The Periodic Table Explained Simply

Groups and Periods

The vertical columns are called groups. The group number tells you how many electrons are in the outer shell. Group 1 elements have 1 outer electron, group 7 elements have 7. This is the single most important idea — the number of outer-shell electrons determines how an element reacts, which is why elements in the same group behave in similar ways.

The horizontal rows are called periods. Moving across a period from left to right, each element has one more proton and one more electron than the one before. The period number tells you how many electron shells the atom has. Period 1 elements have one shell, period 3 elements have three.

Group 1 — The Alkali Metals

Group 1 contains lithium, sodium, potassium, rubidium, caesium and francium. They are soft metals that can be cut with a knife, have low densities and low melting points compared to most metals. Each has one electron in its outer shell, which it loses easily to form a +1 ion. They react vigorously with water, producing a metal hydroxide and hydrogen gas. Reactivity increases as you go down the group because the outer electron is further from the nucleus and is lost more easily. Potassium reacts more violently with water than sodium, and sodium more than lithium.

Group 7 — The Halogens

Group 7 includes fluorine, chlorine, bromine, iodine and astatine. They are non-metals that exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). Each has seven outer-shell electrons and gains one electron to form a -1 ion. Reactivity decreases down the group — the opposite trend to group 1. This is because the outer shell is further from the nucleus, making it harder to attract an extra electron. A more reactive halogen can displace a less reactive one from a solution of its salt (displacement reaction). Chlorine displaces bromine from potassium bromide solution, for example.

Group 0 — The Noble Gases

Group 0 contains helium, neon, argon, krypton, xenon and radon. They are colourless, odourless gases that are extremely unreactive. This is because they have full outer electron shells — helium has 2 outer electrons, the rest have 8. With a stable electron arrangement, they have no tendency to gain, lose or share electrons. Their boiling points increase going down the group because the atoms become larger and the intermolecular forces (London dispersion forces) become stronger.

Why Is It Arranged That Way?

Mendeleev originally arranged elements by atomic mass, but left gaps where he predicted undiscovered elements would fit. When those elements were later found and matched his predictions, the table was accepted. Today we arrange elements by atomic number (number of protons) rather than mass. The beauty of the periodic table is that it groups elements with similar properties together, making it possible to predict how an unfamiliar element will behave based on its position. Metals sit on the left and centre, non-metals on the right. The dividing line runs roughly like a staircase from boron to astatine.