Module 3 Section 1 - Ionisation energies Flashcards
What is the first ionisation energy definition?
The first ionisation energy is the energy needed to remove 1 mole of electrons from 1 mole of gaseous atoms.
What type of reaction is ionisation?
Endothermic since you have to put energy in to ionise an atom or molecule.
First ionisation energy of oxygen equation
O(g). -> O+ (g) +. e-
What are the three factors affecting ionisation energy?
1) nuclear charge
2) atomic radius
3) shielding
How does nuclear charge affect ionisation energy?
The more protons in the nucleus, the more positively charged the nucleus is so there is a stronger attraction to the outer electron. This means ionisation energy is higher.
How does atomic radius affect ionisation energy?
Attractions decreases with distance. An electron close to the nucleus will be much more strongly attracted than one further away. So as atomic radius increases, ionisation energy decreases.
How does shielding affect ionisation energy?
As the number of electrons between outer electrons and the nucleus increase, the outer electrons feel less attraction towards the nuclear charge. The greater the amount of shielding the lower the ionisation energy.
What is the trend of ionisation energy down a group?
As you go down a group in the periodic table, ionisation energies tend to decrease
Why does ionisation energy decrease down a group?
1) elements down a group have extra electron shells. These extra electron shells mean atomic radius is larger so the outer electron is further away so it becomes less attracted to the nucleus.
2) the extra inner shells shield the outer electrons from the attraction of the nucleus.
The positive charge of the nucleus increases, but this effect is overridden by the effect of extra shells.
What is the trend of ionisation energies across a period?
The general trend is that it tends to increase across a period.
Why does ionisation energy increase across a period?
1) the number of protons is increasing. As the positive charge increases, electrons are pulled closer to the nucleus so atomic radius becomes smaller.
2) the extra electrons that the elements gain across a period are added to the outer energy level so they don’t really provide any extra shielding affect.
What are the two exceptions in the trend if ionisation energies ?
1) between Be and B (groups 2 and 3, period 2)
2) between N and O (groups 5 and 6, period 3)
Why is there a drop in ionisation energies between Be and B?
1) the outer electron in group 3 is in a p-orbital rather than an s orbital. A p orbital has a slightly higher energy than an s orbital in the same shell, so on average is found further away from the nucleus.
2) the p orbital also has additional shielding provided by the s electrons.
3) these factors override the feat of increased nuclear charge so the ionisation energy drops slightly.
Why is there a drop in ionisation energies between N and O?
1) in group 5 elements, the electron being removed in is a singly occupied orbital.
2) in group 6 elements, the electron is being removed from an orbital containing two electrons
3) the repulsion between two electrons in an orbital means that electrons are easier to remove from shared orbitals.
What is successive ionisation energy?
The energy required to move an electron one after the other.
