Structure of the Atom

Atom Is Divisible

Do you recall Dalton’s atomic theory? Dalton postulated in his theory that an atom
is indivisible. However, the later discoveries of protons and electrons proved this to be
erroneous.

In 1886, while carrying out an experiment in a gas discharge tube, E. Goldstein discovered
positively charged radiations which led to the discovery of the subatomic particles called
protons. Later, in 1897, J. J. Thomson discovered another type of subatomic particle—the
negatively charged electron. Consequent to these discoveries, an atom was no longer
indivisible; rather, it became a sum total of differently charged subatomic particles.

We know that an atom is neutral. It is made up of an equal number of oppositely charged

particles—protons and electrons. Now, the question that arises is this:

How are the subatomic particles arranged inside an atom?

Many scientists performed varied experiments to develop different models for the
structure of an atom. The first such model was proposed by J. J. Thomson. His atomic model
is compared to a plum pudding and a watermelon; hence, it is known by the names ‘the
plum-pudding model’.

The Plum-Pudding Model of an Atom

Let us understand Thomson’s atomic

model with the help of a slice of a watermelon. The slice consists of a red edible portion
with embedded black seeds. Now, if we liken this watermelon to an atom, then (as per
Thomson’s model) the positive charge in the atom is spread all over the red edible part;
and the negatively charged particles, like the seeds, are embedded in this positively
charged space.
In the same way, we can liken an atom to a plum pudding. In this case, the positive charge is
spread all over the pudding, while the negatively charged particles are embedded like
plums in this positively charged space.

According to Thomson’s atomic model:

1. An atom consists of a positively charged sphere with electrons

embedded in it.

2. The negative and positive charges present inside an atom are equal in magnitude.
Therefore, an atom as a whole is electrically neutral.

Cathode Rays

J.J Thomson discovered that there are small particle present in the atom and that atom is
divisible. J.J Thomson and his colleagues conducted experiments using discharge tube
apparatus.
A discharge tube apparatus consists of a glass tube of about 15 cm length and 3 cm in
diameter, filled with gas at low pressure. The tube is connected with the vacuum pump and
two metal electrodes are fitted to the ends of the tube.
Low pressure was created inside the tube and high voltage was applied to the electrodes of
the tube. This produced greenish glow at the anode end of the tube. The greenish glow at
anode was produced due to the emission of the streams of rays from the cathode. These
rays are known as cathode rays. Cathode rays will emit with blue glow.
When J.J Thomson placed a light paddle wheel inside the tube in the path of the cathode
rays, the wheel started rotating. This led him to conclude that cathode rays are particulate
in nature.

Properties of Cathode Rays

When J.J Thomson applied an electric field in the direction parallel to the path of cathode
rays , he observed that the rays were deflected towards the anode.

This observation led to the conclusion that cathode rays are negatively charged.

When the above experiment was conducted with different gases, same observation were
made and he named these negatively charged particles as electrons.

An electron is lighter than hydrogen atom and has very small mass in comparison to the
mass of an atom.

Thus, J.J Thomson’s experiment and discovery of electron proved that atom is divisible and
is made up of sub – atomic particles.
Know Your Scientist
Sir Joseph John Thomson (1856−1940) was a British physicist. He is known for the
discovery of electrons and for his model of an atom, popularly known as ‘the plum-pudding
model’. He received the Nobel Prize in Physics in 1906 for discovering electrons and for his
research on conduction in gases. In 1912, while working on the composition of canal rays,
he and his colleague (F. W. Aston) found the first evidence for isotopes of neon.

Eugen Goldstein (1850−1930) was a German physicist. He is known for the discovery of
canal rays which led to the discovery of protons. He also investigated comets using gas
discharge tubes. His experiments established that a small object (like a ball) placed in the
path of cathode rays produces emissions, flaring outward just like in case of a comet’s tail.
 After J.J Thomson’s discovery of atom another question arose that: if electrons are
present inside the atom, then how is atom electrically neutral? Does this mean that
there are positively charged particle also present inside the atom?

