COMPOUNDS OF GROUP II ELEMENTS

Key Concepts

  • The vertical columns in the periodic table of the elements are known as groups.
  • The second vertical column from the left in the periodic table is referred to as Group 2.

    Alternative names for Group 2 are:
    (a) alkali earth metals (still commonly used)
    (b) alkaline earth metals, or alkaline-earth metals (1) (still commonly used)
    (c) group IIA (this numbering is no longer used)

  • The name and symbol for the elements in Group 2 are given below:
    Group 2 Elements
    (Alkaline-earth Metals)
    name symbol
    beryllium Be
    magnesium Mg
    calcium Ca
    strontium Sr
    barium Ba
    radium Ra
  • Except for beryllium(2), the Group 2 elements are typical metals:(a) relatively soft, but harder than group 1 metals, shiny solids at room temperature and pressure that are good conductors of heat and electricity(b) Moderately-high melting point.(c) have 2 valence electrons (2 electrons in the highest energy level)(d) are very reactive(e) form cations with a charge of +2 (M2+) when they combine with non-metals in an ionic compound(f) form white ionic compounds (3)
  • Going down group 2 from top to bottom the elements display the following general trends:

    (a) Atomic radius increases.

    (b) First ionization energy decreases.

    (c) Second ionization energy decreases.

    Second ionization energy is about double the first ionization energy for each element.

    (d) Third ionization energy decreases.

    Third ionization energy is much greater, about 4 times greater, than the second ionization energy for each element.

    (e) Electronegativities decrease as successive energy levels (electron shells) are filled resulting in the positive nucleus exerting less of a force of attraction on electrons.

    (f) Chemical reactivity increases

    (g) Densities generally increase.(4)

  • Summary of trends in Group 2 elements:
    Trends: Decreasing Group 2 Elements Trends: Increasing
    First Ionisation Energy Second Ionisation Energy Third Ionisation Energy Electro-
    negativity
    name symbol atomic number atomic radius Reactivity
    largest largest largest highest beryllium Be lowest smallest less reactive
    magnesium Mg
    calcium Ca
    strontium Sr
    smallest smallest smallest lowest barium Ba highest largest most reactive

Table of Data for Group 2 Elements

The table below gives the name, atomic number, electronic configuration of the atom, the first, second and third ionisation energy, melting point, density and electronegativity, of the Group 2 elements (alkaline-earth metals).

Carefully inspect this data to find trends, or patterns, in the properties of group 2 elements.

These patterns, or trends, recur throughout the periodic table and are referred to more generally as periodic trends, or, as periodicity.

Periodic Trends in Properties of Group 2 Elements
Period Name
(Symbol)
Atomic Number (Z) Simple Electronic Configuration Atomic Radius
(pm)
First
Ionization Energy (kJ mol-1)
Second
Ionization Energy (kJ mol-1)
Third
Ionization Energy (kJ mol-1)
Melting point (°C) Density (g cm-3) Electro-
negativity
(Pauling)
2 Beryllium
(Be)
4 2,2 112 899 1757 14,849 1280 1.86 1.57

3 Magnesium
(Mg)
12 2,8,2 160 738 1450 7730 651 1.75 1.31

4 Calcium
(Ca)
20 2,8,8,2 197 590 1145 4941 851 1.55 1.0

5 Strontium
(Sr)
38 2,8,18,8,2 215 549 1064 4207 800 2.6 0.95

6 Barium
(Ba)
56 2,8,18,18,8,2 217 503 965 3420 850 3.6 0.89

7 Radium
(Ra)
88 2,8,18,32,18,8,2 509 978 960 5.0 0.89

Trends in Electronic Configuration of Group 2 Elements

Consider the electronic configuration of group 2 elements. Can you see a trend (a pattern)?

name electronic configuration
beryllium 2,2
magnesium 2,8,2
calcium 2,8,8,2
strontium 2,8,18,8,2
barium 2,8,18,18,8,2
radium 2,8,18,32,18,8,2

Atoms of group 2 elements have just 2 electrons in the highest energy level (also known as the valence shell of electrons).

