6.1 - Atomic Structure & Isotopes

Atomic number, mass number and isotopes

The number of protons determines which element an atom is (e.g. every carbon atom has 6 protons).

Atomic number and mass number

• Atomic number (proton number) - The number of protons in the nucleus, symbol $Z$.
• Mass number (nucleon number) - The total number of protons and neutrons in the nucleus, symbol $A$.
• In a neutral atom, the atomic number also equals the number of electrons.
• Number of neutrons = $A-Z$.

Nuclide notation

A nucleus is written with the mass number above the atomic number, in front of the chemical symbol:

$${}_Z^A\text{X}$$

Where:

• $\text{X}$ = Chemical symbol
• $A$ = Mass number
• $Z$ = Atomic number

For example, ${}_6^{14}\text{C}$ has 6 protons and 14 - 6 = 8 neutrons.

Isotopes

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (same atomic number, different mass numbers).
• Example: carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
• Most elements have multiple isotopes, but usually only one or two are stable.