To find out the answers to such questions, Goldstein conducted an experiment similar to
that of J.J Thomson’s but with some modifications, for example he used perforated cathode
in the discharge tube.

It was observed during the experiment that some rays were travelling in the direction
opposite to that of cathode rays. Goldstein named these rays as anode rays.
When he applied an electric field in the direction parallel to that of the rays he observed
that rays deflected towards cathode, thereby he concluded that anode rays are positively
charged.
However, the deflection of anode rays in the discharge tube was found to be very less than
that of cathode rays, because the emission of cathode rays was not dependent on the nature
of the gas taken in the discharge tube. The deflection was seen highest for the hydrogen
gas, when taken in the discharge tube.
The positive particles of hydrogen were found to be lightest and were named protons.
Their mass is approximately equal to 1840 times that of electron. This mass is assumed
as 1 atomic mass unit. The charge on a proton (+1) is equal to charge on an electron in
magnitude (-1).

Rutherford's Atomic Model

Why the Plum-Pudding Model Failed

The plum-pudding model of an atom was unable to explain the findings of Rutherford’s
experiment while studying radioactivity.

In an experiment with gold foil, Rutherford bombarded the gold foil with alpha particles.
With Thomson’s model as the basis, Rutherford expected small deviations; however, his
findings were different from what was expected.

As we go further into this lesson, we will learn more about Rutherford’s gold-foil
experiment, his observations and his conclusions. We will also learn about the atomic
model that he came up with on the basis of his conclusions.

Set up for Rutherford’s experiment:

1. A thin gold foil, approximately 1000 atoms thick, was taken. Gold was chosen for its high
malleability.
2. A detector screen with a small slit (for emission of radiation from the atom) was placed
around the foil.
3. A source of alpha particles was kept in front of the foil.
4. The foil was bombarded with fast-moving alpha particles.

The set-up for Rutherford’s gold-foil experiment is shown in the figure.

Rutherford’s Expectations and Observations

What Rutherford expected?

Rutherford expected that the alpha particles would pass straight through the foil and only a
small fraction of alpha particles would be deflected. This expectation was in compliance
with Thomson’s atomic model.

What Rutherford observed?

 Rutherford’s findings were contrary to his expectation. He observed that:

1. Most of the fast-moving alpha particles passed straight through the gold foil.

2. Some particles were deflected through the foil by small angles.

3. One out of every 12000 particles rebounded, i.e., they got deflected by an angle of 180°.

What Rutherford Concluded from His Observations

Rutherford then carefully studied his observations and made the following conclusions.

1. Most alpha particles passed through the gold foil without any deflection. This indicates that
most of the space inside an atom is empty.
2. Very few particles suffered a deflection from their path. This means that positive charge
occupies very little space inside an atom.
3. Only a small fraction of particles underwent a 180° deflection. This shows that the entire
positive charge and mass of an atom are present within a very small volume inside the
atom.

Rutherford’s Atomic Model

Based on his conclusions in the gold-foil experiment, Rutherford devised his own atomic
model. The major features of Rutherford atomic model or the nuclear model of an
atom are as follows:

1. An atom consists of a nucleus at its centre and all the protons are present inside this
nucleus.
2. Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small as compared to the size of the atom. As per
Rutherford’s calculations, the nucleus is 105 times smaller than the atom.
4. Since the mass of the electrons is negligible as compared to the mass of the protons, almost
all the mass of the atom is concentrated in its nucleus.
 Know Your Scientist

Ernest Rutherford (1871−1937) was a British chemist and physicist. He is known as ‘the
father of nuclear physics’. He discovered radioactive half-life. He proved that alpha
radiations are nothing but helium ions. He was awarded the Nobel Prize in Chemistry in
1908 for his work on ‘the disintegration of elements’ and ‘the chemistry of radioactive
substances’. He was the first scientist to split an atom in a nuclear reaction. The element
‘rutherfordium’ (atomic number 104) is named after him.
Solved Examples

Hard

Example 1:

What would have been observed if neutrons had been used to bombard the gold foil?

1. The observations of the experiment would have remained the same in spite of the
change in the nature of the bombarding particles.
2. The neutrons would have suffered no deflection from the subatomic particles.
3. All the neutrons would have been absorbed by the gold atoms.
4. All the neutrons would have rebounded.

Solution:
 The correct answer is B.

Neutrons do not carry any charge; so, they do not suffer any repulsion. Hence, if neutrons
had been used to bombard the gold foil, no deflection would have occurred. It is also
possible that some neutrons would have been absorbed by the nucleus.

Medium

Example 2:

State whether the following statements are true (T) or false (F).

1. Increasing the energy of the alpha particles will lead to more deflection.____
2. Speed of the alpha particles can be increased by increasing their energy.____
3. Use of aluminium sheet will lead to the same result as in case of gold foil.____

Solution:

1. T: Increasing the energy of the alpha particles will cause them to strike closer to the
nucleus. Consequently, they will suffer greater deflection.
2. T: The kinetic energy of the alpha particles is directly related to their velocity. So,
increasing their energy will result in an increase in the speed of the particles.
3. F: The positive charge on the nucleus in case of an aluminium foil is much smaller as
compared to that on the gold nucleus. So, the result will vary.

Easy

Example 3:

One of the postulates of Rutherford’s atomic model is that

1. an atom consists of a positively charged sphere.

2. an atom has its mass concentrated in its nucleus.
3. the nucleus of an atom is composed of electrons and protons.
4. the mass of an atom is the sum of the masses of all electrons and protons.

Solution:

The correct answer is B.

The postulates of Rutherford’s atomic model are as follows:

1. An atom consists of a nucleus at its centre and all the protons are present inside this
nucleus.
2. Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.
3. The size of the nucleus is very small as compared to the size of the atom. As per
Rutherford’s calculations, the nucleus is 105 times smaller than the atom.
4. Since the mass of the electrons is negligible as compared to the mass of the protons, almost
all the mass of the atom is concentrated in its nucleus.

Rutherford also noticed that the actual mass of the nucleus was much more higher than the
sum of the masses of protons and electrons. This lead him to predict that nucleus contains
some kind of neutral particle whose mass must be equal to that of proton.

This was experimentally proved by James Chadwick in the year 1932. He proved that
nucleus of atom contains an additional neutral particle and called them neutrons. The
mass of these neutrons is equal to that of protons.

Bohr's Atomic Model

The Shell Model of an Atom

In the late nineteenth century, scientists researched on the dispersal of white light into its
constituent seven colours—known as the spectrum.

The spectrum was used for analyzing newly discovered elements. It was observed that this
spectrum was different for different elements.

Now, the above observation could not be explained using Rutherford’s model of an atom.
Thus, Niels Bohr made some modifications to Rutherford’s model. The modified atomic
model of Niels Bohr is also known as the shell model of an atom.

Let us go through this lesson to learn more about this model.

 Bohr’s Model of an Atom

The postulates of Bohr’s model of an atom are as follows:

1. Only certain special orbits,known as discrete orbits of electrons, are allowed inside the
atom.

2. While revolving in the discrete orbits, the electrons do not radiate energy.

3. An electron can jump from one orbit to another by absorbing or emitting a fixed amount
of energy in the form of radiation.

Bohr named these orbits as energy levels. These orbits (or shells) are represented by the
letters K, L, M, N…, or the numbers n = 1, 2, 3, 4….

Bohr’s model of an atom explains how:

 the electrons are arranged and distributed in the extra-nuclear space of the atom.
 the atom attains stability due to the presence of energy levels around the nucleus, in which
the electrons revolve without radiating energy.
 each energy shell can accommodate only a fixed number of electrons.
 the filling of a shell begins only when the preceding shell has been completely filled.
 this gives the atom a shell-like structure.

