Chapter 8: Journey Inside the Atom

Everything around us—living beings, non-living things, air, water, food, buildings, plants, and animals—is made up of matter. Matter is made up of extremely tiny particles called atoms.

Atoms are so small that we cannot see them with our naked eyes. Earlier, atoms were thought to be the smallest indivisible particles of matter. Later, scientists discovered that atoms are made up of even smaller particles called subatomic particles.

The main questions studied in this chapter are:

• Question: Are atoms the smallest indivisible particles?
• Answer: No. Atoms are made up of smaller particles such as electrons, protons, and neutrons.

• Question: Why did scientists keep modifying atomic models?
• Answer: Scientists modified atomic models because new experiments gave new evidence that older models could not explain.

The idea that matter is made up of tiny particles is very old.

Ancient Indian Idea

The Indian philosopher Acharya Kanada suggested that if matter is divided again and again, we reach a stage where it cannot be divided further. He called these smallest particles parmanus.

According to him:

Matter is made up of extremely small particles called parmanus. These parmanus combine to form larger particles and finally all material objects.

Ancient Greek Idea

Greek philosophers Leucippus and Democritus also proposed that matter is made up of tiny indivisible particles. They called them atomos, meaning “indivisible”.

Dalton’s Atomic Theory

In 1808, John Dalton proposed a scientific atomic theory based on experiments.

According to Dalton:

Atoms are the smallest particles of matter. They are indivisible and cannot be broken into smaller parts.

Dalton’s theory was important because it was the first scientific explanation of the structure of matter. However, later discoveries showed that atoms are divisible and are made up of smaller particles.

Scientists proposed different atomic models to explain the structure of atoms. Each model improved the earlier one.

The journey of atomic models is:

Dalton’s model → Thomson’s model → Rutherford’s model → Bohr’s model → Modern atomic model

Discovery of Electron

In 1897, J. J. Thomson studied the passage of electric current through gases at very low pressure using a cathode ray tube.

He observed rays moving from the cathode to the anode. These rays were called cathode rays.

Thomson concluded that cathode rays were made up of tiny negatively charged particles. These particles were later called electrons.

Important Conclusion

Electrons are present in all atoms. Therefore, atoms are divisible.

Thomson’s Atomic Model

Since atoms are electrically neutral, Thomson thought that the negative charge of electrons must be balanced by positive charge.

He proposed that:

An atom is a positively charged sphere in which negatively charged electrons are embedded.

This model is called the plum pudding model.

It can also be compared with a watermelon:

• The red pulp represents the positive charge.
• The seeds represent electrons.

Limitations

Thomson’s model could explain electrical neutrality of atoms, but it could not explain the results of Rutherford’s gold foil experiment.

In 1911, Geiger and Marsden, working under Ernest Rutherford, performed the famous gold foil experiment.

Experiment

They passed a beam of positively charged alpha particles through a very thin gold foil.

Expected Observation

According to Thomson’s model, positive charge was spread throughout the atom. So alpha particles were expected to pass straight through the gold foil with little or no deflection.

Actual Observations

• Most alpha particles passed straight through the foil.
• Some alpha particles were slightly deflected.
• A very few alpha particles were deflected through large angles or bounced back.

Conclusion

The experiment proved that Thomson’s model was incorrect.

From the gold foil experiment, Rutherford gave a new atomic model.

Main Features of Rutherford’s Model

Most of the atom is empty space because most alpha particles passed straight through.

The positive charge of the atom is concentrated in a very small central region called the nucleus.

The nucleus is dense and contains most of the mass of the atom.

Electrons revolve around the nucleus like planets revolve around the Sun.

This model is also called the planetary model of the atom.

Size of Atom and Nucleus

• Diameter of atom ≈ 10⁻¹⁰ m
• Diameter of nucleus ≈ 10⁻¹⁵ m

This means the nucleus is about one lakh times smaller than the atom.

Rutherford’s model could not explain the stability of atoms.

According to physics, an electron moving in a circular path around the nucleus should lose energy because it is constantly changing direction.

If electrons lose energy, they should spiral inward and fall into the nucleus.

But this does not happen. Atoms are stable.

Therefore, Rutherford’s model was incomplete.

Rutherford showed that the nucleus contains positively charged particles called protons.

Proton

• A proton has a positive charge.
• Its charge is equal in magnitude but opposite to the charge of an electron.
• It is much heavier than an electron.

Electrical Neutrality of Atom

An atom is neutral because:

Number of protons = Number of electrons

For example:

• Helium has 2 protons and 2 electrons.
• Sodium has 11 protons and 11 electrons.

So, their total positive and negative charges balance each other.

In 1913, Niels Bohr proposed a new atomic model to explain atomic stability.

Main Features of Bohr’s Model

Electrons revolve around the nucleus in fixed circular paths called orbits, shells, or energy levels.

