2.3 Atomic structure and the periodic table

An atom is the central nucleus containing neutrons and protons surrounded by electrons in shells.

An atom is made up of subatomic particles:

• Protons - positively charged.

• Neutrons - neutral charge.

• Electrons - negative charge. 

Protons and neutrons are found in the nucleus of an atom

Mass number/ nucleon number is the total number of protons and neutrons in the nucleus of an atom (also known as “A” in the periodic table)

Proton number/ atomic number is the number of protons in the nucleus of an atom (Known as “Z”)

Protons and electrons have equal, opposing charges; the atom's overall charge is zero

Electrons are found in the electron shells that surround the nucleus

In an electron shell, the first shell can hold up to 2 electrons, the second shell can hold up to 8 electrons, and the third shell can also hold up to 8 electrons

Electronic configuration is the arrangement of electrons in an atom

Example:

•  Argon has the electronic configuration: 2,8,8

• · Magnesium has the electronic configuration: 2,8,2

The further the shell is from the nucleus, the higher the energy level.

Note:

  Valence electrons are the number of electrons in the outer shell of an Atom.

The group number is the same as the number of electrons in the outer shell.

The period number shows how many shells there are.

·Group VIII noble gases have a full outer shell.

Written by: Peter Ngugi

Edited by: Yoshicaa