The mass number is primarily used to calculate the number of neutrons in an atom's nucleus.
Understanding Mass Number and Atomic Number
- Mass Number: This represents the total number of protons and neutrons in an atom's nucleus. It's approximately equal to the atomic mass (in atomic mass units or amu) and is usually represented by A.
- Atomic Number: This represents the number of protons in an atom's nucleus. It uniquely identifies an element and is represented by Z.
Calculating the Number of Neutrons
To find the number of neutrons (N) in an atom, you simply subtract the atomic number (Z) from the mass number (A):
N = A - Z
Examples
- Carbon-12 (¹²C): The mass number (A) is 12, and the atomic number (Z) is 6 (because carbon has 6 protons). Therefore, the number of neutrons is 12 - 6 = 6.
- Uranium-235 (²³⁵U): The mass number (A) is 235, and the atomic number (Z) is 92. The number of neutrons is 235 - 92 = 143.
The mass number doesn't directly calculate the number of atoms. It only provides information about the composition of a single atom. To determine the number of atoms, you'd use Avogadro's number (6.022 x 10²³) and the molar mass of the element.