These shells are represented as:

K, L, M, N …

or

n = 1, 2, 3, 4 …

The shell closest to the nucleus is the K-shell.

Electrons can revolve only in certain allowed shells, not between shells.

Electrons do not lose energy while moving in a fixed shell.

The energy of shells increases as we move away from the nucleus.

• K-shell has the least energy.
• L-shell has more energy than K-shell.
• M-shell has more energy than L-shell.

An electron can move from one shell to another by absorbing or releasing a fixed amount of energy.

Importance of Bohr’s Model

Bohr’s model explained why electrons do not fall into the nucleus. It also explained atomic stability better than Rutherford’s model.

Rutherford’s model showed that most of the mass of an atom is present in the nucleus.

Electrons are very light, so their mass is usually ignored.

Scientists found that the mass of atoms could not be explained only by protons. For example, helium has 2 protons, but its mass is about four times that of hydrogen. This suggested that there must be another particle in the nucleus.

In 1932, James Chadwick discovered the neutron.

Neutron

• A neutron has no charge.
• It is electrically neutral.
• Its mass is nearly equal to the mass of a proton.
• It is present in the nucleus of all atoms except ordinary hydrogen.

Subatomic Particles

Contribution to Atomic Mass

The mass of an atom mainly comes from:

Protons + Neutrons

Electrons have negligible mass.

Role of Neutrons

Neutrons help in increasing the stability of the nucleus. They reduce the repulsion between protons and help bind the nucleus together through nuclear force.

Earlier, Dalton used pictorial symbols to represent elements. Later, Berzelius suggested that symbols should be written using letters.

Today, the names and symbols of elements are approved by IUPAC.

Rules for Writing Symbols

The first letter of a symbol is always capital.

The second letter, if present, is always small.

Examples:

• Hydrogen — H
• Aluminium — Al
• Cobalt — Co
• Chlorine — Cl
• Zinc — Zn

Some symbols come from Latin, Greek, or German names.

Examples:

• Iron — Fe from Latin Ferrum
• Sodium — Na from Latin Natrium
• Potassium — K from Latin Kalium
• Gold — Au from Latin Aurum
• Silver — Ag from Latin Argentum
• Mercury — Hg from Greek Hydrargyros
• Tungsten — W from German Wolfram

Importance of Symbols

Chemical symbols are internationally accepted. They help scientists communicate clearly across different languages.

The atomic number of an element is the number of protons present in the nucleus of one atom of that element.

It is represented by Z.

Formula

Atomic number = Number of protons

For a neutral atom:

Number of protons = Number of electrons

Examples

• Hydrogen has 1 proton.
• Atomic number of hydrogen = 1

• Helium has 2 protons.
• Atomic number of helium = 2

• Sodium has 11 protons.
• Atomic number of sodium = 11

Importance of Atomic Number

Atomic number decides the identity of an element.

For example, every atom with atomic number 6 is carbon. Every atom with atomic number 8 is oxygen.

The mass number of an atom is the total number of protons and neutrons present in its nucleus.

It is represented by A.

Formula

Mass number = Number of protons + Number of neutrons

Protons and neutrons together are called nucleons.

Example

Carbon has:

• 6 protons
• 6 neutrons

Mass number = 6 + 6 = 12

So, carbon is written as:

¹²₆C

Here:

• 12 = Mass number
• 6 = Atomic number
• C = Symbol of carbon

Finding Neutrons

Number of neutrons = Mass number − Atomic number

Example:

If an atom has mass number 23 and atomic number 11:

Number of neutrons = 23 − 11 = 12

Bohr and Bury gave rules for the arrangement of electrons in shells.

Rule 1: Maximum Electrons in a Shell

The maximum number of electrons in a shell is given by:

2n²

where n is the shell number.

Rule 2: Outermost Shell Rule

The outermost shell can have a maximum of 8 electrons.

Exception: The first shell, K-shell, can hold only 2 electrons.

Rule 3: Stepwise Filling

Electrons fill shells step by step from inner to outer shells.

Order of filling:

K → L → M → N

The arrangement of electrons in different shells is called electronic configuration.

Electronic Configuration of First 18 Elements

Atoms combine with other atoms to form molecules and compounds.

The combining capacity of an atom is called its valency.

Valence Shell

The outermost shell of an atom is called the valence shell.

Valence Electrons

Electrons present in the outermost shell are called valence electrons.

Octet

If the outermost shell has 8 electrons, it is called an octet.

Atoms with complete octet are stable and mostly unreactive.

Example:

• Neon has electronic configuration 2, 8.
• Its outermost shell is complete.
• So, neon is stable and unreactive.

Helium is stable with 2 electrons because it has only one shell.

Valency

Valency is the number of electrons lost, gained, or shared by an atom to achieve a stable electronic configuration.