It is even easier to see this if we use a short-hand description of the electronic configuration of each atom in which the electrons that make up part of a Noble Gas (group 18) electron configuration are represented in square brackets followed by the number of electrons in the valence shell.
We have done this in the table below:

name short-hand electronic configuration
beryllium [He],2
magnesium [Ne],2
calcium [Ar],2
strontium [Kr],2
barium [Xe],2
radium [Rn],2

If an atom (M) of a group 2 element lost both these valence electrons (2e), then the ion of the group 2 element would have a charge of +2 (M2+) as shown in the equations below:

General equation: M M2+ + 2e
examples: Be Be2+ + 2e
Mg Mg2+ + 2e
Ca Ca2+ + 2e
Ba Ba2+ + 2e
Sr Sr2+ + 2e
Ra Ra2+ + 2e

And, the positively charged ion (cation) formed would have the same electronic configuration as a group 18 (Noble Gas) element, we say that the cation is isoelectronic with the Noble Gas, as shown below:

cation electronic configuration
Be2+ [He]
Mg2+ [Ne]
Ca2+ [Ar]
Sr2+ [Kr]
Ba2+ [Xe]
Ra2+ [Rn]

and the cation of a group 2 element would therefore be chemically very stable (that is, no longer very reactive), just like a Noble Gas (group 18 element).

So, just how likely is it that a group 2 element will lose both valence electrons and form a cation …..

Trends in Ionisation Energy of Group 2 Elements

Ionisation energy (or ionization energy) is the energy required to remove an electron from a gaseous species.

The first ionisation energy (or first ionization energy) refers to the energy required to remove an electron from a gaseous atom.

We can write a general equation to describe the removal of an electron (e) from a gaseous atom (M(g)) to produce a gaseous cation with a charge of +1 (M+(g)) as:

M(g) → M+(g) + e

Second ionisation energy refers to the energy required to remove an electron (e) from the gaseous ion with a charge of +1 (M+(g)) to form a gaseous ion with a charge of +2 (M2+(g)) as shown in the equation below:

M+(g) → M2+(g) + e

If the value of the ionisation energy is high, then lots of energy is required to remove the electron, and the reaction is less likely to occur readily.
If the value of the ionisation energy is low, then little energy is required to remove the electron, and the reaction is more likely to occur readily.

So let’s look at the values of the first and second ionisation energy for each Group 2 element (alkaline-earth metal):

1st Ionisation Reaction 1st Ionisation Energy (kJ mol-1) 2nd Ionisation Reaction 2nd Ionisation Energy (kJ mol-1)
Be(g) Be+(g) + e 899 highest Be+(g) Be2+(g) + e 1757 highest
Mg(g) Mg+(g) + e 738 Mg+(g) Mg2+(g) + e 1450
Ca(g) Ca+(g) + e 590 Ca+(g) Ca2+(g) + e 1145
Sr(g) Sr+(g) + e 549 Sr+(g) Sr2+(g) + e 1064
Ba(g) Ba+(g) + e 503 lowest Ba+(g) Ba2+(g) + e 965 lowest

As you go down group 2 from top to bottom, the value of first ionisation energy decreases, it is progressively easier to remove the first valence electron.

As you go down group 2 from top to bottom, the value of the second ionisation energy decreases, it is progressively easier to remove the second valence electron.

The suggestion here is that the chemical reactivity of the elements increases as you go down group 2 from top to bottom.
That is since it requires less energy to remove the two valence electrons as you go down the group, the chemical activity of these elements will increase going down the group.

You might also notice that the value of the second ionisation energy for each element is about double that of the first ionisation energy.

If we are right and the electronic configuration of a Noble gas (Group 18) element is particularly stable, then it should be very difficult, that is, require a lot more energy, to remove the third electron from each Group 2 element.

Third ionisation: M2+(g) → M3+(g) + e

So, let’s look at the value of each third ionization for each group 2 element:

name First Ionisation Energy (kJ mol-1 Second Ionisation Energy (kJ mol-1 Third Ionisation Energy (kJ mol-1
beryllium 899 (× 1.95 =) 1757 (× 8.45 = ) 14,849
magnesium 738 (× 1.96 = ) 1450 (× 5.33 = ) 7730
calcium 590 (× 1.94 = ) 1145 (× 4.32 = ) 4941
strontium 549 (× 1.94 = ) 1064 (× 3.95 = ) 4207
barium 503 (× 1.92 = ) 965 (× 3.54 = ) 3420

In general, it requires a bit less than twice as much energy to remove the second valence electron than it does to remove the first valence electron from a gaseous atom of each element.

But in general it requires more than double this amount of energy again in order to remove the third electron.
This strongly supports the concept that the electronic configuration of a Noble Gas (group 18) element is remarkably stable and that any atom or ion with this structure will not be chemically reactive.

