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## Chemical properties of the groups

In some groups, the elements display very similar chemical properties and some of the groups are even given special names to identify them. The characteristics of each group are mostly determined by the electron configuration of the atoms of the elements in the group. The names of the groups are summarised in Figure 1

A few points to note about the groups are:

• Although hydrogen appears in group 1, it is not an alkali metal.

• Group 15 elements are sometimes called the pnictogens.

• Group 16 elements are sometimes known as the chalcogens.

• The halogens and the alkali earth metals are very reactive groups.

• The noble gases are inert (unreactive).

Groups in the periodic table

The following diagram illustrates some of the key trends in the groups of the periodic table:

Table 3 summarises the patterns or trends in the properties of the elements in group 1. Similar trends are observed for the elements in the other groups of the periodic table. We can use the information in Table 3 to predict the chemical properties of unfamiliar elements. For example, given the element Francium ($Fr$) we can say that its electronic structure will be $[Rn]7s 1$, it will have a lower first ionisation energy than caesium ($Cs$).

One general trend that is not shown is the melting and boiling points. For the metals (groups 1 to 13) the melting and boiling points increase as you go up the group. For the non-metals the melting and boling points decrease as you go up the group.

You should also recall from chapter Chapter 2 that the metals are found on the left of the periodic table, non-metals are on the right and metalloids are found on the zig-zag line that starts at boron.

Table 1: Summary of the trends in group 1
 Element $​37Li$ $​37Na$ $​37K$ $​37Rb$ $​37Cs$ Electron structure $[He]2s 1$ $[Ne]3s 1$ $[Ar]4s 1$ $[Kr]4s 1$ $[Xe]5s 1$ Group 1 chlorides $LiCl$ $NaCl$ $KCl$ $RbCl$ $CsCl$ Group 1 elements all form halogen compounds in a 1:1 ratio Group 1 oxides $Li 2 O$ $Na 2 O$ $K 2 O$ $Rb 2 O$ $Cs 2 O$ Group 1 elements all form oxides in a 2:1 ratio Atomic radius Increases as you move down the group. First ionisation energy Decreases as you move down the group. Electronegativity Decreases as you move down the group. Melting and boiling point Decreases as you move down the group. Density Increases as you move down the group.

## Exercise 1: Groups in the periodic table

Use Table 3 and Figure 2 to help you produce similar tables for group 2 and group 17.

1)

ElementBeMgCaSrBa
Electron structure $\text{[He]}2{\text{s}}^{2}$ $\text{[Ne]}3{\text{s}}^{2}$ $\text{[Ar]}4{\text{s}}^{2}$ $\text{[Kr]}5{\text{s}}^{2}$ $\text{[Xe]}6{\text{s}}^{2}$
Chlorides ${\text{BeCl}}_{2}$ ${\text{MgCl}}_{2}$ ${\text{CaCl}}_{2}$ ${\text{SrCl}}_{2}$ ${\text{BaCl}}_{2}$
All form chlorides in the ratio 1:2
Oxides $\text{BeO}$ $\text{MgO}$ $\text{CaO}$ $\text{SrO}$ $\text{BaO}$
All form oxides in the ratio 1:1
Atomic radius Increases down the group
First ionisation energy Decreases down the group
Electronegativity Decreases down the group
Boiling and melting point Decreases down the group
Density Increases down the group

2)

ElementFClBrIAt
Electron configuration $\text{[He]}2{\text{s}}^{2}2{\text{p}}^{5}$ $\text{[Ne]}3{\text{s}}^{2}3{\text{p}}^{5}$ $\text{[Ar]}4{\text{s}}^{2}4{\text{p}}^{5}$ $\text{[Kr]}5{\text{s}}^{2}5{\text{p}}^{5}$ $\text{[Xe]}6{\text{s}}^{2}6{\text{p}}^{5}$
Atomic radius Increases down the group
First ionisation energy Decreases down the group
Electronegativity Decreases down the group
Boiling and melting points Increases down the group
Density Increases down the group

Note that the melting and boiling points of the halogens DO NOT follow the trend given.

The following two elements are given. Compare these elements in terms of the following properties. Explain the differences in each case. $​1224Mg$ and $​2040Ca$.

1. Size of the atom (atomic radius)

2. Electronegativity

3. First ionisation energy

4. Boiling point

1. Calcium has a larger atomic radius than magnesium. Calcium has more electrons than magnesium and is filling an extra energy level. This increases the atomic radius.
2. Magnesium has a higher electronegativity than calcium. As you move down a group the valence electrons are further away from the nucleus and so experience less attraction from the nucleus. This decreases the elements pull on the electrons (electronegativity).
3. Magnesium has a higher first ionisation energy than calcium. As you move down a group, the outermost electrons are further away from the nucleus and are not held as tightly. This makes the ionisation energy higher for atoms at the top of a group.
4. Magnesium has a higher boiling point than calcium. Magnesium has stronger forces holding its atoms together and so more energy is needed to make magnesium boil.

Study the following graph and explain the trend in electronegativity of the group 2 elements.

As you move down a group the electronegativity decreases. This is due to the valence electrons being further away from the nucleus and so experience less attractive forces. As the electrons are not held as tightly the element's electronegativity decreases.

Refer to the elements listed below:

• Lithium ($Li$)

• Chlorine ($Cl$)

• Magnesium ($Mg$)

• Neon ($Ne$)

• Oxygen ($O$)

• Calcium ($Ca$)

• Carbon ($C$)

Which of the elements listed above:

1. belongs to Group 1

2. is a halogen

3. is a noble gas

4. is an alkali metal

5. has an atomic number of 12

6. has four neutrons in the nucleus of its atoms

7. contains electrons in the 4th energy level

8. has all its energy orbitals full

9. will have chemical properties that are most similar

1. Lithium

2. Chlorine

3. Neon

4. Lithium (Group 1 metals are known as alkali metals)

5. Carbon

6. Lithium

7. Calcium

8. Neon

9. Magnesium and calcium would have similar chemical properties as they are in the same group of the periodic table.

## Activity 1: Inventing your own periodic table

You are the official chemist for the planet Zog. You have discovered all the same elements that we have here on Earth, but you don't have a periodic table. The citizens of Zog want to know how all these elements relate to each other. How would you invent the periodic table? Think about how you would organise the data that you have and what properties you would include. Do not simply copy Mendeleev's ideas, be creative and come up with some of your own. Research other forms of the periodic table and make one that makes sense to you. Present your ideas to your class.

Circular periodic table

Image from Wikimedia commons