Atomic Number and Mass Number

Atomic Number and Mass Number

1. Atomic Number ($Z$)

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

• It uniquely identifies an element.

• Formula: 

  $$Z=\text{Number of protons}$$

• Example: Hydrogen ($H$) has 1 proton, so its atomic number is 1.

2. Mass Number (A)

• The mass number of an atom is the sum of protons and neutrons in the nucleus.

• Formula:

                               A = Number of protons + Number of neutrons

• Example: Carbon-12 (12_C) has 6 protons and 6 neutrons, so its mass number is 12.

Isotopes and Isobars

1. Isotopes

• Isotopes are atoms of the same element with the same atomic number (Z) but different mass numbers (A).

• Example: Hydrogen Isotopes:

  • Protium (1_H) → 1 proton, 0 neutrons

  • Deuterium (2_H) → 1 proton, 1 neutron

  • Tritium (3_H) → 1 proton, 2 neutrons

Diagram: Examples of Isotopes of Hydrogen

(A labeled diagram showing Protium, Deuterium, and Tritium with their proton and neutron numbers.)

2. Isobars

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

• Example:

  • Carbon-14 (14_C) → Z = 6, A = 14

  • Nitrogen-14 (14_N) → Z = 7, A = 14

Isotopes are important in radioactive dating, medical imaging, and nuclear energy, while isobars help understand nuclear structure and stability.