The Structure of the Atom

Sub-atomic particles …particles that make up an

atom…

Proton

Neutron

+

nucleus

_

Electron

+

_

…some of these particles have charges…

…the nucleus contains most of the mass of the atom.

Electron cloud

+_

Sub-atomic particles

Neutron

Atoms – Atomic Number

•The atom we have just drawn is the element Helium (He). Its position on the periodic table is second, and it has the atomic number 2.

•Each element on the periodic table has a different atomic number.

•The atomic number represents the number of protons an atom of a particular element contains in its nucleus.

•The number of +ve protons = the number of -ve electrons.

Atoms – Mass Number

•Each atom also has a mass number. The mass number represents the number of particles in the nucleus.

•The mass number represents the number of protons + the number of neutrons.

•In actual fact the mass number includes the mass of the electrons as well, but because their mass is so minimal, the mass number is rounded off to the nearest whole number.

•If an atom was the size of a football field, the nucleus would be the size of a pin head. The mass of an atom is therefore condensed into a very small area.

Atoms – Neutrons•We can work out the number of neutrons if we know the mass number and atomic number.

He

4

2

Mass number

Atomic number

•Helium has 2 protons (atomic number) and 4 particles in the nucleus, so therefore 4 (neutrons+protons) - 2 (protons) = 2 (neutrons).

Atoms – Isotopes‘Ordinary’

Hydrogen – 99.98% of H on

earth+

_

H1

1

H2

1

+

_

Deuterium – 0.02% of H

H3

1

+

_

Tritium – does not occur naturally and is radioactive

…have the same number of protons with a different number of neutrons.

Atoms – Ions

•If an atom looses an electron it becomes a positive ion.

•If an atom gains an electron it becomes a negative ion.

•An ion is an atom with a positive or negative charge._

+

+

+_

_

Loss of electron

Overall positive charge on ion

Gain of electron

Overall negative charge on ion