- Atoms are 3 particles:
- 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.
Atomic & Mass number
- 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 → → → → → → → → →
Relationship between protons & electrons
- 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
Elements in Periodic Table - Group
- 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,_
Elements in Periodic Table - Period
- 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
Elements in Periodic Table - Atomic number
- The electron arrangement of an element can also be used to work out the atomic number of that element.
Oxygen - O
- Electron no. = 16
- Proton no. = 16
- Atomic no. = 16
Magnesium - Mg
- Electron no. = 12
- Proton no. = 12
- Atomic no. = 12