PERIODICITY

Welcome back (or hello if you're new) and be warned this section is very simple but very WORDY! If you get one of these in the exam, which you're bound to get, you have gained some free marks (but only if you memorise the rules). Have fun.

First of all: 



Now we can begin.

As you obviously have noticed, cos you're awesome all of you, the periodic table is arranged in a specific order. Well, this is no rocket science but you do need to understand that all the elements are not arranged in an increasing mass order but in an increasing atomic number order. This means that each element along has one more proton and one more electron.

Also as you move along the periods the chemicals there have a repeating chemical and physical property trend. Such as lithium and sodium react similarly when added to water, they also look similar. This is because of the number of electrons in the outer shell, which is very important to note.

Now this repeating pattern is called PERIODICITY.

Like I have mentioned before all the properties of an element depend on its electron structure, especially the arrangement of the outer electrons.

During one of my previous posts I explained how the ionisation energy is affected by these three factors:

• Nuclear charge
• Atomic radii
• Electron shielding

Make sure you have these memorised, but also that you UNDERSTAND them. Once you do, you will fly through Chemistry (at least this unit). Just to ensure you remember I'll go through it again.

NUCLEAR CHARGE: As you go along the period each element along has an extra proton, which increases the attraction on the outer electrons. An extra electron is also added, however it is added to the same shell as the previous one (until you get to the next period then its the next shell) therefore there is no extra shielding. Because of this the ionisation energy INCREASES ACROSS a period.

On top of this as the nuclear charge increases across a period this draws the electrons further in (as there isn't any extra electron shielding - as electrons are added to the same shell). This therefore increases the attraction on the electrons and ionisation increases ACROSS the period.

ELECTRON SHIELDING: A you go DOWN a group there is a new shell added each time. This means there is more repulsion between the electrons and therefore there is a weaker attraction of the nucleus on the outer electron (the electrons act like a shield - shielding the outer electron from the powerful attraction of the nucleus: hence the name being electron shielding). Therefore the ionisation energy DECREASES DOWN a group.

ATOMIC RADII: This is the easiest part. It is linked to the nuclear charge (read above) and extra shells. As you go DOWN a group an extra shell is added so the radius increases. This means that the outer electron experiences a weaker attraction from the nucleus and hence the ionisation energy DECREASES DOWN the group.

The next trend which you need to understand is boiling points. Like I have discussed in previous posts, boiling points are all about how easy it is to break bonds - to make the molecule free to fly off wherever it wants. Also remember when you bring something to its boiling point you do not break the INTRAmolecular forces (the ones the bond the ATOMS together - like covalent bonds as this requires HUGE amounts of energy) but the INTERmolecular forces (the attraction between neighbouring molecules - much lower energy required). Make sure you spot the difference!

I would love to get you a lovely, snazzy table but the blogger doesn't want to cooperate so lets improvise.

Ok. As you go across period 2 we have: 

Li, Be:

• These are giant metallic lattices, which have strong metallic bonding between the POSITIVE IONS AND NEGATIVE DELOCALISED ELECTRONS.
• They have very high melting points, which increase as you go across a period. This is because the nuclear charge increases, so delocalised electrons are more dense (more of them - if you have a +1 metal ion then you need 1 electron to have no charge overall, if you have +2 charge on the ion you need an extra free electron to cancel out the charge). The higher nuclear attraction also draws the electrons closer in and therefore the metallic bonding is stronger.
• Stronger bonding - harder to break the bonds - more energy required - higher boiling/melting point

B, C:

• These are giant covalent lattices, which contain strong covalent bonds BETWEEN ATOMS so a huge amount of energy is required to break them, so high melting/boiling point.
• You don't see molten diamonds often do you? They're made of carbon alone.

N2, O2, F2, Ne:

• These are simple molecular structures (not giant lattices). They only have weak forces between MOLECULES, which are Van der Waals's forces. Weak forces mean low energy required to break them so a low boiling/melting point.

Period 3 is very similar:

Na, Mg, Al: Giant metallic lattice.

Si: Giant covalent lattice.

P4, S8, Cl2, Ar: Simple molecular structure.