Distribution of Electrons or Electronic Configuration

Let us see how the electrons are distributed in different orbits in an atom. Bohr, along with
Charles R. Bury, suggested certain rules to show this electronic distribution. These rules
(known as the Bohr−Bury scheme of electronic configuration) have to be followed while
writing the electronic configuration of an atom.

1. The maximum number of electrons in a shell is given by the formula 2n2, where n is the
orbit number or the energy level index (i.e., 1, 2, 3…).
 Orbit Names of the Numbers of electrons in the
numbers shells shells
1 K-shell 2 × (1)2 = 2

2 L-shell 2 × (2)2 = 8

3 M-shell 2 × (3)2 = 18

4 N-shell 2 × (4)2 = 32

2. The maximum number of electrons that can be accommodated in the outermost shell is 8.
3. The filling of the shells takes place in a stepwise manner. First, one shell is filled
completely, then the next shell, and so on.

Solved Examples

Medium

Example 1:

Which of the following statements is true for Bohr’s model of an atom?

1. Electrons go around the nucleus based on the strength of the force of attraction
extended by the nucleus.
2. Electrons lose energy while travelling in orbits around the nucleus.
3. The energy shells can accommodate only a fixed number of electrons.
4. The energy of the shells is continuous.

Solution:

The correct answer is C.

According to Bohr’s model of an atom:

 Only certain special orbits, known as discrete orbits of electrons, are allowed inside the
atom.
 While revolving in the discrete orbits, the electrons do not radiate energy.
 The energy shells can accommodate only a fixed number of electrons.
 The energy of the shells is discrete, and not continuous.

Easy

Example 2: An element has 12 electrons. How many energy shells does this element
possess?
1. 1
2. 2
3. 3
4. 4

Solution: The correct answer is C.

According to the Bohr−Bury scheme, each energy shell can accommodate 2n2 electrons,
where n is the energy number. In case of any element, the electrons are distributed as
shown in the table.
Shells Energy Numbers of electrons in the
numbers shells
K 1 2 × (1)2 = 2

L 2 2 × (2)2 = 8

M 3 2 × (3)2 = 18

Now, the given element has 12 electrons. So, the K-shell fills first with 2 electrons. The
remaining 10 electrons are divided among the L-shell and the M-shell as follows: 8 in the L-
shell and 2 in the M-shell. Thus, the element has a total of 3 energy shells.

Hard

Example 3:

What is the electronic configuration for an element having 17 electrons?

1. 2, 8, 7
2. 2, 6, 9
3. 2, 10, 5
4. 2, 7, 8

Solution:

The correct answer is A.

According to the Bohr−Bury scheme, each energy shell can accommodate 2n2 electrons,
where n is the energy number. In case of any element, the electrons are distributed as
shown in the table.
Shells Energy Numbers of electrons in the
numbers shells
 K 1 2 × (1)2 = 2

L 2 2 × (2)2 = 8

M 3 2 × (3)2 = 18

Now, the given element has 17 electrons. So, the K-shell fills first with 2 electrons. The
remaining 15 electrons are divided among the L-shell and the M-shell as follows: 8 in the L-
shell and 7 in the M-shell. Thus, the electronic configuration for the element is 2, 8, 7.
Electronic Configuration of Atoms

Bohr postulated that electrons can move from one shell to another by absorbing or
emitting energy in the form of radiation. Let us understand this phenomenon.

As you know, in Bohr’s atomic model, electrons are arranged in energy shells and each of
these shells has a fixed amount of energy. The electrons residing in a particular shell
possess the characteristic energy of the shell. The energy of an electron remains constant
as long as it remains in a particular energy level.

In an atom, shells are arranged in order of increasing energy.

So, the shells in increasing order of energy are as follows:

K<L<M<N

Excitation and De-Excitation of Electrons

Excitation of electrons:
An electron lying in a lower energy shell absorbs energy from incident light to gain energy
corresponding to that of the adjacent higher energy level. This is shown in the figure.

