ionisation energy

what is ionisation energy?

the energy required to remove 1 mole of e^- from 1 mole of gaseous atoms

what do the term first ionisation of an atom mean?

enthalpy change when an e- is removed from a gaseous atom

what is the unit of measurement for (ionisation) energy?

kJmol^-1

what is important to remember when writing equations for the ionisation process?

• always keep the atom on the left and the ion + e^- on the right

• always write the state symbol next to the element - (g)

what are the factors affecting the size of ionisation energy?

• distance of e^- from nucleus

• nuclear charge

• shielding

why does the distance of e^- from nucleus affect ionisation energy?

because the e^- is further away from the nucleus therefore the attraction is weaker

why does the nuclear charge affect ionisation energy?

• the more protons there are in the nucleus, the more positively charged it is

• therefore the more strongly attracted e^- are to the nucleus

why does the shielding affect ionisation energy?

other e^- in inner shells (between nucleus and outer e^-) will repel

are the ionisation energies larger in period 2 or period 3?

ionisation energy of period 2 is higher than period 3

why does boron (group 3) have a lower IE than beryllium (group 2)?

• the outer shell e- in boron is in a higher energy level than beryllium’s (2p)

• therefore the e- is slightly further from the nucleus and attraction is weaker

• 2s orbital also creates more shielding so e- in 2s repel outermost e- in 2p orbital more

why does oxygen (group 6) have a lower IE than nitrogen (group 5)?

• the e- pair in the 2p orbital repel each other so less energy required to remove an e-

• (remember the shielding is the same - 2p)

why does aluminium (group 3) have a lower IE than magnesium (group 2)?

• outermost e^- (being removed) is from 3p orbital whereas e^- in mg is in 3s

• the energy of 3p is greater than 3s

• also 3s e- cause greater shielding and repel 3p e- therefore the attraction between the nucleus and e- is weaker

why does sulfur (group 6) have a lower IE than phosphorus (group 5)?

the e- pair in the 3p orbital repel each other so less energy is required to remove an e-