C3 The periodic table

The chemical elements can be arranged in a periodic table.

It developed as chemists tried to classify the elements. It arranges them in a pattern in which similar elements are grouped together.

John Newlands' table put the elements in order of atomic mass but failed to take account of elements that were unknown at the time.

Dmitri Mendeleev's table left gaps for the unknown elements, and so provided the basis for the modern periodic table.

The atomic (proton) number of an element determines its position in the periodic table.

The number of electrons in the outermost shell (highest energy level) of an atom determines its chemical properties.

The group number equals the number of electrons in the outermost shell.

We explain trends in reactivity as we go down a group in terms of:

• the distance between the outermost electrons and the nucleus
• the number of occupied inner shells in the atoms

Metals react by losing electrons. Non-metals react by gaining electrons.

The elements in Group 1 of the periodic table are called the alkali metals.

Their melting points and boiling points decrease going down the group. 

The metals all react with water to produce hydrogen and alkaline solutions containing the metal hydroxide.

They form 1+ ions in reactions to make ionic compounds. Their compounds are generally white and dissolve in water, giving colourless solutions.

The reactivity of the alkali metals increases down the group.

Compared with the alkali metals, transition elements have much higher melting points and densities. They are also stronger and harder, but much less reactive.

The transition elements do not react vigorously with water or oxygen.

A transition element can form ions with different charges, in compounds that are often coloured.

Transition elements and their compounds are important industrial catalysts.

The halogens all form ions with a single negative charge in their ionic compounds with metals.

The halogens form covalent compounds by sharing electrons with other non-metals.

A more reactive halogen can displace a less reactive halogen from a solution of one of its salts.

The reactivity of the halogens decreases going down the group.