The periodic table - (CCEA)

• Dmitri Mendeleev developed an early version of the periodic table that places elements with similar properties in the same vertical group.
• The alkali metals (Group 1) are a group of highly reactive metals that have distinctive observations when they react with water.
• The halogens (Group 7) are a group of highly reactive non-metals that can react with each other in displacement reactions.
• The noble gases (Group 0) are a group of non-reactive gases, and the transition metals are a block of metals that react to produce colourful compounds.

An element is a substance that is made up of only one type of atom, that cannot be broken down into simpler substances by chemical means.

In the 1800s, many chemists attempted to organise the elements into a logical order.

However, at this time most of the elements had not yet been discovered, so these chemists’ tables were incomplete, and often had mistakes in them.

What was Mendeleev’s periodic table?

In 1869, the Russian chemist, Dmitri Mendeleev published a periodic table that was the starting point for the table we use today.

Mendeleev’s periodic table had three important features:

• He arranged the elements in order of increasing atomic mass.
• He put elements with similar properties in the same group.
• He left gaps for undiscovered elements.

What does the modern periodic table look like?

Scientists have made slight changes to Mendeleev’s periodic table over time:

• The modern periodic table is arranged in order of increasing atomic number, rather than atomic mass.
• There are no gaps in the modern periodic table as more elements have been discovered.
• The noble gases were discovered and added as a group to the modern periodic table.
• The modern periodic table includes actinides and lanthanides.
• The transition metals are arranged in a separate block in the modern periodic table.

The modern table is called a “periodic” table because the elements with similar properties appear at regular intervals.

• The vertical columns are called groups.
• The horizontal rows are called periods.

There are many divisions in the periodic table, but one of the most important is between the metals and the non-metals.

The metal elements are on the left of a ‘stepped line’ that runs below boron (B), silicon (Si), arsenic (As), tellurium (Te), and astatine (At).

You can tell the difference between the metals and non-metals by looking at their properties.

• The non-metal elements hydrogen nitrogen, oxygen, fluorine, chlorine and all the noble gases (Group 0) are gases.

• Bromine and mercury are the only liquid elements.

• Every other element is a solid.

Elements in the same group have the same number of electrons in their outer shell.

For example, every element in Group 1 has 1 electron in the outer shell.

You need to know the names of the following groups:

The Group 1 elements all have similar physical properties:

• Low density - the first three (lithium, sodium and potassium) can all float on water.
• Very soft - they are easily cut with a knife.
• Shiny when first cut, but tarnish rapidly in air.
• Low melting points.

All alkali metals react with water to produce a metal hydroxide and hydrogen.

metal + water→ metal hydroxide + hydrogen

For example, here is the reaction of sodium with water:

sodium + water → sodium hydroxide + hydrogen

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

This reaction is quite dangerous as the Group 1 metals are all highly reactive.

They are stored under oil to prevent any reaction with air or water vapour when in storage.

The following observations are made for the reaction of lithium, sodium and potassium with water:

The following safety precautions must be followed when carrying out the reaction of Group 1 metals with water:

• Use tweezers when lifting alkali metals.
• Use a safety screen and wear safety glasses.
• Use a small piece of metal.
• Use a large volume of water (eg in a trough)

Group 1 metals all react in the same way, because they all have one electron in their outer shell.

This means each of them loses an electron to form a positive ion with a stable full outer shell.

For example, sodium loses one electron to form a sodium ion, Na⁺, with a full outer shell.

This can be represented by a half equation (higher tier only):

Na → Na⁺ + e^-

Group 1 metals become more reactive as you move down the group.

Elements further down a group have larger atoms, and the larger the atom, the further the outer electron is from the nucleus.

The force of attraction between the nucleus and the electron is less than for smaller atoms, and the outer electron is lost more easily.

Group 7 elements exist as simple, diatomic molecules.

Each molecule is made up of a pair of halogen atoms, linked by a single covalent bond.

Halogens are toxic.

You should use a fume cupboard if you are using them in experiments.

Key fact

You will notice that, moving down Group 7, the elements become darker in colour and change from gas to liquid to solid.

This is because elements lower in the group have higher melting points.

Iodine is a grey-black solid at room temperature and pressure.

If heated, it sublimes to form a purple gas.

Sublimation is the change of state from solid directly to gas on heating, without passing through the liquid phase.

Damp universal indictor paper changes to red and then bleaches white in the presence of chlorine gas.

Halogen elements can react with each other in a reaction called a displacement reaction.

This involves a reaction where a more reactive halogen displaces (pushes out) a less reactive halogen from a compound.

For example, chlorine can react with potassium bromide.

In this reaction, just the bromine and the chlorine react (the potassium is just a spectator).

Chlorine is a more reactive halogen than bromine, so it is able to displace bromine from its compound and take its place.

Click through the slides below to see the equations and observations for this reaction.

Chlorine can also displace iodine from a compound.

When a Group 7 element reacts, it gains one electron to form a negative ion with a stable full outer shell.

As we move down the column of Group 7 elements, their reactivity decreases.

In each, the outer shell is further from the nucleus and the incoming electron is not as easily attracted to it.

Half equations can be used to represent these reactions (higher tier only):

Chloride ions from a chlorine molecule:

Cl₂ + 2e^- → 2Cl^-

A chloride ion from a chlorine atom:

Cl + e^- → Cl^-

Group 0 elements have stable electronic configurations – full outer shells of electrons – and are therefore unreactive.

The Group 0 elements are all colourless gases.

All noble gases have low boiling points.

Their boiling points increase as we move down the group.

The transition metals are in the block in the middle of the periodic table, between Groups 2 and 3.

They are all metals and include many common metals such as chromium (Cr), iron (Fe), nickel (Ni) and copper (Cu).

The properties of the transition metals are often quite different to those of the Group 1 metals.

This is shown in the table below.

You will notice under ‘formation of ions’ that the transition metals react to form ions with different charges.

An iron(II) ion has a 2+ charge, and an iron(III) ion has a 3+ charge.

Transition metals tend to form coloured compounds: