WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.2 Basic ideas about atoms

Electrons are held in their shells by their attraction to the positive nucleus, therefore the greater the attraction, the greater the ionisation energy. This attraction depends on three factors: ▪ The size of the positive nuclear charge – the greater the nuclear charge, the greater the attractive force on the outer electron and the greater the ionisation energy. ▪ The distance of the outer electron from the nucleus – the force of attraction between the nucleus and the outer electron decreases as the distance between them increases. The further an electron is from the nucleus, the lower the ionisation energy. ▪ The shielding effect by electrons in filled inner shells – all electrons repel each other since they are negatively charged. Electrons in the filled inner shells repel electrons in the outer shell and reduce the effect of the positive nuclear charge. The more filled inner shells or subshells there are, the smaller the attractive force on the outer electron and the lower the ionisation energy. Evidence for shells and subshells can be seen from a plot of first ionisation energies against the elements (or atomic number). Such a plot is shown below for the first twenty elements.

Key term Shielding effect is the repulsion between electrons in different shells. Inner shell electrons repel outer shell electrons.

2500

5

1

2000

1500 First ionisation energy / kJ mol –1 3 2 1000 500

4

Link

Trends across periods page 63

0

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca

▲ Plot of IE against elements

The most significant features of the plot are: ▪ The ‘peaks’ are occupied by elements of Group 0. ▪ The ‘troughs’ are occupied by elements of Group 1. ▪ There is a general increase in ionisation energy across a period, although this increase is not uniform. ▪ There is a decrease in ionisation energy going down a group. Looking at the plot in detail (remember the three main factors that affect ionisation energy): 1. He > H since helium has a greater nuclear charge in the same subshell so little extra shielding. 2. He > Li since lithium’s outer electron is in a new shell which has increased shielding and is further from the nucleus. 3. Be > B since boron’s outer electron is in a new subshell of slightly higher energy level and is partly shielded by the 2s electrons. 4. N > O since the electron–electron repulsion between the two paired electrons in one p orbital in oxygen makes one of the electrons easier to remove. Nitrogen does not contain paired electrons in its p orbital. 5. He > Ne since neon’s outer electron has increased shielding from inner electrons and is further from the nucleus. 25 DRAFT Study point If the conditions for ionisation energy are 298K and 1atm then the process is known as the standard ionisation energy. All ionisation energies are positive since it always requires energy to remove an electron. Knowledge check State and explain how you would expect the first ionisation energy of phosphorus to compare with the first ionisation energy of sulfur. 9