7.1.1 Classifying elements by group in the periodic table

A group collects elements that behave alike

1. Elements in the same group are placed together because they react in similar ways.
2. Every element in a main group has the same number of electrons in its outer shell.
3. That shared outer arrangement is what makes their chemistry similar.
4. For the main groups the group number gives that count, except for Group 0.
5. Three groups have names of their own: Group 1, Group 7 and Group 0.

Group 1 elements are the alkali metals

1. The alkali metals make up Group 1, and include lithium, sodium and potassium.
2. Each has one electron in its outer shell.
3. Losing that single electron leaves an ion with a charge of $1+$.
4. They are metals, so they are shiny when freshly cut and conduct electricity.
5. Hydrogen sits above them in many layouts but is a non-metal and is not an alkali metal.

Group 7 elements are the halogens

1. The halogens make up Group 7, and include chlorine, bromine and iodine.
2. Each has seven electrons in its outer shell.
3. Gaining one electron completes the shell and gives an ion with a charge of $1-$.
4. They are non-metals, and they exist as molecules of two atoms such as ${Cl}_2$.
5. Their compounds with metals are called halides, as in sodium chloride.

Group 0 elements are the noble gases

1. The noble gases make up Group 0, and include helium, neon and argon.
2. Each has a full outer shell, which is eight electrons for every one except helium.
3. Helium is full with two electrons, because the first shell holds only two.
4. Having a full shell means there is nothing to gain, lose or share, so they barely react.
5. The group number $0$ reflects that they form no ordinary compounds, not that they have no outer electrons.

The electronic configuration places an element in its group

1. Writing out the electron arrangement shows how many electrons occupy the outer shell.
2. Sodium is $2,8,1$, so its single outer electron places it in Group 1.
3. Chlorine is $2,8,7$, so its seven outer electrons place it in Group 7.
4. Argon is $2,8,8$, so its full outer shell places it in Group 0.
5. Two elements with the same outer count sit in the same group however many inner shells they have.

7.1.2 Properties and reactions of the alkali metals

The alkali metals are soft and melt at low temperatures

1. Lithium, sodium and potassium are soft enough to be cut with a knife.
2. A freshly cut surface is shiny, then dulls within seconds as it reacts with air.
3. Their melting points are low for metals, and they fall down the group.
4. Sodium melts at about $98{}^{\circ }C$, which is below the boiling point of water.
5. They are stored under oil to keep air and moisture away from the metal.

Lithium, sodium and potassium all react with water

1. Each of the three gives an alkaline solution and a gas: $$metal+water\to metal\; hydroxide+hydrogen$$
2. Lithium fizzes steadily, floats, and slowly disappears.
3. Sodium melts into a ball that darts across the surface, fizzing rapidly.
4. Potassium reacts so vigorously that the hydrogen ignites with a lilac flame.
5. The solution left behind turns universal indicator purple, because a hydroxide has formed.

Reactivity increases down the group

1. The three reactions differ in vigour, not in the products they give.
2. Lithium is the least vigorous of the three and potassium the most.
3. The order lithium, sodium, potassium is therefore an order of increasing reactivity.
4. The same order appears in how quickly each metal tarnishes in air.
5. A pattern that holds for three members of a group can be extended to the rest of it.

Predicting the reactivity of rubidium and caesium

1. Rubidium lies below potassium, so it reacts with water more vigorously than potassium does.
2. Caesium lies below rubidium, so it is more vigorous still.
3. The products follow the same pattern, so rubidium gives: $$2Rb+2H_{2}O\to 2RbOH+H_2$$
4. Their melting points continue to fall, so caesium melts a little above room temperature.
5. A prediction of this kind is made by extending the trend, not by recalling a result.

Electronic configuration explains the pattern

1. Every alkali metal atom has one electron in its outer shell, and reacting means losing it.
2. Down the group each atom has more shells, so the outer electron sits further from the nucleus.
3. The inner shells also shield the outer electron from the nucleus's positive charge.
4. Distance and shielding together mean the outer electron is held less strongly.
5. An electron that is lost more easily makes the metal more reactive, which is the pattern observed.