Atomic Structure

• Atoms are 3 particles:

protons

neutrons

electrons

• Protons & neutrons are found in the centre of the atom (nucleus).
• The nucleus will have a positive charge & is the heavist part of the atom as it contains the particles with the mass.

• The atomic number = The number of protons
• Atoms of the same element will have the same number of protons.

• The mass number = The sum of protons & neutrons
• Electrons are left out of the mass number as their mass is negligible (nearly zero) compared to the mass of protons & neutrons.

Atomic no. = 11  Proton no. = 11  Electrons = 11

Mass no. = 22.99 (23) Neutron no. = 23 - 11 = 12

• Isotopes are atoms with the same number of protons (element) but with different number of neutrons.

Mass no.      1         2           3

  H H H

Atomic no.    1      1         1

Protons =  1 1 1

Neutrons =  0 1 2

getting heavier  → → → → → → → → →

• In all atoms:

no. protons = no. electrons

positive charge = negative charge

• This is why atoms have no overall charge.

Atomic no. = 15

Protons = 15 Charge = +15

Electrons = 15  Charge = -15

Protons - electrons = Charge = 0

• In an atom, the electron spin around the nucleus in energy levels called orbits.

1st shell = 2 electrons  2nd shell = 8 electrons  3rd shell = 8 electrons

4 + shell = 18 electrons

• All elements position in the Periodic Table are linked to their electron arrangement.
• The Group no. = no. of electrons in the outershell.

Group 1 -  Group 6 - 

Li = 2,1  O = 2,6

Na = 2,8,1  S = 2,8,6

K = 2,8,8,1  Se = 2,8,8,6

Group 0 - (full outershell)

He = 2,

Ne = 2,8,

Ar = 2,8,8,_

• All elements positions in the Periodic Tables are linked to their electron arrangement.
• The Period no. = no. of shells with electrons in

Period 1 -  Period 4 -

He = 2 Se = 2,8,8,6

H = 1 K = 2,8,8,1

Period 3 -

Mg = 2,8,2

Na = 2,8,1

• The electron arrangement of an element can also be used to work out the atomic number of that element.

Oxygen - O

• 2,8,6
• Electron no. = 16
• Proton no. = 16
• Atomic no. = 16

Magnesium - Mg

• 2,8,2
• Electron no. = 12
• Proton no. = 12
• Atomic no. = 12