Atomic Number, Isotopes and Isobars 

1.0Introduction to Atomic Number, Isotopes and Isobars

In atomic structure, three basic ideas form the foundation:

• Atomic Number – the identity of an element.
• Isotopes – same element, different masses.
• Isobars – different elements, same mass number.

These concepts are directly linked to the periodic table, nuclear chemistry, and atomic models, making them very important for JEE Main and JEE Advanced Chemistry.

2.0What is Atomic Number?

The atomic number (symbol Z) is the number of protons in the nucleus of an atom. It defines the element’s identity. For example, hydrogen has Z = 1, helium has Z = 2, and so on. In JEE Chemistry, understanding atomic number is foundational because:

• It determines the element’s position in the periodic table.
• It dictates an atom’s chemical properties and reactivity.
• It equals the number of electrons in a neutral atom, influencing electronic configuration and bonding.

Importance of Atomic Number

1. Identity of an Element: Atomic number decides which element it is.
2. • Example: Z = 1 → Hydrogen, Z = 6 → Carbon.
2. Position in the Periodic Table: Mendeleev’s table was later modified to arrange elements by increasing atomic number, not mass.
3. Chemical Properties: Since electrons control reactivity, atomic number determines the valency, bonding, and chemical nature.

Examples of Atomic Number

• Hydrogen (H): Z = 1 → 1 proton, 1 electron.
• Oxygen (O): Z = 8 → 8 protons, 8 electrons.
• Sodium (Na): Z = 11 → 11 protons, 11 electrons.

3.0What are Isotopes?

Isotopes are atoms of the same element (same Z) with different mass numbers (different neutrons). Thus, isotopes share chemical properties but differ in mass and nuclear stability

Characteristics of Isotopes

• Same number of protons.
• Different number of neutrons.
• Identical chemical properties (due to same electron arrangement).
• Different physical properties (mass, density, stability).

Examples of Isotopes

Hydrogen Isotopes:

Protium (¹H): 1 proton, 0 neutrons.

Deuterium (²H): 1 proton, 1 neutron.

Tritium (³H): 1 proton, 2 neutrons (radioactive).

Carbon Isotopes:

¹²C and ¹³C (stable).

¹⁴C (radioactive, used in carbon dating).

Uranium Isotopes:

²³⁵U (used in nuclear reactors).

²³⁸U (most abundant but not fissile).

Applications of Isotopes

• Medicine:
• • Iodine-131 → thyroid treatment.
  • Cobalt-60 → cancer therapy.
• Archaeology:
• • Carbon-14 → dating fossils and ancient materials.
• Industry:
• • Isotopes are used as tracers in chemical reactions and pipelines.
• Nuclear Energy:
• • Uranium-235 and Plutonium-239 are used in reactors and weapons.

4.0What are Isobars?

Definition

Isobars are atoms of different elements that have the same mass number (A) but different atomic numbers (Z).

Characteristics of Isobars

• Same mass number.
• Different atomic number.
• Belong to different elements.
• Different chemical properties (since electrons differ).
• May have similar nuclear properties.

Examples of Isobars

Applications of Isobars

• Important in nuclear physics and radioactivity studies.
• Used in nuclear energy production.
• Help in understanding nuclear stability.

5.0Difference Between Isotopes and Isobars