In the 1800s, scientists tried to sort out the elements and put them into some sort of order.

They knew nothing about protons, neutrons and electrons, so they used their relative atomic masses to organise the elements into a list.

This meant that some elements were put in the wrong order, for example, argon was placed before potassium because it has a greater relative atomic mass.

Newlands noticed that every eighth element had similar properties, so in 1863, he started listing the elements in rows of seven. Unfortunately, he did not leave gaps for unknown elements, so the pattern went wrong.

In 1869, a Russian called Mendeleev followed Newlands ideas, but left gaps in order to keep the elements with similar properties listed in groups.
This meant he could predict the properties of the missing elements.

His table was remarkably accurate.

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The Modern Periodic Table 

In the modern periodic table, all the elements are listed in order of their atomic (proton) number.

Elements with similar properties are listed in vertical groups.

The rows across the table are called periods.

From left to right across the table, a particular energy level (shell) is being filled with electrons.

In the next period, the next energy level is filled.

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

This is why they have similar properties.

All elements in Group 1 have one outer electron, Group 2 have two outer electrons, and so on.

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Metals and Non-Metals

More than three-quarters of the elements are metals, shown on the left hand side of the table. Less than one quarter are non-metals, shown on the right hand side of the table.

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Group 1

Group 1 elements are known as the alkali metals.

They have a low density and the first three metals, lithium, sodium and potassium, float on water.

They form metal ions with a +1 charge, by losing one electron.

They react with non-metals to form ionic compounds, which are white solids that dissolve in water to form a colourless solution.

They are very reactive and react with cold water to form a metal hydroxide and hydrogen gas. The gas causes the metal to move around the surface of the water.

The hydroxide dissolves in water to give an alkaline solution.

sodium   +   water      sodium hydroxide   +   hydrogen

The reaction is exothermic, which can cause the metal to melt or catch fire.

The more reactive the metal, the more vigorous its reaction with water.

The test for hydrogen is to put a lighted splint into the gas.

The hydrogen burns in the air with a squeaky explosion.

The further down Group 1 an element is, the more reactive it is and the lower its melting and boiling points.

The pattern in reactivity is due to the sizes of the atoms, and how far from the nucleus the outer electrons are.

In larger atoms, the outer electrons are further from the positively charged nucleus, so are less strongly attracted to it.

These electrons are therefore more easily lost, so the element is more reactive.

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The Transition Metals

This block of metals is in the centre of the table and includes iron and copper.

Compared to alkali metals, these elements:

• Are much less reactive and do not corrode as quickly with oxygen and/or water
• Are hard, tough and strong
• Have high melting points, except for mercury, which is a liquid at room temperature

Transition metals are therefore useful as structural materials, e.g. iron, usually in the form of steel, and for making things that must conduct heat or electricity, e.g. copper for electrical cables.

Many transition metals are used as catalysts, e.g. iron in the Haber process and platinum in the manufacture of nitric acid.

Many transition metals form coloured compounds which are used in pottery glazes.

Weathered copper forms green coloured compounds.

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Group 7 

The non-metal elements in this group are called the halogens.

They are all poisonous and produce coloured vapours.

They are made up of molecules which are pairs of atoms, e.g. Cl_2.

They form halide ions which have a –1 charge, by gaining an electron.

They react with metal ions to form ionic salts.

They form covalent bonds with other non-metallic elements to form molecular compounds, e.g. hydrogen chloride gas.

The further down Group 7 an element is, the less reactive it is and the higher its melting and boiling points.

Fluorine and chlorine are gases at room temperature, bromine is a liquid and iodine is a solid.

A more reactive halogen can displace a less reactive one from an aqueous solution of its salt.

e.g. chlorine  +   potassium bromide      bromine   +   potassium chloride

The pattern in reactivity is due to the sizes of the atoms, and how far from the nucleus the outer energy level is.

In larger atoms, the outer energy level is further from the positively-charged nucleus, so electrons are less strongly attracted to it.

Extra electrons are harder to gain if the atom is bigger, so non-metal elements are less reactive the further down a group.

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Group 0 

The elements in Group 0 are known as the noble gases.

They are all chemically very unreactive, because they have a full outer shell of electrons.

They exist as single atoms (monatomic), rather than diatomic molecules like other gaseous elements.

The full outer shell means they have no tendency to gain, lose or share electrons. 

Because they are inert (unreactive), they are used in light bulbs so the filament doesn’t burn, and in electrical discharge tubes where they glow brightly when a current is passed through. 

Helium is less dense than air, so it is used in airships and party balloons.

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