Atoms, Elements and Compounds

Atomic Structure and Electronic Configuration

Cambridge IGCSE Chemistry

Protons, neutrons and electrons

An atom: protons and neutrons in the nucleus, electrons in shells around it.

The nucleus contains protons and neutrons; electrons are in shells around the nucleus.

Particle	Relative charge	Relative mass
proton	+1	1
neutron	0	1
electron	−1	1/1840

Proton number and nucleon number

Proton number (atomic number): the number of protons in the nucleus of an atom.
Nucleon number (mass number): the total number of protons and neutrons in the nucleus of an atom.

Particle	Number in an atom	Number in an ion
protons	the proton number	the proton number
neutrons	nucleon number minus proton number	nucleon number minus proton number
electrons	the same as the number of protons	positive ion: fewer than the protons, by the charge; negative ion: more than the protons, by the charge

A positive ion is positive because it has more protons than electrons.

Electronic configuration

Electrons fill the shells from the inside out: the first shell holds up to 2, the second up to 8, the third takes 8 before the fourth shell starts. The second shell is full with 8 electrons before the third shell fills.
Write the number in each shell, inner shell first, separated by commas: silicon 2,8,4, argon 2,8,8, calcium 2,8,8,2.
For an ion, remove electrons for a positive ion and add electrons for a negative ion, by the size of the charge: Al³⁺ is 2,8. The ion has the configuration of a noble gas.

A sodium atom, 2,8,1.

Noble gas	Configuration	Positive ions with it	Negative ions with it
Ne	2,8	Na⁺, Mg²⁺, Al³⁺	N³⁻, O²⁻, F⁻
Ar	2,8,8	K⁺, Ca²⁺	P³⁻, S²⁻, Cl⁻

Group and period

In Groups I to VII, the number of outer shell electrons is the group number. Two elements in the same group have the same number of outer electrons.
The number of occupied electron shells and the period number: they are the same. Two elements in one group but different periods have different numbers of shells.
The Group VIII noble gases have a full outer electron shell.

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