Ionisation energies Flashcards
What does ionisation energy measure?
How easily an atom loses electrons to form positive ions
What is the first ionisation energy?
The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of an element to form 1 mole of gaseous 1+ ions
What affects ionisation energy?
The attraction between the nucleus and outer electrons of an atom
What are the 3 factors affecting attraction between a nucleus and electrons?
1) Atomic radius
2) Nuclear charge
3) Electron shielding
How does atomic radius affect ionisation energy?
The greater the distance between the nucleus and the electrons, the less the nuclear attraction to the electron
How does nuclear charge affect ionisation energy?
The more protons in the nucleus, the greater the attraction to the electrons
How does electron shielding affect ionisation energy?
Electrons are negatively charged so inner-shell electrons repel outer-shell electrons - this repulsion (shielding effect) reduced attraction between the nucleus and outer electrons
How many ionisation energies does an element have?
As many as there are electrons
Why are successive ionisation energies always larger?
After the first electron is lost, the other electrons are pulled closer to the nucleus so the nuclear attraction on the remaining electrons increases and more ionisation energy will be needed to remove the further electrons
What is second ionisation energy?
The energy required to remove one electron from each ion in 1 mole of gaseous 1+ ions of an element to form 1 mole of gaseous 2+ ions
How do successive ionisation energies show shell structure?
Large increases between ionisation energies suggest that the next electron must be removed from a different shell, closer to the nucleus with less shielding
What 3 predictions can be made from successive ionisation energies?
1) The number of electrons in the outer shell
2) The group of the element in the periodic table
3) The identity of an element
What 2 key patterns are shown by the trends in first ionisation energies?
1) A general increase in the first ionisation energy across each period
2) A sharp decrease in first ionisation energies between the end of one period and the start of another
What happens to first ionisation energy down a group?
It decreases
Why do ionisation energies decrease down a group?
Although the nuclear charge increases, its effect is outweighed by the increased radius and increased shielding
What is the overall trend down a group?
Atomic radius increases
More inner shells so shielding increases
Nuclear attraction on outer electrons decreases
First ionisation energy decreases
What is the overall trend across a period?
Nuclear charge increases
Same shell so similar shielding
Nuclear attraction increases
Atomic radius decreases
First ionisation energy increases
Why is there a fall in ionisation energy from beryllium to boron?
It marks the start of filling of the 2p subshell
Why is there a fall in first ionisation energy from nitrogen to oxygen?
It marks the start of electron pairing in the p orbitals of the 2p subshells
