameter of 6 cm, then the diameter of atom will be
B. 300 m.
C. 600 m.
Choose the correct answer.
4. Calculate the mass ratio of an electron to a proton and neutron.
5. A zinc atom has a radius of r 1.35 �10 nm and an atomic mass of 65 u.
a) Calculate the the density of the zinc atom.
b) In reality, the atomic mass concentrates on the nucleus with a radius of r 2 �10 nm .
Calculate the density of the nucleus of zinc atom.
Given that Vsphere = .
The relationship between the number of nuclear charge
units and the number of protons electrons.
How is the mass number of a nucleus calculated?
What is a chemical element, isotope, atomic mass,
everage atomic mass?
I ATOMIC NUCLEUS
1. Nuclear charge
a) Proton has a charge of 1+, if the nucleus has Znprotons, thenit has total charge of +Z anh
the number of nuclear charge units is Z
b) Atom is electrically neutral, thus the number of protons in the nucleus is equal to that of
electrons of the atom.Therefore, in an atom.
The number of nuclear charge units Z = the number of protons = the number of electrons
Example: The number of nuclear charge units of nitrogen atom is 7, therefore, nitrogen atom
has 7 protons and 7 electrons.
2. Mass number
a) Mass number (symbol A) is the total number of protons (symbol Z) and neutrons( symbol
N) in an atom:
A= Z + N
Example, the atomic nucleus of lithium has 3 protons and 4 neutrons, then the mass number
of the nucleus of a lithium atom is:
A= 3 + 4 = 7
b) A nucleus, as well as an atom, is characterized by the number of nuclear charge units Z
and the mass number A, because when Z and A of an atom are given , we know the number
of protons, of electrons and of neutrons in the atom (N=AZ)
Example: Sodium atom has A=23 and Z=11, therefore, sodium atom has 11 protons, 11
electrons and 12 neutrons.
II CHEMMICAL ELEMENTS
The chemical properties of an element depend on the number of electrons in the atom of the
element and therefore depend on the number of nuclear charge units Z of the atom. Hence,
the atom with the same number of nuclear charge units Z have the same chemical properties.
Definition: A chemical element is a apecies of atoms wich have the same number of nuclear
Example: All atom, which have the number of nuclear charge units of 11, belong to sodium
elemen.They all have 11 protons and 11 electrons.
92 chemical elements occurring n nsture have been identified up to now and about 18
artificial ones have been synthesized in nuclear laboratories. There are about 110 elements in
2. Atomic number
The number of atomic nuclear charge units of an element is called the tomic number of the
element, symbol Z.
3. Atomic notation
The number of nuclear charge units and the mass number are considered the basic
characteristics of an atom
To symbolize atoms , the symbols of characteristic indices are usually put on the left of
elemen symbol X with the mass number A above, the atomic number Z below:
Mass number A
Atomic number Z
The above notation shows that:
The atomic number of Na elements is 11, so the number of nuclear charge units is 11. There
are 11 protons in the nucleus and 11 electrons in the atomic shell of Na atom . The mass
number of Na atom is 23 indicating that sodium has 12 (2311=12) neutrons in the nucleus.
The atom of the same chemical element can have different mass number because these atoms
contain the same number of protons in their nucleus but different number of neutrons.
The isotopes of the same chemical element are atom that have the same number of protons
but different numbers of neutrons, therefore, their mass numbers A are different.
The isotopes are arranged in he same position( element cell) in the Periodic Table
For example, hydrogen has three isotopes:
a) Protium ( 1 H )
b) Deuterium( 2 H )
The nucleus consists of 1 The nucleus contains
proton(the only case
1 proton and 1
when the atom does not
neutron, it accounts
have nuetron), and it
for 0.016% of natural
accounts for 99,984% of
natural hydrogen atoms.
c) Tritium( 1 H )
The nucleus has 1 proton and 2
neutrons (the only case that the
number of neutrons is 2 times
greater than the number of
protons). This isotope only
accounts for about 10 % of
natural hydrogen atoms.
Figure 1.5. Diagram of atomic structure of 3 hydrogen istopes.
