The Periodic Table: Chemistry's Master Map

In 1869, a Russian chemist named Dmitri Mendeleev arranged the known elements into a table — and left deliberate gaps. He predicted that unknown elements would one day be discovered to fill those gaps. He was right. Within 15 years, three of his predicted elements — gallium, scandium, and germanium — were found and matched his descriptions almost exactly. Think about that: a table on paper predicted the existence of substances not yet discovered. How could a chart of data be that powerful? By the end of this lesson, you will understand exactly why.

**What is an element?** Everything around you is made of matter. Matter is built from atoms, and an element is a pure substance made of only one type of atom. Gold is an element — every atom in a gold ring is a gold atom. Water is not an element because it is made of hydrogen and oxygen atoms bonded together. There are 118 known elements. Each one has a unique atomic number — the number of protons (positively charged particles) inside the nucleus, or centre, of one of its atoms. Hydrogen has 1 proton, so its atomic number is 1. Carbon has 6 protons; its atomic number is 6. No two elements share the same atomic number. **How is the table arranged?** The periodic table lists all 118 elements in order of increasing atomic number, reading left to right and top to bottom — just like lines of text. This creates rows and columns with a very important pattern. • A **period** is a horizontal row. Elements in the same period have the same number of electron shells (the layers around the nucleus where electrons are found). Period 1 has just 2 elements (hydrogen and helium). Period 2 has 8 elements, including carbon, nitrogen, and oxygen. • A **group** is a vertical column. Elements in the same group have the same number of electrons in their outermost shell. This is crucial: the number of outer electrons largely determines how an element behaves chemically. Elements in the same group therefore share similar chemical properties. **Key groups to know** Group 1 — the **alkali metals** (e.g. lithium, sodium, potassium): soft, shiny metals that react vigorously with water, producing hydrogen gas. Sodium is the element in table salt. Group 17 — the **halogens** (e.g. fluorine, chlorine, iodine): non-metals that are highly reactive and typically form salts when they react with metals. The fluoride added to toothpaste is a halogen compound. Group 18 — the **noble gases** (e.g. helium, neon, argon): colourless gases that are almost entirely unreactive because their outer electron shell is completely full. Helium fills balloons; neon glows in illuminated signs. **Metals, non-metals, and metalloids** About 75 % of elements are metals, found on the left and centre of the table. Non-metals sit on the upper right. A staircase-shaped line separates them. Elements touching that line — such as silicon and germanium — are called metalloids (or semi-metals); they show properties of both metals and non-metals. Silicon is the foundation of every computer chip you have ever used.