Atomic structure
Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells. The number of subatomic particles in an atom can be calculated from the atom's atomic number and mass number.
Atomic number and mass number
Atomic number
The number of protons in an atom of an element is its atomic number. Remember that:
- all atoms of a given element have the same number of protons
- atoms of different elements have different numbers of protons
An atom contains equal numbers of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms are have no overall electrical charge.
For example, the atomic number of sodium is 11. Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges.
Mass number
Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.
Calculating numbers of subatomic particles
The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.
To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number:
- number of protons = atomic number
- number of electrons = atomic number
- number of neutrons = mass number - atomic number
- Question
The atomic number of a sodium atom is 11 and its mass number is 23. Calculate the number of protons, neutrons and electrons it contains.
Number of protons = 11
Number of electrons = 11
Number of neutrons (mass number - atomic number) = 23 - 11 = 12