As a result, Group 2 elements form ionic compounds in which the group 2 cation has a charge of 2+. (5)

But why is it easier to remove these valence electrons as you go down group 2 from top to bottom….

Trends in Atomic Radius of Group 2 Elements

First, let us think about the number of electron shells (or energy levels) being filled to make an atom of each group 2 element:

name electronic configuration Number of occupied energy levels
beryllium 2,2 2
magnesium 2,8,2 3
calcium 2,8,8,2 4
strontium 2,8,18,8,2 5
barium 2,8,18,18,8,2 6
radium 2,8,18,32,18,8,2 7

As you go down group 2 from top to bottom, you are adding a whole new “electron shell” to the electronic configuration of each atom.
Surely that will increase the size of each atom as you go down the group?
We record the “size” of an atom using its “atomic radius”.
Consider the values for the atomic radius of each of the atoms in group 2 as shown in the table below:

name atomic radius (pm) Trend
beryllium 112 smallest
magnesium 160
calcium 197
strontium 215
barium 217 largest

As you go down group 2 from top to bottom the radius of the atom of each successive element increases.
This means that the negatively charged valence electrons get further away from the positively charged nucleus and we say that these electrons are ‘shielded’.
So, the positively charged nucleus has less of a “pull” on the valence electrons as you go down the group.
Therefore, the valence electrons are easier to remove, and therefore the ionisation energy decreases down the group as discussed in the previous section.

All of this means that the reactivity of Group 2 elements increases as you go down the group from top to bottom…

 

Trends in Reactivity of Group 2 Elements (alkaline-earth metals)

All the group 2 elements (M(s)), except beryllium, react with water (H2O(l)) to form hydrogen gas (H2(g)) and an alkaline (basic) aqueous solution (M(OH)2(aq)) as shown in the balanced chemical equations below:

Mg(s) + 2H2O(l) H2(g) + Mg(OH)2(aq)
Ca(s) + 2H2O(l) H2(g) + Ca(OH)2(aq)
Sr(s) + 2H2O(l) H2(g) + Sr(OH)2(aq)
Ba(s) + 2H2O(l) H2(g) + Ba(OH)2(aq)

The reaction between magnesium and water is usually slow because magnesium readily reacts with oxygen and a protective layer of magnesium oxide forms over the metal.
The reactions between other Group 2 elements and water is vigorous.

Beryllium and magnesium do not combine directly with hydrogen, however, calcium, strontium and barium will combine directly with hydrogen:

Ca(s) + H2(g) CaH2(s)
Sr(s) + H2(g) SrH2(s)
Ba(s) + H2(g) BaH2(s)

Reactions with water and hydrogen as described above indicate that there is a general trend in the chemical reactivity of group 2 elements: the reactivity of the group 2 elements increases as you go down the group from top to bottom.

The group 2 metals (M(s)) react with oxygen gas (O2(g)) at room temperature and pressure to form oxides with the general formula MO as shown in the balanced chemical reactions below:

2Be(s) + O2(g) 2BeO(s)
2Mg(s) + O2(g) 2MgO(s)
2Ca(s) + O2(g) 2CaO(s)
2Sr(s) + O2(g) 2SrO(s)
2Ba(s) + O2(g) 2BaO(s)

Group 2 metals (M(s)) react with halogens (group 17 elements) to form halides with the formula MX2.
For example, group 2 elements react with the halogen chlorine gas (Cl2(g)) to form an ionic chloride(6) (MCl2(s)) as shown in the balanced chemical equations below:

Be(s) + Cl2(g) BeCl2(s)
Mg(s) + Cl2(g) MgCl2(s)
Ca(s) + Cl2(g) CaCl2(s)
Sr(s) + Cl2(g) SrCl2(s)
Ba(s) + Cl2(g) BaCl2(s)

Group 2 elements will also combine with sulfur to form sulfides with the general formula MS:

Be(s) + S BeS
Mg(s) + S MgS
Ca(s) + S CaS
Sr(s) + S SrS
Ba(s) + S BaS

and they will combine with nitrogen to form nitrides with the general formula M3N2:

3Be(s) + N2(g) Be3N2
3Mg(s) + N2(g) Mg3N2
3Ca(s) + N2(g) Ca3N2
3Sr(s) + N2(g) Sr3N2
3Ba(s) + N2(g) Ba3N2

ASSIGNMENT : COMPOUNDS OF GROUP II ELEMENTS ASSIGNMENT MARKS : 10  DURATION : 1 week, 3 days

 

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