The mass number is an important part of atoms that helps determine how they behave.

The mass number (A) tells us the total number of protons and neutrons in an atom's center, which is called the nucleus.

You can find the mass number using this formula:

$A = Z + N$

Here, $Z$ stands for the atomic number, which is the number of protons, and $N$ is the number of neutrons.

How Mass Number Affects Atoms:

1. Stability:

   • Isotopes are atoms that have the same number of protons but different mass numbers.
   • For example, Carbon has isotopes like Carbon-12 ($^{12}C$) and Carbon-14 ($^{14}C$).
   • Carbon-12 is stable, but Carbon-14 is radioactive and changes into a different type of atom through a process called beta decay.

2. Abundance:

   • Different isotopes can be found in different amounts in nature.
   • About 98.9% of Carbon is Carbon-12, while Carbon-14 makes up only 1.1%.

3. Physical Properties:

   • The mass of an atom affects how dense it is.
   • Usually, heavier isotopes make an element denser.
   • For example, Deuterium ($^{2}H$) is about twice as heavy as Protium ($^{1}H$).

4. Chemical Behavior:

   • Isotopes of the same element act similarly in chemical reactions.
   • However, factors like their mass and how common they are can slightly change how fast reactions happen.

Knowing how mass number works is essential for understanding how atoms behave in chemistry.