Besides about 340 natural isotopes,more than 2400 artificial isotopes have been
synthesized.Many artificial isotopes are used in medicine, agriculture,scientific research…
IV - ATOMIC MASS AND AVERAGE ATOMIC MASS OF CHEMICAL ELEMENTS
1. Atomic mass
Atomic mass is the relative mass of the atom
The atomic mass of an atom indicates how many time the mass of that element is greater
than atomic mass unit
The mass of an atom is the sum of the mass of protons , neutrons and electrons in the
atom.Since the mass of electron is too small as compared to that of the nucleus, it can be
dismissed. Therefore, the mass of an atom is considered the sum of the mass of protons and
neutrons in the atomic nucleus.
So , the atomic mass is considered to be aqual to the mass number( when high precision is
Example: Determine the atom mass of P,given that P has Z=15,and N=16
The atomic mass of P is 31.
2. The average atomic mass
Most chemical element have many naturally occurring isotopes, so the atomic mass of the
these elements is the average mass of their isotopes.
Assuming that an element has two isotopes which are X and Y,X is the atomic mass (mass
number) of the isotope X; Y is the atomic mass of the isotope Y; a is the percent abundance
of X isotope; b is the percent abundance of Y isotope.
The formula for the average atomic mass A is:
A = 100
In the calculations not requiring high precision, the mass number can be used instead of the
Example: Chlorine is a mixtue of two stable isotopes :
Cl , accounting for 75.77% and
Cl , accounting for 24,23% of total number of atoms in nature.
The everage atomic mass of chlorine is:
75.77 �35 24.23 �37
1. A chemical element is a species of atoms that have the same
A. Mass number.
C. Number of neutrons.
B. Number of protons.
D. Number of neutrons and protons.
Choose the correct answer.
2. Atomic symbol expresses full characteristics of the atom of a chemical element because it
A. Mass number A.
B. Atomic mass of an atom.
C. Atomic number Z.
D. Mass number A and atomic number Z.
Choose the correct answer.
See section 1, Subheading / material page 14.
In the Periodic Table, more precision is given as 35.45.
3. Carbon element has two stable isotopes: C6 , accounting for 98.89% and C6 , accounting
for 1.11%. The average atomic mas of carbon is
4. Determine the nuclear charge, the number of protons, neutrons, electrons and atomic mass
of the atoms given below:
3 Li , 9 F , 12 Mg , 20 Ca
5. Copper has two stable isotopes, 29 Cu and 29 Cu . The average atomic mass of copper is
63.54. What is the isotopic composition of copper?
6. Hydrogen has the average atomic mass of 1.008. How many hydrogen-2 atoms are there in
1 ml of water (assumed that there are only 1 H and 1 H isotopes in water)?
(Given that the density of water is 1g/ml)
7. Natural oxygen is a mixture of isotopes: 99.757% of
O ; 0.039% of 17O ; 0.204% of 18O .
Calculate the number of oxygen atoms of each isotope when there is one O atom.
8. Argon which is separated from the air is a mixture of three isotopes: 99.6% of Ar ,
0.063% of Ar , 0.337% of Ar . Calculate the volume of 10 g Ar at STP.
THE APPLICATIONS OF RADIOISOTOPES
AND THE USE OF NUCLEAR ENERGY FOR PEACE
1. Nuclear fission – radioactive decay and fission reaction
Radioactivity is the property of several unstable atomic nuclei that can be self-transformed and emit
nuclear radiations (usually called radioactive rays). Radioactive atoms are called radioisotopes and
the non-radioactive atoms are called stable isotopes. The elements containing only radioisotopes
(unstable isotopes) are called radioactive elements.
Radioactive rays can be the beams of positively charged particles such as α particles (α radiation)
and protons negatively charged particles such as electron beams (β radiation), electrically neutral
particles such as neutron or γ ray (which has the same nature as the light but with much greater
energy). The self-transformation of an atomic nucleus is usually called radioactive decay or nuclear
Fission reaction is the process in which the nucleus of radioactive atoms with a high atomic mass
such as U splits into smaller nuclei, accompanied by the liberation of neutrons and several other
elementary particles. Fission reaction is also a from of nuclear fission.
In fission reaction and radioactive decay, there is a mass loss; namely, the total mass of smaller
particles formed is less than the original mass of the nucleus. This loss of mass converted into a
∆E = ∆mc (1)
Where, ∆E (J) is the energy released in the nuclear fission (this energy is included in the kinetic
energy of the particles escaping in nuclear fission and the energy of γ-radiation); ∆m (g) is the mass
loss; c 2.988 �10 m/s is the speed of light in the vacuum.