How to Find Valency

If valence electrons are 1, 2, or 3:

Valency = Number of valence electrons

If valence electrons are 5, 6, or 7:

Valency = 8 − Number of valence electrons

If valence electrons are 4:

Valency = 4

If valence electrons are 8 or 2 in helium:

Valency = 0

Examples

• Sodium: 2, 8, 1
• Valence electrons = 1
• Valency = 1

• Oxygen: 2, 6
• Valence electrons = 6
• Valency = 8 − 6 = 2

• Carbon: 2, 4
• Valence electrons = 4
• Valency = 4

• Neon: 2, 8
• Valence electrons = 8
• Valency = 0

Valency of First 18 Elements

Dalton had said that all atoms of an element are identical and have the same mass. Later, scientists discovered that atoms of the same element can have different numbers of neutrons.

Definition

Isotopes are atoms of the same element having the same atomic number but different mass numbers.

This means:

• They have the same number of protons.
• They have the same number of electrons.
• They have different numbers of neutrons.
• They have different mass numbers.

Isotopes of Hydrogen

Hydrogen has three isotopes:

Isotopes of Carbon

Carbon has three isotopes:

• ¹²₆C
• ¹³₆C
• ¹⁴₆C

All have 6 protons and 6 electrons, but different numbers of neutrons.

Chemical Properties of Isotopes

Isotopes have similar chemical properties because they have the same number of electrons and the same electronic configuration.

Physical Properties of Isotopes

Isotopes may have different physical properties because they have different masses.

Uses of Isotopes

• Uranium-235 is used as fuel in nuclear reactors.
• Cobalt-60 is used in cancer treatment.
• Iodine-131 is used to treat goitre and thyroid cancer.
• Carbon-14 is used to find the age of ancient fossils and artefacts.

Some elements occur naturally as a mixture of isotopes.

For example, chlorine occurs mainly as two isotopes:

³⁵Cl and ³⁷Cl

They occur in the ratio 3 : 1.

This means:

• 75% chlorine atoms are ³⁵Cl
• 25% chlorine atoms are ³⁷Cl

Weighted Average Atomic Mass of Chlorine

• Average atomic mass
• = 35 × 75/100 + 37 × 25/100
• = 26.25 + 9.25
• = 35.5 u

So, the average atomic mass of chlorine is 35.5 u.

This does not mean that one chlorine atom has mass 35.5 u. It means that the average mass of a large sample of chlorine atoms is 35.5 u.

Definition

Isobars are atoms of different elements having the same mass number but different atomic numbers.

This means:

• They belong to different elements.
• They have different numbers of protons.
• They have the same mass number.

Examples

Calcium, potassium, and argon can have mass number 40:

• ⁴⁰₂₀Ca
• ⁴⁰₁₉K
• ⁴⁰₁₈Ar

They have the same mass number, 40, but different atomic numbers.

So, they are isobars.

Atomic Number

Z = Number of protons

For a neutral atom:

Number of protons = Number of electrons

Mass Number

A = Number of protons + Number of neutrons

Number of Neutrons

Number of neutrons = Mass number − Atomic number

Maximum Electrons in a Shell

Maximum electrons = 2n²

where n = shell number.

Atoms are the building blocks of matter.

Earlier, atoms were considered indivisible, but later discoveries proved that atoms contain smaller particles.

J. J. Thomson discovered electrons and proposed the plum pudding model of the atom.

Rutherford’s gold foil experiment proved that most of the atom is empty space and that positive charge and most mass are concentrated in a small nucleus.

Rutherford proposed the nuclear or planetary model of the atom.

Rutherford’s model failed to explain atomic stability.

Bohr proposed that electrons revolve around the nucleus in fixed energy levels or shells without losing energy.

The shells are named K, L, M, N and so on.

Rutherford discovered the proton.

James Chadwick discovered the neutron.

The three main subatomic particles are electrons, protons, and neutrons.

Atomic number is the number of protons in an atom.

Mass number is the total number of protons and neutrons.

Electrons are distributed in shells according to certain rules.

The maximum number of electrons in a shell is given by 2n².

The outermost shell is called the valence shell.

Valency is the combining capacity of an atom.

Atoms become stable by completing their outermost shell.

Isotopes are atoms of the same element with the same atomic number but different mass numbers.

Isobars are atoms of different elements with the same mass number but different atomic numbers.

The modern atomic model says that electrons do not move in fixed circular paths but exist in regions called electron clouds.

• Atomic number = protons
• Mass number = protons + neutrons
• Neutrons = mass number − atomic number
• Neutral atom: protons = electrons
• Valency depends on valence electrons
• Isotopes: same Z, different A
• Isobars: same A, different Z
• Stable atoms usually have 8 electrons in the outermost shell
• Helium is stable with 2 electrons
• Protons and neutrons are present in the nucleus
• Electrons revolve around the nucleus in shells according to Bohr’s model