Atomic Structure

Atomic Number and Mass Number

Atoms are defined by two key numbers: the atomic number and the mass number. These numbers are fundamental for understanding the identity and properties of elements.

• Atomic Number (Z): The atomic number is the number of protons in the nucleus of an atom. It determines the identity of the element. For example, all carbon atoms have Z = 6.

• Mass Number (A): The mass number is the total number of nucleons (protons and neutrons) in the nucleus. It is given by the sum of protons and neutrons.

The relationship between these quantities can be expressed as:

• Number of neutrons = Mass number (A) - Atomic number (Z)

For example, the isotope carbon-12 is represented as:

$$

• Atomic number (Z): 6 (number of protons)

• Mass number (A): 12 (number of protons + neutrons)

• Number of neutrons: 12 - 6 = 6

Key Definitions:

• Proton: A positively charged subatomic particle found in the nucleus.

• Neutron: A neutral subatomic particle found in the nucleus.

• Nucleon: A collective term for protons and neutrons in the nucleus.

Example: For the element carbon, the atomic number is 6 and the mass number for the most common isotope is 12. This means a carbon-12 atom has 6 protons and 6 neutrons.

Summary Table:

Key Point: The atomic number (Z) uniquely identifies an element, while the mass number (A) distinguishes between isotopes of the same element.