2. Applications of radioisotopes
Although is was not until 1896 that the radioactive phenomenon was disscovered by the French
scientist Becquerel, radioactive isotopes have quickly had a significant role in the development
history of the twentieth and twenty-first centuries.
The applications of radioisotopes in various fields of engineering and life have been on two factors:
(1) Strong interaction of radioactive rays with the medium that they pass through; (2) Due to the
emission of radioactive rays, the radioactive isotopes are easily detected by radioactive detectors, so
they can play the role of labeled atoms. Some examples of applications of radioisotopes are given
a. In the biological and agricultural research
In the recent brilliant achievements in genetics, genetic decoding, and understanding the transport of
amino acids in an organism…, the labeled atoms playa very important role. Radioactive rays with
great energy can cause genetic mutations to from new species with various outstanding properties.
This is the basis of the green revolution in the world. The γ-ray of Co isotope is an agent used for
sterilization, and as antifungal in the preservation of food, foodstuff and seeds.
b. The application of radioisotopes in medicine
In medicine, radioactive isotopes are widely used in research, diagnostics and treatment.
Radiolabelled compounds provides information about anatomy of human organs, about the activities
of individual functional organs for diagnosis.
Radioactive rays are used in the tomography methods. The I isotope has been used in diagnosis
and treatment of thyroid disease for a long time. γ-Rays can be converged to form a beam with a
huge energy, which is used as a sharp blade (gamma knife) in bloodless surgery for tumors located
deeply in the brain, so patients do not have to be anesthetized and can walk immediately after
surgery… In 2005, a ‘‘gamma knife’’ like that was brought into use in Vietnam (at Hue Medical
c. The application of radioisotopes in industry and scientific research
Isotope labeling method is widely used to track the passage of surface water, groundwater, to test
the rate of infiltration into dukes and dam, to explore oil and gas to study the mechanism of complex
reactions and to measure physicochemical constants.
γ-Rays (with high penetrating ability) are used for inspecting the density of concrete and
agglomerated materials, and for detecting such defects as internal cracks or fractures without
breaking the material.
The energy of radioactive rays can cause chemical changes leading to the modification of material
to create new materials with truly unique properties.
Nuclear methods capable of detecting impurities in very small concentrations 9 10 - 10 ) have
caused significant changes in the aspects of modern analytical chemistry. Isotope analysis enables
the determination of the age of rock or fossil samples…
3. Using fission energy
Nuclear fission releases a huge energy. From equation (l), it is calculated that the of with a volume
equal to the size of a tennis ball is equivalent to the energy obtained by burning 2000 tonnes of coal
(this amount of coal equal to 200 loads of 10-tonne trucks, or the energy releasing in the explosion
20000 tonnes of TNT. The fission energy of uranium is used in nuclear power plants. In 2005, this
energy provided about 16% of the world's electricity output.
Nuclear power's almost non-emission of CO2 and harmful gases and low costs can be a reasonable
choice for the sustainable development of our country and many other countries.
Radioactive rays can seriously aftect the
health of human beings, animals and plants.
When working with radioactive isotopes, it
is necessary to comply with the nuclear
safety regulations strictly
The radioactive waste must be treated under
strict procedures and buried in specially
built repository. For wastes with high
reactivities, the waste repositories must be
safe for thousands of years.
Nuclear Research Institute, Da Lat City,(Lam
REVIEW: COMPOSITION OF ATOMS
To consolidate knowledge about:
- Composition of atoms, atomic nucleus, size, mass,
electric charge of particles.
- Definition of chemiscal elements, atomic notation,
isotopes, molecular mass, average molecular mass.
To pracuice determining the number of electrons,
protons, nutrons and molecular mass when the atomic
notation is given.
A KNOWLEDGE TO MASTER
1. An atom is composed of electrons and nucleus. A nucleus is composed of protons and
qe 1,602 �1019 C , is equal to 1- ; me �0.00055u .
q p 1, 602 �1019 C
, is equal to 1+; m p �1u .
2. In atoms, the number of nuclear charege units Z = the number of protons = the
number of electrons.
Mass number A = Z + N
Molecular mass is cosidered to be equal to the sum of the number of protons and the number
of neutrons (approximately).
Molecular mass of an element that has several isotopes is the average molecular mass of the
Chemical elements are the atoms that have the same Z
The isotopes of a chemical element are the atoms that have the same Z, but different N.
