Mendeleev: He arranged elements in order of atomic mass and lined up in groups with similar
properties. Left gapsif group properties did not fit for elements not discovered and if some atomic
mass measurements were wrong. Potassium and argon are not arranged in increasing of atomic
mass, but rest of first twenty are.
• Periodic Table Now: Elements arranged in order of increasing atomic number to show properties
of rest of group.
• Groups: Each element in a group has same number of outer-shell electrons and similar properties.
• Periods: The number of the periods gives the number of the highest energy electron shell.
• Periodicity: A repeating trend in properties of the elements across each period of periodic table.
Ionisation Energies
• First Ionisation Energy: The energy required to remove one electron from each atom in one mole
of gaseous atoms of an element to form one mole of gaseous 1+ ions.
• Na (g) -> Na+
(g) + e-
.
• Aluminium has smallest third ionisation energy as is first element in period with electron in 3p
shell.
D
Factors Affecting Ionisation Energy: The electron lost in the highest energy level and will
experience the least attraction from the nucleus. The three factors…
• Atomic Radius: The greater the distance between nucleus and outer electrons, the less the nuclear
attraction.
• Nuclear Charge: The more protons in nucleus of atom, the greater the attraction between nucleus
and outer electrons.
• Electron Shielding: Electrons are negatively charged so inner- shell electrons repel outer- shell
electrons. This is called shielding effect and reduces net attractive forces between nucleus and
outer shell electrons.
• Second Ionisation Energy: Energy required to remove one
electron from each atom in one mole of gaseous 1+ ions of
an element to form one mole of gaseous 2+ ions.
• Na+
(g) -> Na2+ (g) + e-
. Second IE = 2+ ion produced.
• Successive Ionisation Energies:
– The second ionisation energy is greater than the first.
– The Na+ ion is smaller than Na atom (only write if it is),
radius decreases.
– So shielding is decreases.
– Attraction between the remaining electron and nucleus increases.
– So more ionisation energy needed to remove this electron.
• When talking about second ionisation energy and beyond, no longer atomic radius but ionic radius
and not Mg but Mg+
.
• When drawing these diagrams, make the jump really big.
• Always mention electron closer/ radius, nuclear attraction and shielding.
• Comparing two elements across period… Nothing to do with Be or B stuff below.
– 2
nd ionisation energy of Rb involves removing electron from shell closer to nucleus.
– Stronger nuclear attraction on outermost electron of Rb compared to Sr.
– Outermost electron of Rb experiences less shielding compared to Sr.
• Comparing two elements down period…
– Mg has smaller radius.
– Outermost electron of Mg experiences less shielding.
– So more nuclear attraction on outermost shell of Mg.
• Different Shells: If a large increase of ionisation energies marks removed electron from a different
shell closer to nucleus so less shielding. Give examples.
• Electrons in inner shell will have the largest ionisation energies. Remember first ionisation energy
is from outer electrons.
• Trends in First Ionisation Energies down a Group:
– Down a group, the atomic radius increases. R.
– More inner shells so shielding increases. S.
– Nuclear attraction on outer electrons decreases. N.
– So first ionisation energy decreases so easier to remove electron.
– For the different groups, although the nuclear charge increases, the effect is outweighed by
increased radius and shielding.
• Trends in First Ionisation Energy across a Period: Increased nuclear charge is most important
factors for the general increase.
– Across a period, nuclear charge greater. C.
– Outermost electrons added to same shell so experiences similar shielding. S.
– Atomic radius decreases. R.
– Greater nuclear attraction on outermost shells. N.
– So first ionisation energy increases.
• Sub- Shell Trends: A sharp decrease between the end of one period and start of new. Although
first ionisation energy shows general increase across period 2 and across 3, it does fall in two
places in each period.
• Learn above.
• Comparing Beryllium and Boron: Also Magnesium and Aluminium but it is the 3s and 3p sub shells
involved.
– The first ionisation energy of boron is less than the first ionisation energy of beryllium. As
marks start of filling of the 2p sub- shell.
– Be has outer electron in 2s sub shell.
– B has outer electron in 2p sub shell.
– 2p sub shell has higher energy than 2s sub shell and so electron easier to remove.
• Comparing Nitrogen and Oxygen: Also P and S but it is the 3p sub shell involved.
– The first ionisation energy of oxygen is less than the first ionisation energy of nitrogen.
– Marks the start of electron pairing in the p- orbitals of the 2p sub- shell.
– In O, one of the 2p orbitals contains paired electrons in highest energy level whereas in N, all
three orbitals are singly occupied.
– The paired electrons in O repel and electron is easier to remove.