Chemistry: Ionisation Energies Flashcards
What is meant by ionisation energy?
a measure of energy required to remove electrons from an atom
What is nuclear charge?
the actual (relative) charge due to protons in nucleus
Define the term first ionisation energy
energy required to remove one mole of electrons from one mole of isolated, gaseous atoms to form one mole gaseous positive ions
What is ionisation energy measured in?
kjmol-1
Show equations for the successive ionisation energies of element X
X (g) -> X+ (g) + e-
X+ (g) -> X2+ (g) + e-
X2+ (g) -> X3+ (g) + e-
Why does aluminium have a lower first ionisation energy compared to magnesium?
magnesium: 1s2 2s2 2p6 3s2
aluminium: 1s2 2s2 2p6 3s2 3p1
increase in nuclear charge BUT aluminium in higher energy level 3p rather than 3s therefore less energy to remove it
Why is the first ionisation energy of sulfur less than that of phosphorous?
phosphorous: 1s2 2s2 2p6 3s2 3p3
sulfur: 1s2 2s2 2p6 3s2 3p4
each three 3p orbitals in phosphorus contain one electron
in sulfur one of the 3p orbitals must have two electrons
mutual repulsion, easier to remove despite increase in nuclear charge
Outline the general trend in first ionisation energy down group one?
value decreases because:
greater nuclear charge
outer s electron easy to remove
increased shielding
greater distance from nucleus
Outline the trend in first ionisation energy down group two
value decreases because
nuclear charge increases
distance between electron and nucleus greater, weaker attraction
increased shielding
Why do first ionisation energies increase across period two?
all electrons in same energy level
extra proton in nucleus, greater nuclear charge
more energy needed to remove electron
atoms smaller, greater attraction
Why does the first ionisation energy fluctuate across period three?
increases from 14 protons onwards
electron lost from either s or p orbital
p orbital higher energy level, easier to remove electron
group 6 elements have lower than group 5 due to mutual repulsion in p orbital
Which elements deviate from the trend across period three?
aluminium / phosphorous / sulfur
What is meant by the term second ionisation energy?
energy required to remove one mole of electrons from one mole of gaseous unipositive ions to form one mole of gaseous dipositive ions
What are the features to second ionisation energy?
-successive ionisation energy always greater
-large increase occur with change in shell
What can a large increase in second ionisation energy determine?
a group of an unknown element
