Shirley McQueen, of Dunedin, asks :-

I was given a jigsaw puzzle of the Periodic Table of all known elements, and in the course of putting it together realised that it is mostly arranged in rows of 18 elements with each element assigned a number. The exceptions are row 1 which has only Hydrogen (1) and Helium (2), and rows 2 and 3 which have 8 elements with gaps of ten between Beryllium (4) and Boron (5), and Magnesium (12) and Aluminium (13).

Why are there fewer elements, in rows 1, 2 and 3, and why is the numerical order interrupted between 56 and 72, and 88 and 104?

Keith Gordon, a chemist at the University of Otago, responded.

The periodic table groups in the columns elements that have similar chemical properties and as you read left to right and along each row the atomic number, Z, (the number of protons in the nucleus and also the number of electrons in orbit) increases by one.

The design comes from how the electrons arrange around the nucleus and thus determine the atom's chemical properties. The electrons organise into levels or shells) and then orbitals within each level. So at the top the first shell only has an s-orbital (called the 1s); two electrons fit into this. Hydrogen has one s electron (1s1), helium has two (1s2). Once that is filled excess electrons occupy the second shell. In the periodic table this is shown by coming down to the next row. The second shell has two types of orbitals, the s which are now filled and a second group called the p-orbitals. There are three p-orbitals accommodating six electrons so the row is eight electrons wide. This takes us from lithium at the left-hand side of the row through to neon at the far right.

The left-hand column consists of atoms that have one electron outside full shells so are chemically very reactive (eg hydrogen, lithium, sodium, etc). The right-hand column are atoms which have full shells so are chemically unreactive (the noble gases helium, neon, argon etc). The second-to-right column are atoms that are one short of having full shells so are chemically very reactive. If a member of the left hand column (eg sodium) can get together with a member of the second to right column (eg Cl) they readily combine together (one gives an electron to the other so both now have full orbital shells) thus forming a stable compound (NaCl or common salt).

There are two further structural dislocations in the table. On the fourth row down scandium fills the electrons into a new family of orbitals called the d-orbitals. As there are five of these they accommodate 10 electrons, so the row is 18 elements wide. Finally in row six there is an odd shift from barium (Z = 56) to hafnium (Z = 72). This is because a new set of orbitals start to fill (the f-orbitals). Without this shift the table would appear 32 elements wide as there are 7 f-orbitals.