De-excitation of electrons:

An electron lying in a higher energy shell emits energy in the form of electromagnetic
radiation to lose energy corresponding to that of the adjacent lower energy level. This is
shown in the figure.

The emitted radiation may lie in any region of the electromagnetic spectrum, depending
upon the frequency of the radiation.

Sub-Shells in an Atom and Discovery of Neutron

Sub-Shells in an Atom:

Further research on atomic structure indicated that the energy shells K, L, M, N, etc., are
further divided into sub-shells of different shapes and energies. The division of shells into
sub-shells is as follows:

Main energy Sub-shell(s) in the main

levels levels
K (1st) s
L (2nd) s, p
M (3rd) s, p, d

N (4th) s, p, d, f
 In an atom, these sub-shells are arranged in order of increasing energy and are filled
successively

Discovery of Neutrons:

 In 1932, J. Chadwick discovered the sub-atomic particles known as neutrons.

 Neutrons are present in all atomic nuclei, except that of hydrogen.
 The mass of a neutron was found to be 1.6749 × 10−27 kg, which is slightly more than that
of a proton.
 Mass of an atom = Mass of all neutrons + Mass of all protons

Did You Know?

A hydrogen atom contains only one electron and one proton. It does not have any neutron
in its nucleus. The removal of the electron from the atom leaves behind the single-proton-
containing nucleus; so, H+ is sometimes referred to as simply ‘proton’.
Know Your Scientist

Neils Bohr (1885−1962) was a Danish physicist. He made major contributions to the
understanding of the atomic structure and quantum mechanics. He was awarded the Nobel
Prize in Physics in 1922. He is credited with developing the planetary model of an atom.
While working on quantum mechanics, he postulated that electrons jump from one energy
level to another by absorbing or emitting discrete amounts of energy. He also identified the
uranium isotope U-238.
James Chadwick (1891−1974) was a British physicist. He is credited with the discovery
of neutrons, for which he received the Nobel Prize in Physics in 1935. He was part of the
Manhattan Project in the US and helped in the development of the atomic bombs that were
dropped on Hiroshima and Nagasaki during the Second World War.
Atomic Number and Mass Number
Atomic Number and Mass Number

In the 1830s, representation of elements and compounds was a major concern for
chemists.

Many symbolic notations for elements were devised during this period. Gradually, the
representations became standardized. Currently, the general symbolic notation for an
element is:

. Now, take for example the specific symbolic notations for oxygen and nitrogen.

Element Symbolic notation

Oxygen
Nitrogen

Wondering what these symbolic notations represent? Go through this lesson to find out.

You know that the symbolic notation of oxygen is . In this notation, the letter ‘O’
symbolises the element ‘oxygen’; the number ‘16’ represents the mass number of oxygen;
and the number ‘8’ indicates the atomic number of oxygen.

Thus, in the general symbolic notation of an element , the letter ‘E’ is the symbol of
the element, the letter ‘A’ is its mass number, and the letter ‘Z’ is its atomic number.

The atomic number is the number of protons present in the nucleus of an atom. It is
denoted by Z.
The total number of the protons and the neutrons present in the nucleus of an atom is
known as mass number. It is denoted by A.

Symbolic Notations of Some Elements

Elements Symbolic Symbols Atomic Mass

notations numbers numbers
Hydrogen H 1 1
Helium He 2 4
Lithium Li 3 7
Beryllium Be 4 9
Boron B 5 11
Carbon C 6 12
Nitrogen N 7 14
Oxygen O 8 16
Fluorine F 9 19
Neon Ne 10 20

Symbolic Notations of Some Elements

Elements Symbolic Symbols Atomic Mass

notations numbers numbers
Sodium Na 11 23
Magnesium Mg 12 24
Aluminium Al 13 27
Silicon Si 14 28
Phosphorus P 15 31
Sulphur S 16 32
Chlorine Cl 17 35
Argon Ar 18 40
Potassium K 19 39
Calcium Ca 20 40

Relation between Atomic Number and Mass Number

 Mass number (A) of an atom = Number of protons + Number of neutrons

Therefore, Mass number (A) = Atomic number (Z) + Number of neutrons

Therefore, Number of neutrons = A - Z

Hence, the number of neutrons can be calculated if the atomic number and mass number of
an element are known.

