The Group II metals- Alkaline Earth metals
this group consists of the elements in descending order, Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr) and Barium (Ba).
The metals in this group are considered 'S block' elements because their valance electrons are located in s-orbitals (subshells).
Trend: Increases down the group
Explaination: As you move down the group, each atom of each element has more electrons, therefore creating more electron shields.
However, compared to Group I metals, the atomic radius of group 2 elements decrease.
This is because as you move along the period, each element has an increase in nuclear charge of +1. This causes the electrostatic attraction between the nucleus and valence electrons to increase. With the same amount of sheilding, the stronger attraction draws the electron shells closer to the nucleus, therefore reducing the radius.
Trend: Decreases down the group (for first ionisation energy)
(which means it's easier to remove electrons for metals down the group)
Explaination: The increase in electrons per atom of each element as you move down the group creates more electron sheilding between the nucleus and valence electrons. Increases sheilding therefore reduces the electrostatic attraction between these electrons and the nucleus. This means that less energy will be needed to remove a valence electron from the element as the force holding it there is weaker than in the elements higher up in the group.
😲 😲 🤔 🤔
However, the sucessive ionisation energies increase down the group
This could be due to the electron being removed coming from a higher energy subshell where less sheilding increases the attraction between the nucleus and the valence electrons and so more energy is required to overcome this attraction to remove another electron.
Also once an electron has been removed a positive ion remains. After the first ionisation this will be a +1 ion, which increases by +1 per electron removed. This causes there to be a higher proton to electron ratio after each successive ionisation and creates a stronger, more positive nuclear charge. In return, this increases the attraction between the nucleus and the valence electrons and makes it more difficult for another electron to be removed from the atom of the element.
Trend: Decreases down the group
(excluding Mg, which has a different crystaline structure)
Explaination: As you move down the group,the size of the ions formed by each element increases. This means that the delocalised electrons from each metal ion are further away from the nucleus as you move down the group, therefore reducing the eletrostatic attraction between the nucleus and these electrons. This allows weaker metallic bonds to be formed which require less energy to overcome.
Trend: Increases down the group
Explaination: It is easier to remove electrons to form the 2+ ion. This is due to an increase in number of electrons and sheilding between the valence electrons and the nucleus which reduces the electrostatic attraction, making it easier for electrons to be lost during a reaction. (see first ionisation energy)
Reaction with Water:
Metal + Water --> Metal Hydroxide + Hydrogen
M(s) + H2O(l) --> M(OH)2(aq) + H2(g)
In this redox reaction, the oxidisation state of the magnesium has increased from 0 to +2, meaning that the magnesium has been oxidised, therefore magnesium is the reducing agent. It has reduced the hydrogen from +2 in water to 0 as a gas, making hydrogen the oxidising agent. This reaction becomes increasingly vigorous down the group.
Reaction with steam:
magnesium + steam → magnesium oxide + hydrogen
Mg(s) + H2O(g) → MgO(s) + H2(g)
Magnesium reacts very slowly with water. However, it reacts vigorously with steam.
As a general rule, if a metal reacts with cold water, the metal hydroxide is produced. If it reacts with steam, the metal oxide is formed. This is because the metal hydroxides thermally decompose to the oxide and water.
SOLUABILITIES OF GROUP II Hydroxides/Sulphates IN WATER:
Trends:The metal hydroxides become increasingly more soluable whilst sulfates become less soluable down the group.
Explaination for the soluability of the group 2 hydroxides:
As you move down the group, the size of the ions increases which creates a lower charge density. This means that the larger ions do not hold onto the OH- ions as much. More OH- ions then get released into the water, causing the metal hydroxide to dissipitate and be surrounded in water (soluable) and gives the solution a higher pH level.
Hydroxides: Magnesium hydroxide is virtually insoluble, whereas barium hydroxide will readily dissolve in water.
Sulphates: Magnesium sulfate is readily soluble in water whereas barium sulfate is insoluble.
Applications of the group 2 hydroxides:
Magnesium hydroxide: Most commonly known as 'milk of magnesia' is used as a laxative in medicine. It is also used to relieve acid indigestion by neutralises the hydrochloric acid in the stomach.
Calcium hydroxide: Also known as slaked lime, it is used in agriculture to raise the pH of the soil.
Applications of the group 2 sulfates:
Barium sulphate: Also known as a 'Barium meal' it is used as a radiocontrast agent to help take X-Ray images of the digestive system. It's insoluability means that it is not absorbed into the body when swallowed, which would otherwise be fatal because it's toxic. Barium is also a very good absorber of X-Rays, and it helps define structures of the digestive system to aid in diagnosis.
Drafted by Eunice Wong (Chemistry)