3. An atom is characterized by atomic number Z and mass number A
Atomic notation: Z X
1. According to the data in table 1 of lesson, page 8:
a) Calculate the mass (g) of a nitrogen atom (which contains 7 protons, 7 neutrons and 7
(This is an approximate calculation.)
b) Calculate the ratio of the mass of electrons in a nitrogen atom to the mass of the whole
2. Calculate the average molecular mass of a potassium atom, given that in nature, the
isotopes of potassium, 19 K , 19 K , 19 K have a percentage of: 93.258%;0.012% and 6.730% ,
3. a)Define a chemical alement.
b) What characteristics of an atom does the atomic notation show? Take a potassium
atom as an example.
4. Why is it known for sure that there are only 90 elements between hydrogen (Z=1) and
5. Calculate the approximate radius of a calcium atom, given that the volume of 1 mole of
crystalline calciumis is 25.87 cm3.
( Given: in crystals, calcium atoms only occupy 74% of the volume, the rest is empty
6. Write the formula of different forms of copper (II) oxide, given that copper and oxygen
have the following isotopes:
Cu , 2963Cu, 168 O, 188 O
THE STRUCTURE OF ATOMIC SHELL
How do electrons move in atoms?
What is the maximum number of electron in each shell,
I – THE MOVEMENT OF ELECTRONS IN ATOMS
In the early years of the twentieth century, people believed that electrons moved around
atomic nucleus in circular or oval paths like the orbits of planets orbiting the Sun.
That was the planetary model of atoms proposed by Rutherford, N. Bohr and A.
This model played an important role
in the development of the theory of
atomic structure, but it was
insufficient to explain all the
properties of an atom.
Nowadays, people have known that
the electrons move very fast (they
have a speed of thousands of km/s) in
the area around the atomic nucleus
not following any determinedFigure 1.6. The planetary model of atoms by
trajectory(1) to form atomic shell.
Rutherford, Bohr and Sommerfeld
The number of electrons in the atomic shell is equal to the number of protons in the atomic
nucleus also equal to the atomic number Z or the ordinal number of that element in the
Periodic Table. For example, hydrogen’s atomic shell (Z = 1) has 1 electron, chlorine’s
atomic shell (Z = 17) has 17 electrons, gold’s atomic shell (Z = 79) has 79 electrons… So
how are the electrons arranged?
Research results showed that they must be arranged according to certain rules.
See the concept of atomic orbital (Further reading “The Concept of atomic Orbitals”, page 22).
II – ELECTRON SHELL AND ELECTRON SUBSHELL
1. Electron shell
Electrons in atoms from the around state successively occupy the lowest to higher energy
levels and are arranged into shells. Those electrons closer to the nucleus are attracted to the
nucleus move strongly. Therefore, electrons at the inner shells have lower energy levels than
electrons at the outer shells.
Electrons on the same shell have nearly equal energy level.
Arranged in the order from low to high energy levels, these electron shells are denoted by
integers n = 1, 2, 3, 4… with the names of: K, L, M, N…
2. Electron subshell
Each electron shell is subdivided into subshells.
Electrons on the same subshell have equal energy level.
The subshells are designated by lowercase letters s, p, d and f.
The number of subshells in each shell is equal to the shell number.
The first shell (K shell, n = 1) has one subshell, the 1s;
The second shell (L shell, n = 2) has two subshells, the 2s and 2p;
The third shell (M shell, n= 3) has three subshells, the 3s, 3p and 3d; etc.
Electrons at s subshell and p subshell are calleds electrons and p electrons, respectively…
III – THE MAXIMUM NUMBER OF ELECTRONS IN ONE SUBSHELL, ONE
The maximum number of electrons in one subshell is as follows:
- s subshell contains a maximum of 2 electrons;
- p subshell contains a maximum of 6electrons;
- d subshell contains a maximum of 10 electrons;
- f subshell contains a maximum of 14 electrons;
A subshell containing the maximum number of electrons is calls saturated electron subshell.
Hence, we infer the maximum number of electrons in one shell:
1. The first shell (K shell, n = 1) has 1 subshell 1s, with a maximum of 2 electrons.
2. The second shell (L shell, n = 2) has 2 subshells, 2s and 2p:
- 2s subshell contains a maximum of 2 electrons;
- 2p subshell contains a maximum of 6 electrons;
Hence, the second shell contains a maximum of 8 electrons.
3. The third shell (M shell, n= 3) has 3 subshells: 3s, 3p and 3d:
- 3s subshell contains a maximum of 2 electrons;
- 3p subshell contains a maximum of 6 electrons;
- 3d subshell contains a maximum of 10 electrons;
Hence, the third shell contains a maximum of 18 electrons.