An atom of sodium contains 11 protons and 12 neutrons. Can you calculate the mass
number of a sodium atom?

Now, mass number (A) = number of protons + number of neutrons

Therefore, mass number of sodium atom = 11 + 12 = 23

Hence, the mass number of sodium is 23.

An atom of carbon is represented as . Can you tell the number of neutrons and
protons present in carbon atom?

It is seen from the symbolic notation of carbon that the atomic number and mass number of
carbon atom is 6 and 12 respectively.

Now, number of neutrons = mass number − atomic number = 12 − 6 = 6

Since the number of protons is equal to the atomic number of that element. Thus, the
number of protons present in a carbon atom is 6.

Solved Examples

Easy

Example 1: What is the symbol of the element sodium?

1. Na
2. N
3. So
4. S

Solution: The correct answer is A.

The symbol of sodium is Na. It is derived from the Latin name for the element, i.e.,
‘natrium’.
 Example 2: What is the atomic number of an element having five protons and six
neutrons?

1. 11
2. 9
3. 6
4. 5

Solution:

The correct answer is D.

The atomic number of an element is the number of protons or electrons present in an atom
of the element. Since an atom of the given element has five protons, its atomic number is 5.

Medium

Example 3: What is the number of neutrons in an element having 39 protons and 89

as its mass number?

1. 45
2. 50
3. 55
4. 60

Solution:The correct answer is B.

We know that:

Mass number = Number of protons + Number of neutrons

In case of the given element:

Mass number = 89

Number of protons = 39

So,

89 = 39 + Number of neutrons

=> Number of neutrons = 89 -39 = 50

Hard
 Example 4: What is the symbol of the element having 22 neutrons and 40 as its mass
number?

1. Al
2. Mg
3. Ar
4. Ca

Solution:The correct answer is C.

The given element has:

Mass number = 40

Number of neutrons = 22

We know that:

Mass number = Number of protons + Number of neutrons

So,

40 = Number of protons + 22

=> Number of protons = 40 - 22 = 18

Also,

Atomic number = Number of protons = 18

Argon is the element having 18 as its atomic number and 40 as its mass number. The
symbol of argon is Ar.
Did You Know?

 Water is the major constituent of the human body. It is made up of two elements: hydrogen
and oxygen.
 Almost all the mass of our body is made up of the following six elements.

1. Oxygen (65%)
2. Carbon (18%)
3. Hydrogen (10%)
4. Nitrogen (3%)
5. Calcium (1.5%)
6. Phosphorus (1%)
 Some of the other elements found in our body are:
 Sulphur (0.25%)
 Sodium (0.15%)
 Magnesium (0.05%)
 Zinc (0.7%)

Whiz Kid

The Periodic Table

The periodic table is a table classifying all the known elements.

It is divided into 18 columns (called groups) and 7 rows (called periods).

The elements are arranged in the rows or periods by order of increasing atomic number.

The elements in the columns or groups display similar chemical and physical properties.
This feature of the periodic table makes it easy to study the vast number of elements.

The periodic table is shown in the figure.

Isotopes
In 1910, Frederick Soddy recorded the existence of elements having different atomic
masses to show similar properties. These elements came to be known as isotopes.
Isotopes are defined as atoms of same element having the same atomic number, but
 different mass numbers. These atoms contain an equal number of protons and electrons,
but a different number of neutrons.
For example, in nature, hydrogen is found in three forms with different mass numbers,
namely protium (11H), deuterium(12H), and tritium ( 13H). These are the three naturally
occurring isotopes of hydrogen. The atomic number of each isotope is 1, but the mass
number varies i.e. it is 1, 2, and 3 respectively. Some other examples of isotopes include C-
12 and C-14, which are isotopes of carbon, and Cl-35 and Cl-37, which are isotopes of
chlorine.

