Atoms and ions - protons, neutrons and electrons

Atoms having the same number of protons in their nucleus have the same chemical properties. The number of protons in the nucleus is known as the atomic number (Z).

Elements are substances for which all atoms have the same atomic number. The table shows some example atoms.

Element	Symbol	Atomic Number	Mass (nucleon) Number
hydrogen	H	1	1
carbon	C	6	12
sodium	Na	11	23
oxygen	O	8	16
chlorine	Cl	17	35

Atoms have no electrical charge, thus the number of protons equals the number of electrons.

As you will see later electrons are particularly significant in determining chemical properties.
Example: The number of electrons for all atoms of Cl is 17.

The number of neutrons in an atom can be deduced from the mass (nucleon) number (A) of the atom.

Remember that protons and neutrons are referred to as nucleons.

The mass (nucleon) number (A) equals the sum of the numbers of protons and neutrons in the nucleus.
The mass (nucleon) number for a particular atom is given as a superscript preceding the atom (see below).
This in combination with the atomic number can be used to calculate the number of neutrons present in that atom.

Example: The number of protons for all atoms of chlorine is 17 (as see above). The number of neutrons for ³⁵Cl is 18 (35 - 17).

Monatomic ions have the same numbers of neutrons and protons as their parent atom, but they have a different number of electrons.

Anions have more electrons than the parent atom (Cl^– has 18 electrons).
Cations have fewer electrons than the parent atom (Na⁺ has 10 electrons, one fewer than Na atoms.)