Interactive Periodic Table

What each tile tells you

Every tile carries four pieces of information. The atomic number is the number of protons in the nucleus, and it is what defines the element: an atom with six protons is carbon, always. The symbol is the one or two letter abbreviation used in formulas and equations. The name follows, and beneath it the atomic mass in unified atomic mass units.

Atomic mass is the one that puzzles people, because it is rarely a whole number. Carbon is listed as 12.011 rather than 12. That is because the figure is a weighted average across the element's naturally occurring isotopes, forms of the same element with different numbers of neutrons. Chlorine sits at 35.45 because natural chlorine is roughly three quarters chlorine-35 and one quarter chlorine-37.

Groups: the columns

The eighteen vertical columns are called groups, and elements within a group behave alike because they have the same number of valence electrons in their outer shell. Since bonding is decided almost entirely by those outer electrons, a group is really a family of elements that react in similar ways.

Four families are worth knowing by name. Group 1 holds the alkali metals, soft, reactive metals that lose one electron easily. Group 2 holds the alkaline earth metals, which lose two. Group 17 holds the halogens, which are one electron short of a full shell and are correspondingly aggressive at taking one. Group 18 holds the noble gases, whose outer shells are already full, which is why they barely react at all.

Periods: the rows

The seven horizontal rows are periods, and the period number tells you how many electron shells the atoms have. Elements in period 3 have three occupied shells; elements in period 6 have six.

Reading across a period, you watch properties change in an orderly way rather than repeat. A row typically begins with a highly reactive metal on the left, passes through less reactive metals, then metalloids, then nonmetals, and ends with an unreactive noble gas. Period 1 is the odd one out, containing only hydrogen and helium, because the first shell holds just two electrons.

The four blocks

The table's shape comes from the order in which electron subshells fill, which is why it divides into four blocks. The s-block is groups 1 and 2 plus helium, two columns wide because an s subshell holds two electrons. The p-block is groups 13 to 18, six columns wide. The d-block is groups 3 to 12, the transition metals, ten columns wide. The f-block, fourteen columns wide, holds the lanthanides (elements 57 to 71) and actinides (89 to 103).

The f-block is normally printed as two separate rows below the main body. That is purely for convenience: set in place, the table would be so wide it would not fit on a page. The row markers in periods 6 and 7 show where those elements really belong.

The trends that run across it

The table's real power is that position predicts behaviour. Four trends do most of the work, and all four come from the same tug of war: a stronger nuclear charge pulls electrons in, while additional shells and the shielding they provide push the outer electrons away.

• Atomic radius decreases across a period and increases down a group. Moving right adds protons without adding a shell, so the nucleus pulls the same shell tighter.
• Ionization energy, the energy needed to remove an outer electron, increases across a period and decreases down a group.
• Electronegativity, an atom's pull on shared electrons, increases across and decreases down, peaking at fluorine with a value of 3.98.
• Metallic character does the opposite of the other three: it decreases across a period and increases down a group.

Notice that three of the four point the same way, toward the top right. That is not a coincidence. Each is a different measure of how tightly an atom holds its outer electrons.

Metals, nonmetals and metalloids

Most of the table is metal. Metals occupy the left and centre, conduct heat and electricity, are malleable and lustrous, and tend to lose electrons to form positive ions.

Nonmetals cluster in the upper right. As solids they are dull and brittle, they insulate rather than conduct, and they tend to gain electrons to form negative ions. Several are gases at room temperature.

Between them, along the staircase line, sit the metalloids: boron, silicon, germanium, arsenic, antimony and tellurium. They behave as a mixture of both, and their intermediate conductivity is precisely what makes silicon and germanium useful as semiconductors.

Why the table looks the way it does

Dmitri Mendeleev published the arrangement that became the modern table in 1869, ordering the known elements by atomic mass and grouping those with similar properties. His genuine insight was to leave gaps where the pattern demanded an element that nobody had found, and to predict what those missing elements would be like. When gallium, scandium and germanium were later isolated, their properties matched his predictions closely, which is what convinced chemists the arrangement described something real.

One thing has changed since. The modern table is ordered by atomic number, not atomic mass, following Henry Moseley's work on X-ray spectra. That resolved the handful of pairs where mass order put elements in the wrong family, tellurium and iodine among them. Ordering by proton count also explains why the blocks are 2, 6, 10 and 14 columns wide, since those are exactly the numbers of electrons the s, p, d and f subshells hold.