• Created by: Hannah.C
  • Created on: 17-05-14 15:00

Keywords to remember!

Electronegativity – the power of an atom to withdraw electron density from a covalent bond

First ionisation energy – the energy required to remove one electron from a gaseous atom.

Molecular formula - actual number of atoms of each element in a molecule.

Empirical formula - simplest ratio of atoms of each element in a compound.

Relative atomic mass – average mass per atom of an atom divided by 1/12 of a carbon-12 atom

Ionic bond – an electrostatic attraction between oppositely charged ions. The ions are formed by the transfer of electrons from metal atoms to non-metal atoms.

Covalent bond – happens between non-metals and involves the sharing of electrons.

Co-ordinate bond – a covalent bond where the shared pair of electrons is donated by one atom to an empty orbital of another.

Isotope – atoms of the same element with the same atomic number but a different mass number.

Atomic number – the number of protons in the nucleus of an atom.

Mass number – the number of protons and neutrons in the nucleus of an atom.






The value of 1st ionisation energy depends on:

  • The size of the nuclear charge

  • The energy of the electron that is removed

As size of nuclear charge INCREASES ionisation energy will INCREASE. As the energy of the electron INCREASES (further away from nucleus) ionisation energy will DECREASE.

  • As we go across a period the size of first ionisation energy shows a general INCREASE. This is because the electron removed comes from the same principal energy level but the size of the nuclear charge steadily increases as we go across the period.

Trend in 1st ionisation energy across period 3:

The general trend from Na to Ar is an INCREASE. However there are two breaks in the pattern:

  • The outer electron removed from Mg is in the 3s sublevel. The outer electron removed from the Al is in the 3p which has a higher energy level so is easier to remove.

  • P has 3 electrons in the 3p sublevel. S has 4 electrons in the 3p sublevel. This means that in S the electrons have started pairing up, the mutual repulsion makes it easier to remove an electron.


    Periodicity: trends in properties across period three

  • Atomic radius decreases because nuclear charge is getting bigger with the electron being added to the same principal energy level, increased nuclear charge attracts the electrons more strongly and pulls them closer.

  • Mpt and Bpt increases (Na to Mg to Al) due to increasing number of delocalised electrons, the strength of the metal-metal bond increases.

  • Si has very high Mpt and Bpt because of macromolecular structure.

  • P4,S8,Cl2, simple molecules with weak vdWs’. Mpt and Bpt decreases.

  • Argon has lowest Mpt and Bpt as is monoatomic.


Structural isomers are compounds with the same molecular formula but with a different structural formula.

The different structures can arise in any of 3 ways:


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