The periodic table: an introduction

There are 118 known elements. Learning the properties of each one separately would be hopeless. Chemists therefore looked for a pattern, a way to arrange the elements so that similar ones fall together. The result is the periodic table, one of the most useful charts in all of science.

Early attempts

• Dobereiner noticed groups of three similar elements, which he called triads, such as lithium, sodium and potassium.
• Newlands arranged the elements by increasing atomic mass and found that every eighth element resembled the first, like the notes of music. He called this the law of octaves, but it worked only for the lighter elements.
• Dmitri Mendeleev, a Russian chemist, made the great breakthrough in 1869. He arranged the 63 elements then known in order of atomic mass, in a table where similar elements fell in the same column. Boldly, he left gaps for elements not yet discovered and predicted their properties. When gallium, scandium and germanium were found later, they matched his predictions closely.

The modern periodic law

Mendeleev's table had a few elements out of place. In 1913 Henry Moseley showed that the true basis of an element is not its atomic mass but its atomic number, the number of protons. This gave the modern periodic law:

The properties of elements are a periodic function of their atomic numbers. That is, when elements are arranged in order of atomic number, similar properties recur at regular intervals.

How the table is built

The modern table has 7 horizontal rows called periods and 18 vertical columns called groups.

• Periods: all elements in a period have the same number of electron shells. Elements of period 2 have two shells; those of period 3 have three. The first period has only 2 elements, hydrogen and helium; the second and third have 8 each.
• Groups: elements in a group have the same number of valence electrons and therefore similar chemical properties.

Some families to know

• Group 1, alkali metals (lithium, sodium, potassium): soft, very reactive metals with one valence electron.
• Group 2, alkaline earth metals (magnesium, calcium).
• Group 17, halogens (fluorine, chlorine, bromine, iodine): reactive non-metals with seven valence electrons.
• Group 18, noble gases (helium, neon, argon): their outer shells are full, so they hardly react at all.

Metals lie on the left and in the middle of the table, non-metals on the right. Along a zig-zag line between them lie the metalloids, such as silicon, which show properties of both.

Trends in the table

• Atomic size decreases from left to right across a period, because the growing charge of the nucleus pulls the electrons closer. It increases down a group, because a new shell is added at each step.
• Metallic character decreases across a period and increases down a group.
• Valency across a period first rises from 1 to 4 and then falls to 0.

The first twenty elements

Students are expected to know these in order: hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, neon, sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, argon, potassium, calcium.