Concepts: early classification and the modern periodic table

2.1 Why classify at all

By the middle of the nineteenth century more than sixty elements were known, each with its own set of properties, and chemistry needed a system. A good classification would place similar elements together, expose patterns in their properties and predict elements that had not yet been found. The early attempts failed for lack of a good ordering principle, and the modern table succeeds because it orders by atomic number and therefore by electronic configuration.

2.2 Early attempts

Dobereiner arranged elements in groups of three called triads. In each triad the atomic mass of the middle element was roughly the average of the other two, and the three shared similar chemical behaviour. Lithium, sodium and potassium form a familiar triad. Only a few such triads could be found, so the scheme could not cover the whole list of elements.

Newlands arranged the elements in increasing atomic mass and noticed that every eighth element resembled the first, much like the notes of a musical scale. His law of octaves worked for the light elements but broke down beyond calcium, and it did not leave room for elements discovered later.

2.3 Mendeleev's periodic law and table

Mendeleev put the known elements in order of increasing atomic mass and saw that their properties returned in a regular pattern. In his law, mass was the ordering quantity and chemistry repeated with it. He grouped elements of similar behaviour in one vertical column and left gaps wherever no known element fitted.

The table had real predictive power. Mendeleev left gaps for eka-aluminium, eka-silicon and eka-boron, and when gallium, germanium and scandium were discovered their properties matched his predictions closely. He also corrected the atomic masses of several elements whose values did not fit their position, which showed that the table worked as a reasoning tool rather than a bare list.

The limitations appeared as the table grew. Hydrogen was difficult to place. The noble gases had no obvious place when they were discovered, because their valency is zero. Some pairs of elements had to be placed out of strict mass order so that their chemistry would line up: argon with potassium and cobalt with nickel, and tellurium with iodine. Isotopes were the deeper problem. Isotopes of one element have different atomic masses, but they have identical chemical behaviour, so a table ordered by mass alone cannot keep a family inseparable.

2.4 The modern periodic law

The modern periodic law orders the elements by atomic number and states that their properties are a periodic function of that number. The change looks small but it removes every mass-based anomaly. Isotopes of an element share the same atomic number, so they occupy one place. Argon and potassium, cobalt and nickel, and tellurium and iodine fall in the right order because their atomic numbers, not their masses, decide the position.

2.5 The long form of the periodic table

The modern table arranges elements in horizontal periods and vertical groups. Period 1 has two elements, periods 2 and 3 have eight each, periods 4 and 5 have eighteen each, and periods 6 and 7 have thirty-two each. The extra length of the later periods appears because the d and f subshells fill in the middle of them. The groups are numbered 1 to 18.

The table is divided into four blocks by the subshell that receives the last electron. The s-block holds groups 1 and 2, with a helium exception in group 18 for its configuration, though helium is usually placed with the noble gases. The p-block holds groups 13 to 18. The d-block holds groups 3 to 12 and contains the transition metals, with three series of ten elements each. The f-block holds the lanthanoids and actinoids and is placed separately at the bottom of the table.

Mendeleev's original table did not name the new groups clearly, so the groups of the long form were needed. The modern table also separates metals from non-metals and metalloids by a rough diagonal boundary, with metals on the left and non-metals on the right.

2.6 Nomenclature

The IUPAC system names elements with atomic numbers above 100 by a root for each digit and the ending ium. The roots are nil for 0, un for 1, bi for 2, tri for 3, quad for 4, pent for 5, hex for 6, sept for 7, oct for 8 and enn for 9. Thus element 118 is ununoctium in the systematic older form; the full modern names were later assigned by IUPAC after the elements were confirmed.

2.7 Cause of periodicity

The properties of an element depend on its outer electrons. Moving across a period, the outer shell stays the same while the nuclear charge grows, so the effective nuclear charge rises and the atoms pull their electrons closer. Moving down a group, a new shell is added, so the outer electrons lie farther from the nucleus and are shielded by the inner electrons. These two effects explain nearly every trend in the next page and give the periodic table its shape.