Atomic Number And Mass Number

Key notes :

Definition of an Atom:

• An atom is the smallest unit of an element that retains the chemical properties of that element.
• It is made up of subatomic particles: protons, neutrons, and electrons.

Atomic Number (Z):

• The atomic number is defined as the number of protons in the nucleus of an atom.
• It determines the element’s identity and its position on the periodic table.
• For a neutral atom, the atomic number also equals the number of electrons.
• Example: Hydrogen has an atomic number of 1 (one proton), and carbon has an atomic number of 6 (six protons).

Mass Number (A):

• The mass number is the total number of protons and neutrons in the nucleus of an atom.
• It is calculated as: Mass Number (A) = Number of Protons (Z) + Number of Neutrons (N).
• Electrons have negligible mass, so they are not included in the mass number.
• Example: An oxygen atom with 8 protons and 8 neutrons has a mass number of 16.

Notation:

• Atoms are often represented using the notation: Element Symbol A Z

where A is the mass number and Z is the atomic number.

• For example, carbon can be written as 12 6.

Isotopes:

• Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to varying numbers of neutrons.
• Example: Carbon-12 (¹2C) and Carbon-14 (¹4C) are isotopes of carbon with different numbers of neutrons.

Importance of Atomic Number:

• The atomic number determines the chemical properties of an element and its place in the periodic table.
• Elements are arranged in order of increasing atomic number.

Calculation Examples:

• If an atom has 11 protons and 12 neutrons:
  • Atomic Number (Z) = 11
  • Mass Number (A) = 11 + 12 = 23
• This atom would be represented as 23 11 Na