Alkali Metals

Alkali metals: definition

Alkali metals: elements

Alkali metals are lithium, sodium, potassium, rubidium, caesium, and francium. However, you will most likely only encounter the first three in school as the others are far too reactive and therefore dangerous. They are called alkali metals as when they react with water, they form an alkaline solution.

Alkali metals: properties

Let's see some properties of the alkali metals:

• They can all be cut with a knife, despite being metals!
• They are shiny when you freshly cut them, you will notice that they are very shiny. However, they will become much more matte and dull in a short space of time as they react with the oxygen in the air, forming an oxide.
• Low density. The densities of the first three alkali metals (Li, Na, K) are all less than 1g/cm³, making them less dense than water so they all float on water.
• Low melting and boiling points. For metals, they have low melting and boiling points. The melting points and boiling points decrease as you go down the group. The melting point for francium is only 27ºC!
• Good conductors of heat and electricity. As they are metals, they conduct heat and electricity very well. They form white compounds. When they react with substances to produce a compound, these compounds are white.
• Very reactive, due to their electron configuration. They even react vigorously with water.

Elements in the same group have similar properties due to their electron configuration; electrons are responsible for how an element reacts and what properties they have.

All elements in group 1 have 1 electron in their outermost shell and as a result, they have similar properties.

Alkali metals reactivity

The alkali metals are very reactive. The way an atom of any element reacts depends on its electron configuration. In all reactions, all elements want to become stable. That is their life goal! The way that atoms achieve this is by getting a full outer shell. For alkali metals which all have only 1 electron in their outer shell, the quickest way for them to obtain a full outer shell is by losing this 1 electron. This way, they are left only with the next outermost shell which is already full (remember electrons can only fill up the next shell if the previous one is completely full). Because they only need to lose 1 electron, it is very easy for an alkali metal to lose it to end up with a full outer shell, then becoming stable.

Reactions with alkali metals

The alkali metals are all very reactive. Take a look at what happens when an alkali metal reacts with various substances:

In reactions, the alkali metals lose the 1 electron in their outermost shell, forming ions with a 1+ charge.

$$ Li \rightarrow Li^{+}$$

$$Na \rightarrow Na^{+}$$

$$K \rightarrow K^{+}$$

Alkali metals reacting with water Lithium in oil, Wiki Commons

Because the first three alkali metals have a lower density than water, they will all float. However, the reactions with water become more vigorous as you go down group 1.

Here is a table showing what happens when lithium, sodium, and potassium react individually with water.

As you can see from the reactions getting more vigorous, the reactivity increases as you go down group 1.

By the end of the reactions, all of the alkali metals dissolve in the water, producing metal hydroxides. This final solution is alkaline, which is why they are called ‘alkali metals’!

Alkali metals reacting with chlorine

When the alkali metals react with chlorine, they react vigorously to form white metal chloride salts.

The reactions become more vigorous as you go down the group. They react similarly with other group-7 elements (halogens) - fluorine, bromine, and iodine - to form metal halides.

Alkali metals reacting with oxygen

The alkali metals burn vigorously with oxygen to form their metal oxides. These metal oxides are white.

Trends in alkali metals

In reactions, elements want to become stable by obtaining a full outer shell. Alkali metals do this by losing the 1 electron in their outermost shell.

Therefore, the more easily they can lose the electron in their outermost shell, the more reactive they are.

The nucleus is positively charged and the electrons are negatively charged. This electrostatic attraction keeps the electron in its shell.

Trends as you go down the group

• Melting points and boiling points ↓

  • The melting points and boiling points decrease as you go down the group. Lithium has a melting point of 180.5oC compared to francium which has a melting point of only 27oC.

• Densities ↑

  • The alkali metals become more dense as you go down the group.

• Reactivity ↑

  • Alkali metals get more reactive as you go down the group. From the reactions with water, you can see that lithium only fizzed whereas, with potassium, there was a lilac flame.