However, it was F. W. Aston who, in 1919, discovered various stable isotopes for a number
of elements. In that year, he was successful in proving the existence of two isotopes of
neon:
neon-20 and neon-22.

The isotopes of an element are species with different numbers of neutrons but the same
numbers of protons and electrons.

Isotopes are pure substances just like elements and compounds.

We know that the atomic mass of an element is the sum total of its protons and neutrons.
Now, in case of an element having isotopes, the number of neutrons does not remain fixed.
The atomic mass of such an element includes the atomic masses of its isotopes. This gives
rise to the term ‘average atomic mass’. The average atomic mass of an element with
isotopes takes into account the atomic masses of all its isotopes with respect to their
abundance in nature.

Applications of Isotopes

1. An isotope of uranium exhibits nuclear fission properties. It is used in nuclear reactions as

a fuel.
2. An isotope of cobalt is used for treating cancer.
3. An isotope of iodine is used for treating goitre.
4. An isotope of carbon is used in radiocarbon dating to determine the age of an organic
sample.
5. An isotope of calcium is used in biomedical research on cellular functions and bone
formation in mammals.
6. An isotope of iron is used for detecting sulphur in air.
7. An isotope of hydrogen (tritium) is used in estimating the age of water bodies and the rate
of their replenishment through precipitation.

Isobars and Isotones

Isobars
 Isobars are elements having the same mass number but different atomic numbers. Take,
for example, . They have the same mass number (i.e., 13) but different atomic
numbers (i.e., 6 and 7).

One can also say that isobars have an equal number of nucleons but different numbers of
protons, neutrons and electrons.

Here are two more examples of isobars:

1.
2.

Isotones

Isotones are atoms of different elements having the same number of neutrons.

Here are some examples isotones:

1.
2.
3.

Solved Examples

Easy

Example 1: Which of the following atomic pairs are isotopes of each other?

A.
B.
C.
D.

Solution:

The correct answer is B.

Isotopes are those species of the same element which possess the same number of protons
but different numbers of neutrons. In other words, isotopes have the same atomic number
but different atomic masses. Among the given pairs, only is an atomic pair
with the same number of protons

Medium

Example 2:

are the three stable isotopes of magnesium. The average atomic

mass of magnesium is 24.3. The relative abundance of and are 78.7% and
11.2% respectively. What percentage of magnesium existing in nature is ?

A. 10%
B. 12%
C. 14%
D. 16%

Solution:

The correct answer is A.

Let us take:

Average atomic mass of magnesium = M

Atomic mass and relative abundance of = m1 and p

Atomic mass and relative abundance of = m2 and q

Atomic mass and relative abundance of = m3 and r

Now, we know that:

M = 24.3

m1 = 24 and p = 78.7%

m2 = 25
 m3 = 26 and r = 11.2%

The average atomic mass of magnesium can be found as:

Hence, 10% of the magnesium existing in nature is .

Radioactivity

Radioactivity is the phenomenon wherein the nucleus of an unstable atom loses energy
after emitting ionized particles or radiations. In this process, the original atom transforms
into a new stable atom.

In nature, there are many unstable isotopes that emit radiations. The common types of
radiations are:

1. Alpha radiation

1. It consists of a stream of positively charged particles that are generally helium ions, having
atomic mass as 4 and carrying a +2 charge.
2. It results in a decrease in atomic number by 2 and a decrease in mass number
by 4.

2. Beta radiation

1. It consists of a stream of electrons. So, it is negatively charged.

2. It results in an increase in atomic number by 1, but mass number remains unchanged.

3. Gamma radiation
1. It is a photon (packet of light) with very high energy.
2. It does not result in a change in either atomic number or mass number.