0.3 The periodic table  (Page 2/5)

Investigation : the properties of elements

Refer to [link] .

1. Use a periodic table to help you to complete the last two diagrams for sodium ( $\mathrm{Na}$ ) and potassium ( $\mathrm{K}$ ).
2. What do you notice about the number of electrons in the valence energy level in each case?
3. Explain why elements from group 1 are more reactive than elements from group 2 on the periodic table (Hint: Think about the 'ionisation energy').

It is worth noting that in each of the groups described above, the atomic diameter of the elements increases as you move down the group. This is because, while the number of valence electrons is the same in each element, the number of core electrons increases as one moves down the group.

Periods in the periodic table

A period is a horizontal row in the periodic table of the elements. Some of the trends that can be observed within a period are highlighted below:

• As you move from one group to the next within a period, the number of valence electrons increases by one each time.
• Within a single period, all the valence electrons occur in the same energy shell. If the period increases, so does the energy shell in which the valence electrons occur.
• In general, the diameter of atoms decreases as one moves from left to right across a period. Consider the attractive force between the positively charged nucleus and the negatively charged electrons in an atom. As you move across a period, the number of protons in each atom increases. The number of electrons also increases, but these electrons will still be in the same energy shell. As the number of protons increases, the force of attraction between the nucleus and the electrons will increase and the atomic diameter will decrease.
• Ionisation energy increases as one moves from left to right across a period. As the valence electron shell moves closer to being full, it becomes more difficult to remove electrons. The opposite is true when you move down a group in the table because more energy shells are being added. The electrons that are closer to the nucleus 'shield' the outer electrons from the attractive force of the positive nucleus. Because these electrons are not being held to the nucleus as strongly, it is easier for them to be removed and the ionisation energy decreases.
• In general, the reactivity of the elements decreases from left to right across a period.
• The formation of halides follows the general pattern: $\mathrm{X}{\mathrm{Cl}}_{\mathrm{n}}$ (where $\mathrm{X}$ is any element in a specific group and $\mathrm{n}$ is the number of that specific group.). For example, the formula for the halides of group 1 will be $\mathrm{X}\mathrm{Cl}$ , for the second group the halides have the formula $\mathrm{X}{\mathrm{Cl}}_2$ and in the third group the halides have the formula $\mathrm{X}{\mathrm{Cl}}_3$ . This should be easy to see if you remember the valency of the group and of the halides.

The formation of oxides show a trend as you move across a period. This should be easy to see if you think about valency. In the first group all the elements lose an electron to form a cation. So the formula for an oxide will be ${\mathrm{X}}_2\mathrm{O}$ . In the second group (moving from left to right across a period) the oxides have the formula $\mathrm{X}\mathrm{O}$ . In the third group the oxides have the formula ${\mathrm{X}}_2{\mathrm{O}}_3$ .