From the above example, it can be concluded that: The maximum number of electrons of nth
shell is 2n2.
Based on this formula, it can be concluded that the fourth shell (N shell, n = 4) contains a
maximum of 2 x 42 = 32 electrons.
A shell containing the maximum number of electrons is called a saturated electron shell.
Table 2. The maximum number of electrons in shells and in subshells ( n = 1 to 3)
The distribution of
electrons in subshells
K shell (n = 1)
L shell (n = 2)
M shell (n = 3)
Example: Determine the number of electron shells of the following atoms:
The maximum number
of electrons in one shell
The number of nuclear charge units of a nitrogen atom is 7, therefore the nucleus has 7
protons, the atomic shell has 7 electrons which are arranged as follows: 2 electrons on the K
shell (n = 1), 5 electrons on the L shell (n = 2).
With the same explanation, for a magnesium atom, the nucleus has 12 protons, the atomic
shell has 12 electrons which are arranged as follows: 2 electrons on the K shell (n = 1), 8
electrons on the L shell (n = 2) and 2 electrons on the M shell (n=3) (see figure 1.7).
Figure1.7. Diagram showing the distribution of electrons in nitrogen and magnesium
atom’s shells. (The innermost circle represents the nucleus containing neutrons (n) and
protons (p), and the outer circles represent electron shells)
An atom M has 75 electrons and 110 neutrons. The atomic notation of M is
Choose the correct answer.
Which of the following atoms contains 20 neutrons, 19 protons and 19 electrons
Choose the correct answer.
The number of nucleus charge units of a fluorine atom is 9. In a fluorine atom, the
number of electrons at the highest energy level is
Choose the correct answer.
The electrons of X atom are arranged on 3 electrons shells and the third shell contains 6
electrons. The number of nuclear charge units of element X is
Choose the correct answer.
a) What are electron shell and subshell? What is the difference between electron shell
b) Why does N shell contain a maximum of 32 electrons?
The notation of an argon atom is 4018Ar.
a) Determine the number of protons, neutron and electrons of the atom.
b) Determine the distribution of electrons in electron subshells.
THE CONCEPT OF ATOMIC OBITALS
One electron of a hydrogen atom moving very fast in the area around atomic nucleus forms
an electron cloud ( figure 1.8).
The charge density of this electron cloud is the largest inside a sphere with the radius of
0.053 nm. In this area, the ability of finding electrons is about 90%. It is called 1s orbital.
Figure 1.8. The sphere electron cloud of a hydrogen atom
Hence: Atomic orbital is the space area around the nucleus, where the probability of the
existence of electrons (or the probability of finding electrons) is about 90%.
Atomic orbital is shortened to AO.
The s subshell has 1s orbital. The p subshell has 3 px, py, pz orbitals which are perpendicular
to each other in the space on the x, y, z axes of Cartesian coordinate system. The p orbitals
have a spiral (dumb-bell) shape (figure 1.9).
Figure 1.9. The shape of s and p orbitals
The d subshell has 5 d orbitals and the shape is more complex.
Each orbital contains a maximum of 2 electrons, so:
The s subshell has 1 s orbital, containing a maximum of 2 electrons.
The p subshell has 3 p orbital, containing a maximum of 6 electrons.
The d subshell has 5 d orbital, containing a maximum of 10 electrons.
The concept of atomic orbitals helps us to understand orther concepts such as: the
hybridization of orbitals, bond (sigma), bond (pi) which you will study later.
THE ATOMIC ELECTRON CONFIGURATION
How are electrons arranged in the atomic shell of elemenst?
What is an electron configuration? How to write an
Characteristics of the outermost electron shell.
I THE ORDER OF THE ATOMIC ENERGY LEVELS
The electrons in atoms, starting from the
ground state, successively occupy the
lowest to higher energy levels.
From the innermost to outermost shells
and subshells, the energy levels of
electron shells rise in the order from 1 to
7 and energy of electron subshells rises
in the order of s,p,d,f.
This is the arrangement of electron
subshells in order of increasing energy,
which is determined experimentally and
1s2s2p3s3p4s3d4p5s… ( figure 1.10).
When the nuclear charge increases, there is an insert of the energy level, so the 4s energy
level is lower than the 3d.
