Periodic Table Explained: Mendeleev, Groups, Periods

A card game of the elements

In 1869 the Russian chemist Dmitri Mendeleev wrote each known element on a card, with its atomic mass and its properties. He laid the cards out in order of mass and noticed that similar properties came back again and again, like notes in an octave. The pattern was neat, but it only worked if he left empty places. Rather than hide the gaps, he declared that elements were missing there and had not been found yet. The picture draws the first rows of his table, with question marks where the cards were absent. Around 63 elements were known to him at that time.

Predictions that came true

Mendeleev went further and described the missing elements. For the gap under aluminium he predicted a metal with an atomic mass near 68. In 1875 the French chemist Paul-Émile Lecoq de Boisbaudran found it and named it gallium. It melts at about 30 °C, so a lump can melt in your hand. For the gap under silicon Mendeleev predicted a mass near 72 and a density near 5.5 g/cm³. In 1886 Clemens Winkler found germanium: mass 72.6, density 5.35 g/cm³. Such close hits made scientists trust the table. In 1913 Henry Moseley then showed that the true order is by atomic number, the count of protons.

Periods and groups

Each row of the table is a period, and there are seven of them. Each column is a group, and there are eighteen. Elements in one group have the same number of outer electrons, so they behave alike. Across a period, each step adds one proton and one electron. Pick a property in the sandbox, atomic mass, electronegativity or atomic radius, and the table turns into blocks whose heights show it. Atomic mass climbs almost steadily. Electronegativity, the pull an atom has on shared electrons, peaks at fluorine with 3.98 on the Pauling scale. Radius shrinks across a period and grows down a group. See the ridges and valleys? That repeating pattern is periodicity.

Metals, non-metals and noble gases

Most elements are metals: shiny, bendable and good at carrying electricity, like iron, copper and gold. Non-metals such as carbon, oxygen and sulfur are usually brittle as solids and poor conductors. On the far right sit the noble gases, helium, neon, argon and their relatives, whose outer shells are full, so they hardly react at all. Group 1, the alkali metals, is the opposite: each atom has one loose outer electron. Reactivity rises down the group, lithium below sodium and sodium below potassium. That is why these metals are stored under oil and shown only by trained teachers behind a shield. The picture draws it as a staircase of eagerness.

118 and counting

Today 118 elements are confirmed, from hydrogen, number 1, to oganesson, number 118. Roughly 90 of them occur in nature. The heaviest ones are made in laboratories and exist for only a fraction of a second. In 2016 the international union IUPAC approved names for the four newest: nihonium (113), moscovium (115), tennessine (117) and oganesson (118), which completed the seventh row. Open the school's periodic table page to explore every element in detail. Mendeleev's table has grown a great deal, but its shape and its logic are the same as in 1869.

The periodic table