Class 9 | journey inside the atom

Rediscovering Atomic Theory

The idea that matter is made up of tiny particles is very old. Scientists gradually developed the modern atomic theory through observations and experiments.

Kanada’s Idea of Atoms

Kanada was an ancient Indian philosopher and scientist associated with the Vaisheshika school of philosophy.

• He proposed that matter is made up of extremely small particles called Parmanu.
• According to his ideas, a Parmanu was the smallest indivisible unit of matter.
• He suggested that these tiny particles could combine to form larger forms of matter.
• His ideas were developed through philosophical reasoning, rather than modern laboratory experiments.

John Dalton's Atomic Theory

John Dalton developed a scientific atomic theory in the early 19th century based on experimental observations and the laws of chemical combination.

Main Postulates of Dalton's Atomic Theory

1. Matter is made up of very small particles called atoms.
2.  Atom of the same element are identical in their mass and chemical properties.
3. Modern science has shown that atoms of the same element can have different masses because of isotopes.
4. Atoms of different elements have different properties.
5. Atoms combine in simple whole-number ratios to form compounds.
6. Atoms are neither created nor destroyed during a chemical reaction. They are rearranged.

Example

Hydrogen and oxygen combine to form water:

Hydrogen + Oxygen → Water

The atoms combine in a fixed ratio to form molecules of water.

Thomson's Model of Atom

After the discovery of the electron, scientists knew that atoms were not completely indivisible.

J. J. Thomson proposed a model of the atom in 1904.

His model is popularly known as the Plum Pudding Model.

Plum Pudding Model

According to Thomson:

An atom is a sphere of uniformly distributed positive charge.

Electrons are embedded inside this positively charged sphere.

The total positive charge is equal to the total negative charge of electrons.

Therefore, an atom as a whole is electrically neutral.

Why is it called the Plum Pudding Model?

Thomson imagined the atom somewhat like a plum pudding:

Pudding → positive charge

Plums/raisins → electrons

Limitation of Thomson's Model

Thomson's model could not explain the results of Rutherford's alpha-particle scattering experiment.

Rutherford's experiment later showed that:

• Most of the atom is empty space.
• Almost all the positive charge and most of the mass are concentrated in a tiny nucleus.

Therefore, Thomson's Plum Pudding Model was eventually rejected and replaced by Rutherford's nuclear model.

Discovery of Proton

The proton is a positively charged subatomic particle present inside the nucleus of an atom.

Discovery

• Eugen Goldstein discovered the positively charged particles in 1886 using a modified discharge tube.
• He observed rays travelling opposite to cathode rays and called them canal rays or anode rays.
• Later, these positively charged particles were identified as protons.

Properties of Proton

• Symbol: p⁺
• Charge: +1 elementary charge
• Relative mass: approximately 1 u
• Location: Nucleus
• The number of protons determines the atomic number of an element.

Important:

If an atom has 6 protons, it is carbon. If it has 8 protons, it is oxygen. Thus, the identity of an element depends on its number of protons.

Discovery of Neutron

The neutron is a subatomic particle having no electrical charge.

Discovery

• James Chadwick discovered the neutron in 1932.
• He bombarded beryllium with alpha particles and observed a highly penetrating radiation.
• He concluded that this radiation consisted of neutral particles having a mass nearly equal to that of a proton.

Properties of Neutron

• Symbol: n⁰
• Charge: 0
• Relative mass: approximately 1 u
• Location: Nucleus
• It contributes to the mass of an atom.

Nucleus

The nucleus contains:

• Protons → positively charged
• Neutrons → electrically neutral

Therefore, protons and neutrons together are called nucleons.

Bohr's Model of Atom

In 1913, Niels Bohr proposed a model of the atom to explain how electrons are arranged around the nucleus.

According to Bohr:

1. Electrons revolve around the nucleus in certain definite circular paths called shells or energy levels.
2. These shells have fixed energies.
3. An electron does not continuously lose energy while moving in an allowed shell.
4. Electrons can move from one energy level to another by absorbing or releasing energy.
5. The shells are represented by the letters: K, L, M, N

They correspond to:

K → 1st shell

L → 2nd shell

M → 3rd shell

N → 4th shell

Energy of Shells

The shell closest to the nucleus has the lowest energy.

Energy increases as the distance from the nucleus increases:

K < L < M < N

Thus, an electron in the N shell generally has more energy than an electron in the K shell.

Electron Transition

When an electron:

• moves from a lower energy level to a higher level → energy is absorbed
• moves from a higher energy level to a lower level → energy is released

Symbols of Elements — IUPAC Rules

The International Union of Pure and Applied Chemistry (IUPAC) approves the names and symbols of chemical elements.

Rules for Writing Symbols

1. The symbol usually consists of one or two letters.
2. The first letter is always capitalized.
3. If there is a second letter, it is always written in lowercase.

Examples:

Hydrogen → H

Carbon → C

Oxygen → O

Calcium → Ca

Aluminium → Al

Chlorine → Cl

Magnesium → Mg

Symbols Derived from Latin Names

Some element symbols are based on their Latin names, so their symbols may not resemble their English names.

Examples:

• Sodium → Na → Natrium
• Potassium → K → Kalium
• Iron → Fe → Ferrum
• Copper → Cu → Cuprum
• Silver → Ag → Argentum
• Gold → Au → Aurum
• Mercury → Hg → Hydrargyrum
• Lead → Pb → Plumbum

Important

Co = Cobalt

CO = Carbon monoxide

Therefore, capital and lowercase letters are very important in chemical symbols and formulae.

Atomic Number (Z)

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

It is represented by Z.

Formula

Z = Number of protons

For a neutral atom:

Number of protons = Number of electrons

Therefore:

Z = Number of protons = Number of electrons

for a neutral atom.

Examples

Hydrogen has 1 proton:

Z = 1

Carbon has 6 protons:

Z = 6

Oxygen has 8 protons:

Z = 8

Sodium has 11 protons:

Z = 11

Why is Atomic Number Important?

The atomic number uniquely identifies an element.

For example:

Every carbon atom has 6 protons.

Every oxygen atom has 8 protons.

If the number of protons changes, the element itself changes.

Mass Number (A)

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

It is represented by A.

Formula

A = Number of protons + Number of neutrons

Since:

Z = Number of protons

Therefore:

Number of neutrons = A − Z

Example: Carbon

Carbon-12 has:

• Protons = 6
• Neutrons = 6

Therefore:

A = 6 + 6 = 12

and

Z = 6

Nuclear Notation

An element can be represented as:

ᴬZX

where:

A = Mass number

Z = Atomic number

X = Symbol of element

For carbon-12:

¹²₆C

This tells us:

Protons = 6

Electrons = 6 in a neutral atom

Neutrons = 12 − 6 = 6