1.5.k. explain trends in the following properties of the element from periods 2 and 3 of the periodic table: melting temperature of the elements, ionisation energy versus atomic number.

● Why is there a general increase in first ionisation energy across a period? ● Why do first ionisation energies decrease down a group?

● Ionisation increases across a period, so it becomes harder to remove electrons. ○ This is the result of the increasing positive nuclear charge across the period, without the addition of any energy levels to shield the outer electrons. ○ The atomic radius gets smaller and the electrons are held more firmly, so more energy is required to bring about information. ● As one goes down a group, the outer electrons are found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller

● Why does helium has the highest first ionisation energy?

● Its first electron is in the first shell closest to the nucleus and has no shielding effects from inner shells so the pull from the nucleus is very strong. ○ Helium has a bigger first ionisation energy than hydrogen as it has one more proton, so helium has the largest proton:neutron ratio on its row, the nucleus has the largest pull.

● What elements are there in period 2? ● What elements are there in period 3?

● Period 2: Li, Be, B, C, N, O, F, Ne ● Period 3: NA, Mg, Al, Si, P, S, Cl, Ar

Ionisation of period 2 ● Why is there a small drop from Be to B? ● Why is there a small drop from N to O?

● The 1st ionisation of Beryllium is larger than borons. ○ Boron is starting to fill a 2p sub-level, whereas Beryllium has its outer electrons in the 2s sub-level. ○ The electrons in the 2p sub-level are slightly easier to remove because the 2p electrons are higher in energy and are also slightly shielded by the 2s electrons. ● With oxygen there are 4 electrons in the 2p sub shell and the 4th is being paired up with the first 2p orbital. ○ When the second electron is added to a 2p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove.

Ionisation of period 3 ● Why is there a small drop from Mg to Al?

● The 1st ionisation of magnesium is larger than aluminium. ○ Al is starting to fill a 3p sub-level, whereas Mg has its outer electrons in the 3s sub-level. ○ The electrons in the 3p sub-level are slightly easier to remove because the 3p electrons are higher in energy and are also slightly shielded by the 3s electrons.

● The first ionisation of an element is directly related to what?

● The first ionisation of an element is directly related to the attraction of the nucleus to the most loosely bound of the outer electrons. ○ The more tightly the outer electrons are held, the higher the first ionisation energy.

Period 3 ● Na, Mg and Al: melting and boiling points

● The melting and boiling points increase for the three metallic elements from sodium to aluminium. ○ For Na, Mg, Al- Metallic bonding : strong bonding - gets stronger the more electrons there are in the outer shell that are released to the sea of electrons. A smaller positive centre also makes the bonding stronger. More energy is needed to break the stronger metallic bonds, so melting and boiling points are higher.

● Increase in proton number across periods

● The number of protons in the nucleus of the atoms increases across the periods. ○ This increase in the number of protons increases the nuclear charge of the atoms. ○ The nucleus has stronger attraction for the electrons, pulling them in closer and so the atomic radius decreases across the period.

● Why has Na a much lower first ionisation energy than Neon?

● This is because Na will have its outer electron in a 3s shell further from the nucleus and is more shielded. ○ Na's outer electron is easier to remove and has a lower ionisation energy.

Ionisation of period 3 ● Why is there a small drop from P to S?

● With sulphur there are 4 electrons in the 3p sub shell and the 4th is being paired up with the first 3p orbital. ○ When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove.