The Periodic Table

8.1 Arrangement of elements

The Periodic Table arranges the elements in order of increasing proton number (atomic number).

• A period is a horizontal row. The period number is the number of occupied electron shells.
• A group is a vertical column. For Groups I to VII, the group number is the number of outer-shell electrons. Group VIII (the noble gases) has a full outer shell.

Metals and non-metals

The metals are on the left and in the centre of the table, and the non-metals are on the right. A stepped line separates them. Moving from left to right across a period, the elements change from metallic to non-metallic character. In Period 3, sodium, magnesium and aluminium are metals, silicon is in between, and phosphorus, sulfur, chlorine and argon are non-metals.

The oxides change too: metal oxides are basic, aluminium oxide is amphoteric, and non-metal oxides are acidic.

Groups and ions

The group number tells you the charge on the ion that the element forms:

The properties change in a regular way down a group, because the outer electrons are in shells that get further from the nucleus. This means you can predict the properties of an element from its position, and find a trend in a group from the data you are given.

The melting points fall down the group, so the melting point of caesium is lower than $63\,^\circ\text{C}$ (it is about $29\,^\circ\text{C}$).

8.2 Group I properties

The Group I alkali metals are lithium, sodium and potassium (then rubidium and caesium). They are relatively soft metals (they can be cut with a knife), shiny when freshly cut but they tarnish quickly in air, so they are stored under oil. They have low densities for metals, and low melting points, and they have one outer electron, which is why they form $1+$ ions and are very reactive.

Trends down Group I

Reaction with water

All the alkali metals react with water to give hydrogen and a metal hydroxide (an alkaline solution, which turns universal indicator purple):

$\ce{2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)}$

$\ce{2K(s) + 2H2O(l) -> 2KOH(aq) + H2(g)}$

Prediction. Rubidium and caesium are more reactive than potassium (a very violent reaction), have lower melting points and are denser.

8.3 Group VII properties

The Group VII halogens are non-metals made of diatomic molecules ($\ce{Cl2}$, $\ce{Br2}$, $\ce{I2}$). They have seven outer electrons and form $1-$ ions (halide ions).

Trends down Group VII

• The density increases (gas, then liquid, then solid).
• The melting point and boiling point increase.
• The reactivity decreases. Fluorine is the most reactive and astatine the least.

Explanation. A halogen atom reacts by gaining one electron. Going down the group, the outer shell is further from the nucleus, so the attraction for an incoming electron is weaker. The atom gains an electron less easily, so it is less reactive.

Displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its halide.

$\ce{Cl2(aq) + 2KBr(aq) -> 2KCl(aq) + Br2(aq)}$

$\ce{Cl2(aq) + 2Br-(aq) -> 2Cl-(aq) + Br2(aq)}$

$\ce{Br2(aq) + 2KI(aq) -> 2KBr(aq) + I2(aq)}$

The halogen is reduced (it gains electrons) and the halide ion is oxidised (it loses electrons), so these are redox reactions. The order of reactivity is chlorine > bromine > iodine.

No reaction. Bromine is less reactive than chlorine, so it cannot displace chlorine from chloride ions.

Astatine is below iodine, so it is a black solid (a darker colour, with a higher melting point) and it is less reactive than iodine.

8.4 Transition elements

The transition elements form a block of metals in the middle of the Periodic Table, between Group II and Group III (for example iron, copper, zinc, chromium, manganese and nickel).

Compared with the Group I metals, transition elements are harder and stronger, have much higher melting points and densities, and are much less reactive (they react only slowly with water and oxygen).

8.5 Noble gases

The Group VIII noble gases (helium, neon, argon, krypton, xenon) are unreactive, monatomic gases (single atoms, not molecules).

Because they are unreactive and do not form bonds, they exist as single atoms. The boiling points increase down the group.

Uses that depend on being unreactive or of low density: helium in balloons and airships (low density and non-flammable, unlike hydrogen), argon in filament light bulbs (to stop the hot filament from burning) and for welding, and neon in advertising signs.