3.2.6 Group 2, the Alkaline Earth Metals - Trends in physical properties
Students should:
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Physical trends in the group 2 metals
The physical trends explain how quantities which do not involve chemical change vary on descending the group. The group II metals are:
- Be - beryllium
- Mg - magnesium
- Ca - calcium
- Sr - strontium
- Ba - barium
- Ra - radium
However, as beryllium is very small and atypical of the group it is not usually included in trends. Radium is a radioactive element that occurs in minute quantities in the uranium bearing mineral pitchblende (extracted by Marie Curie for one of her Nobel prizes) and as such is also unsuitable for inclusion in the trends here.
Hence, the trends focus on the elements magnesium to barium
Atomic radius
The group II elements are all metals and have an atomic radius defined as half the distance between two adjacent atomic centres in the metal lattice.
As we descend the group the number of electron shells increases and the atomic radius also increases.
Magnesium (electronic configuration 2, 8, 2) has only three occupied energy shells
Calcium (electronic configuration 2, 8, 8, 2) has four occupied energy shells
etc.
First ionisation energy
The first ionisation energy is the energy required to remove the outermost electron from one mole of gaseous atoms to produce 1 mole of gaseous singly charged ions.
M(g) 
M+(g) + 1e
The first ionisation energy decreases on descending the group as the inner electron shells become increasing better at shielding the outer electrons from the nucleus.
| magnesium | calcium | strontium | barium |
| 736 kJ | 590 kJ | 550 kJ | 503 kJ |
This causes the elements to increase in reactivity as the group is descended.
Melting point
The melting point is the temperature at which the liquid state and solid states exist in equilibrium. It is the temperature at which a solid turns to a liquid. The melting point depends on the strength of the interparticular forces within a structure
All of the group II metals have a metallic structure consisting of a lattice of metal ions surrounded by a sea of delocalised electrons. The force of attraction between the negative charge cloud and the positive ions holds the structure together.
| magnesium | calcium | strontium | barium |
| 922 K | 1112 K | 1042 K | 998 K |
In general, as the size of the ions increases on descending the group from Mg to Ba, the distance of the delocalised electrons from the ions' nuclear centres increases causing a decrease in the force of attraction. Hence the general melting point decrease on descending the group.
However, this effect is opposed by the ease of loss of the outer electrons to the charge cloud. From Mg to Ca it is this second effect which is more important. But, the arrival of the 'd' electrons in strontium shield the delocalised cloud from the nucleus more efficiently and the melting points decrease.
