How the Periodic Table Organises the Elements

Every element is defined by the number of protons in its atoms. This number is called the atomic number. Hydrogen has one proton, helium has two, lithium has three and the pattern continues across the table.

Changing the proton number changes the element itself. An atom with six protons is carbon, while one with seven is nitrogen. Atoms of the same element can have different numbers of neutrons, creating isotopes, but their proton number stays the same. The total number of protons and neutrons gives most of an atom's mass, while electrons occupy the space around the nucleus and control much of its chemistry.

Chemical symbols provide short labels, such as O for oxygen and Fe for iron. Some symbols come from older or Latin names, which is why they do not always match the first letters of the English word. Humanity apparently decided chemistry needed a small vocabulary ambush.

The horizontal rows are called periods. As you move across a period, the atomic number rises one step at a time. Electrons are added in an organised pattern around the nucleus, and this changing arrangement affects the element's properties.

Elements near the left side of a period are often metals that lose electrons relatively easily. Elements toward the right include nonmetals, many of which tend to gain or share electrons. The period ends with a noble gas, whose outer electron arrangement is especially stable. These are broad patterns rather than promises about every element. Transition metals in the middle of the table, for example, can form several kinds of ions and show more varied behaviour.

A new row begins when electrons start filling a new main energy level. This is why the table has its distinctive shape rather than one neat rectangle. The shape follows the underlying structure of atoms.

The vertical columns are called groups. Elements in the same group often have similar chemical behaviour because they have similar patterns in their outer electrons. Those outer electrons are especially important during bonding and reactions. Group members are not identical, but their repeated outer-electron pattern gives chemists a useful starting point for predictions.

• •Group 1 contains highly reactive alkali metals.
• •Group 2 contains alkaline earth metals.
• •Group 17 contains reactive halogens.
• •Group 18 contains mostly unreactive noble gases.
• •The centre contains transition metals with varied properties.

When Dmitri Mendeleev published his periodic table in 1869, dozens of elements were still unknown. Instead of forcing every known element into a neat sequence, he left gaps where the pattern suggested something was missing. He predicted the properties of some of these unknown elements, including their approximate masses and densities and the kinds of compounds they would form. When gallium, scandium and germanium were discovered in the following years, they fitted his predictions closely, which persuaded many chemists that the table reflected something real.

Mendeleev ordered elements mainly by atomic mass, and a few pairs sat awkwardly. Tellurium is heavier than iodine, yet it clearly belongs in the column before it. He trusted chemical behaviour over mass and swapped them. The reason became clear in the early twentieth century, when Henry Moseley used X-rays to show that each element has a characteristic atomic number. Ordering by proton number removed the awkward exceptions and confirmed why Mendeleev's instinct had worked.

Group 1 metals all have one outer electron, yet they do not react equally. Drop a small piece of lithium into water and it fizzes steadily. Sodium darts across the surface and melts into a ball. Potassium reacts so strongly that the hydrogen released catches fire with a lilac flame. Moving down the group, each atom has an extra shell of electrons, so the outer electron sits further from the nucleus and is shielded by more inner electrons. It is held less tightly, lost more easily and the metal reacts more readily.

The halogens in group 17 show the opposite trend, which follows from the same reasoning. Halogens react by gaining an electron. In a small atom such as fluorine, the incoming electron can get close to the nucleus and is strongly attracted, so fluorine is the most reactive of all. Further down, chlorine, bromine and iodine have larger atoms and attract an extra electron less strongly. You can see this in displacement reactions: chlorine water added to a solution containing bromide ions releases brown bromine, but bromine cannot displace chlorine.

The two separate rows printed below the main table are not extra elements bolted on. They belong inside periods 6 and 7, between groups 2 and 3, and are pulled out only to keep the table a convenient shape. Put back in place, they make the table 32 columns wide, too long for most pages and posters. These rows hold the lanthanides and actinides. In them, electrons are added to an inner set of orbitals rather than the outermost shell, so the outer pattern barely changes from one element to the next. That is why the lanthanides are so alike and were notoriously hard to separate.

The bottom of the table has its own history. Elements heavier than uranium are found in nature only in tiny traces or not at all, and they have been made in laboratories, often by smashing lighter nuclei together in particle accelerators. The heaviest survive for tiny fractions of a second, and some have been produced only a few atoms at a time. In 2016 the names of elements 113, 115, 117 and 118, nihonium, moscovium, tennessine and oganesson, were approved, completing the seventh row. Starting an eighth row would require making heavier nuclei still, which has so far proved extremely difficult.

The table can also tell you how elements combine. Metals in group 1 tend to lose their single outer electron and form ions with a charge of plus one, group 2 metals form plus two ions, and aluminium in group 13 forms plus three. On the right, group 17 elements gain one electron to become minus one ions, and group 16 elements gain two to become minus two. In an ionic compound the charges must cancel out. Sodium and chlorine therefore combine one to one as NaCl, and magnesium and oxygen as MgO, while calcium needs two chloride ions, giving CaCl2.

Harder cases work the same way. Aluminium oxide needs two aluminium ions, carrying six positive charges in total, to balance three oxide ions, carrying six negative charges, so its formula is Al2O3. A similar pattern helps with covalent molecules made from non-metals. Carbon in group 14 usually forms four bonds, nitrogen three, oxygen two and hydrogen one, which is why methane is CH4, ammonia NH3 and water H2O. The method has limits. Transition metals can form ions with different charges, so iron makes both Fe2+ and Fe3+, which is why chemists write names such as iron(III) oxide to say which one is present.

The periodic table arranges elements by atomic number while grouping repeating chemical patterns. Read across to follow changing electron structures, and read down to find families with related behaviour. Once you see the pattern, the table becomes a map rather than a memory test.