1. WHAT IS AN ATOM?

An atom is the smallest particle of an element that can take part in chemical reactions and still keep the properties of that element.

Everything around you—air, water, food, metals, your body—is made of atoms. Atoms are extremely small; millions of atoms can fit across the width of a human hair.

2. STRUCTURE OF AN ATOM

An atom has a central nucleus and electrons moving around it. The structure can be compared to a solar system, where planets move around the Sun.

2.1 Nucleus of the Atom

The nucleus is the core (center) of the atom.

Properties of the nucleus:

• Very small in size

• Very dense

• Contains almost all the mass of the atom

• Positively charged (overall)

(A) Protons

• Found inside the nucleus

• Charge: +1 (positive)

• Mass: 1 atomic mass unit (amu)

• Symbol: p⁺

Importance of protons:

• The number of protons decides the element

• This number is called the atomic number

• No two elements have the same atomic number

Example:

• Hydrogen → 1 proton

• Helium → 2 protons

• Carbon → 6 protons

(B) Neutrons

• Found inside the nucleus

• Charge: 0 (neutral)

• Mass: 1 amu

• Symbol: n⁰

Importance of neutrons:

• Increase the mass of the atom

• Help keep the nucleus stable

• Different numbers of neutrons form isotopes

2.2 Electrons

Electrons are found outside the nucleus.

Properties of electrons:

• Charge: –1 (negative)

• Mass: extremely small (almost negligible)

• Symbol: e⁻

Electrons move very fast around the nucleus in specific energy regions, not in random paths.

Electron Cloud

The region around the nucleus where electrons are likely to be found is called the electron cloud. It occupies most of the volume of the atom, meaning most of the atom is empty space.

3. ATOMIC NUMBER AND MASS NUMBER

Atomic Number (Z)

• Number of protons

• Identifies the element

• In a neutral atom:number of protons = number of electrons

Mass Number (A)

• Total number of protons + neutrons

• A = p + n

4. DIFFERENT SORTS OF ATOMS

Atoms can be classified into different types based on their structure and behavior.

4.1 Atoms of Different Elements

Each element has atoms with a unique number of protons.

Examples:

• Hydrogen → lightest atom

• Oxygen → supports combustion

• Iron → strong and magnetic

Though atoms of different elements may look similar, their internal structure makes them behave differently.

4.2 Isotopes

Isotopes are atoms of the same element that have:

• Same number of protons

• Same number of electrons

• Different number of neutrons

Example (Hydrogen isotopes):

• Protium → 1 proton, 0 neutrons

• Deuterium → 1 proton, 1 neutron

• Tritium → 1 proton, 2 neutrons

Uses of isotopes:

• Carbon-14 → dating fossils

• Iodine-131 → medical treatment

• Uranium-235 → nuclear energy

4.3 Ions

An ion is a charged atom.

Formation of ions:

• Atoms lose or gain electrons to become stable

Positive ions (Cations):

• Formed by losing electrons

• Example: Na⁺, Ca²⁺

Negative ions (Anions):

• Formed by gaining electrons

• Example: Cl⁻, O²⁻

👉 Protons and neutrons never change in ions.

5. ARRANGEMENT OF ELECTRONS

Electrons are arranged in shells (energy levels) around the nucleus.

5.2 Electron Configuration

Electron configuration means distribution of electrons in shells.

Examples:

• Hydrogen (1) → 1

• Helium (2) → 2

• Carbon (6) → 2, 4

• Oxygen (8) → 2, 6

• Sodium (11) → 2, 8, 1

• Chlorine (17) → 2, 8, 7

5.3 Valence Electrons

Electrons in the outermost shell are called valence electrons.

Importance:

• Decide reactivity

• Decide bonding

• Decide chemical properties

Atoms with:

• 1 or 7 valence electrons → highly reactive

• 8 valence electrons → very stable

5.4 Octet Rule

Atoms try to achieve 8 electrons in their outer shell to become stable (except hydrogen and helium).

They do this by:

• Losing electrons

• Gaining electrons

• Sharing electrons

This leads to chemical bonding.