Mass number

The mass number of an atom is the total number of protons and neutrons in its nucleus. It is typically represented by the symbol A. The mass number is used to determine the atomic mass of an atom, which is the average mass of all the isotopes of that element.

Definition:

The mass number (A) of an atom is the total number of protons and neutrons in its nucleus.

Formula:

```
Mass number (A) = Number of protons (Z) + Number of neutrons (N)
```

Relationship with Other Atomic Properties:

- Atomic number (Z): The atomic number represents the number of protons in the nucleus, which determines the chemical identity of the element.
- Number of electrons (E): In a neutral atom, the number of electrons is equal to the atomic number, but this can change in ions.

Significance:

The mass number is an important property because it:

- Distinguishes between isotopes of the same element, which have the same number of protons but different numbers of neutrons.
- Allows for the calculation of the mass of an atom (in atomic mass units or amu).
- Provides information about the stability of an atom: isotopes with high mass numbers tend to undergo radioactive decay.

Examples:

- Carbon-12 (¹²C): Mass number = 12 (6 protons + 6 neutrons)
- Uranium-235 (²³⁵U): Mass number = 235 (92 protons + 143 neutrons)