II ATOMIC ELECTRON CONFIGURATION
1. Atomic electron configuration
An atomic electron configuration expresses the electron distribution in electron subshells of
different electron shells.
By convention, the atomic electron configuration is written as follows:
− The number of electron shell is writter in digits (1,2,3,…).
− Eletron subshells are written in lowercase letters (s, p, d, f).
− The number of electrons in one subshell is written in superscript numbers on the top right
of subshells ( s2, p6,…).
Steps in writing atomic electron configuration:
Step 1. Determine the number of electrons in the atom .
Step 2. The electrons are arranged successively into subshells in order of increasing energy
in the atom (1s 2s 2p 3s 3p 4s 3d 4p 5s…) following the rules: the subshell contains a
maximum of 2 electrons; the p subshell contains a maximum of 6 electrons; the d subshell
contains a maximum of 10 electrons; and the f subshell contains a maximum of 14
Step 3: write electron configuration to express the electron distribution in the subshells of
different shells (1s 2s 2p 3s 3p 3d 4s 4p…)
A hydrogen atom, Z=1, has 1 electron. The eletron configuration of an H atom is 1s1.
A helium atom, Z=2, has 2 electrons. The eletron configuration of a He atom is 1s2,
A lithium atom, Z=3, has 3 electrons. The eletron configuration of a Li atom is 1s22s1. The
last electron of a lithium atom is written in s subshells. Lithium is an s element.
A chlorine atom, Z= 17, has 17 electrons. The electron configuration of Cl atom is written as
follows: 1s22s22p63s23p5. Or in abbreviation: [Ne]3s23p5.
The last electron of chlorine atom is written in p subshell. Chlorine is a p element .
[Ne] is the electron configuration notation of a neon atom. It is the closest noble gas standing
An iron atom , Z= 26, has 26 electron. The electrons of an iron are distributed as follows:
The last electron of the Fe atom is written in d subshell. Iron is a d element.
The electron configuration of an Fe atom: 1s22s22p63s23p63d64s2.
Or in abbreviation: [Ar] 3d64s2
The s elements are the ones whose atoms have the last electron written in the s subshell.
The p elements are the one whose atoms have the last electron written in the p subshell.
The d elements are the ones whose atoms have the last electron written in the d subshell.
The f elements are the ones whose atoms have the last electron written in the f subshell.
2. Atomic electron configuration of the first 20 elements
Number of electron
symbol shell shell shell N shell
(n=1) (n=2) (n=3)
Electron configuration can be writter in the form of electrons per shell. For example: electron
configuration of Na is 1s22s22p63s1 or it can be written in the form of electron per shell as :
3. Characteristics of the outermost electron shell
In the atoms of all elements, the outermost electron shell contains a maximum of 8 electrons.
The atoms with 8 electrons in the outermost shell (ns2np6) and helium atom (1s2) are not
involved in chemical reaction (except in some special conditions) because the electron
configurations of these elements are every stable. They are the atoms of noble gas elements.
In nature, noble gas molecules contain only one atom.
The atoms with 1, 2, 3 electrons in the outermost shell that lose electrons easily are the atoms
metal elements (except H, He and B).
The atoms with 5, 6,7 electrons in the outermost shell that lose electrons easily are the atoms
of metal or non-metal elements .
The atoms with 4 outer electrons may be the atoms of metal or non-metal elements (see the
Hence, given the atomic electron configuration, we can predict the type of an element.
1. Element with Z=11 is:
A. s element.
B. p element.
C. d element.
D. f element.
Choose the correct answer.
2. The electron configuration of a sulfur element( Z=16) is:
Choose the correct answer.
3. The electron configuration of an aluminum atom (Z=13) is…Hence:
The first shell (K shell) has 2 electrons.
The second shell (L shell) has 8 electrons.
The third shell (M shell) has 3 electrons.
The outermost shell has 1 electron.
Find the incorrect answer.
4. The total number of protons, neutrons and electrons in an element’s atom is 13.
a) Determine the atomic mass.
b) Write the electron configuration of the element.
(Given: Elements that have the atomic numbers from 2 to 82 in the Periodic Table will have
5. How many electrons are there in the outermost shells of the atoms of elemments with the
atomic numbers equal to 3,6,9,18, respectively?
6. Write the electron configurations of the element pairs whose atomic nuclei have the
number of protons equal to:
a) 1, 3
b) 8, 16
c) 7, 9
Which elements are metals? Which elements are non-metals? Why?