Learn how 118 elements are organised into a single table, what periods and groups tell you about an element's behaviour, and how to use trends to predict properties you have never seen before.
The periodic table is an arrangement of all known elements in order of increasing atomic number (the number of protons in the nucleus). Dmitri Mendeleev published the first version in 1869, arranging elements so that those with similar properties fell into the same vertical columns. The modern periodic table has 118 confirmed elements and is one of the most powerful tools in all of science.
What makes the periodic table useful is not just that it lists elements — it is that its structure reveals patterns. An element's position in the table tells you about its electron arrangement, its reactivity, its physical state, and how it bonds with other elements.
A period is a horizontal row in the periodic table. There are seven periods. As you move across a period from left to right, the atomic number increases by one each time. All elements in the same period have the same number of electron shells. Period 2 elements (lithium to neon) all have two electron shells; period 3 elements (sodium to argon) all have three.
Properties change significantly as you cross a period. Elements on the left of a period are metals; those on the right are non-metals. The transition happens gradually through the metalloids (semi-metals) in the middle-right area.
A group is a vertical column. There are 18 groups. Elements in the same group have the same number of electrons in their outermost shell — and this is why they have similar chemical properties. The group number (for groups 1–7) tells you directly how many outer electrons the element has.
Group 1 metals are soft, low-density metals that react vigorously with water and oxygen. Reactivity increases going down the group — potassium is more reactive than sodium, which is more reactive than lithium. This is because the outer electron is further from the nucleus in larger atoms and is lost more easily.
Halogens are reactive non-metals that exist as diatomic molecules (F₂, Cl₂, Br₂, I₂). Reactivity decreases going down the group — fluorine is the most reactive, iodine the least reactive halogen. This is because the outer shell is further from the nucleus in larger atoms, so attracting an extra electron becomes harder.
Noble gases have a full outer electron shell, which makes them extremely stable and unreactive. They do not form compounds under normal conditions and exist as single atoms (monatomic). Their uses exploit their unreactivity: argon fills light bulbs, helium fills balloons, neon is used in signs.
Moving left to right across a period, elements change from metals to metalloids to non-metals. Moving down a group, metallic character generally increases. The most metallic elements are in the bottom-left corner (e.g., caesium, francium); the most non-metallic are in the top-right corner (e.g., fluorine, oxygen).
For metals, reactivity increases down a group because the outer electron is further from the nucleus and experiences more shielding from inner shells. It is lost more easily, making the metal more reactive.
For non-metals, reactivity increases going up a group because the outer shell is closer to the nucleus, so gaining an electron is easier. Fluorine (top of Group 7) is the most reactive non-metal on Earth.
Atomic radius increases down a group (more electron shells added) and decreases across a period from left to right (more protons pull the same-shell electrons closer in).
Each element in the periodic table has an information box containing:
From the atomic number alone you can work out: electron arrangement, number of neutrons (mass number − atomic number), and — for groups 1–7 — how many bonds the element typically forms.
Question: Element X is in Period 3, Group 2. State the number of electron shells, the number of outer electrons, and whether X is a metal or non-metal.
Period 3 → 3 electron shells.
Group 2 → 2 outer electrons.
Group 2 is on the left side of the table → X is a metal.
Element X is magnesium (Mg).
Question: Which is more reactive — sodium or potassium? Give a reason based on electronic structure.
Answer: Potassium is more reactive. It is below sodium in Group 1, so its outer electron is in a higher energy shell, further from the nucleus and more shielded by inner electrons. It is therefore lost more easily, making potassium more reactive.
Question: Chlorine gas is bubbled into potassium bromide solution. Describe and explain what happens.
Observation: The solution turns orange-brown as bromine is produced.
Explanation: Chlorine is above bromine in Group 7 and is therefore more reactive. It displaces bromide ions from the solution.
Equation: Cl₂ + 2KBr → 2KCl + Br₂
Question: An element has atomic number 17. State its period, group, and electron arrangement.
Electron arrangement: 2, 8, 7 (fills shells in order: 2 in shell 1, 8 in shell 2, 7 in shell 3).
Period: 3 (three shells filled).
Group: 7 (7 outer electrons).
Element: Chlorine.
The periodic table rewards students who understand its logic rather than memorise facts in isolation. Once you see how electron arrangement drives every trend, predictions come naturally — and exam questions become much more manageable.