The periodic table explained simply, groups, periods and trends

This guide is for you if the periodic table looks like a wall of letters. It is taught in ICSE Classes 9 and 10, in GCSE and IGCSE chemistry, and in CBSE Class 11 (Classification of Elements and Periodicity in Properties), with CBSE Class 9 Structure of the Atom laying the groundwork through electron arrangements. Once you see how it is organised, the table stops being something to memorise and becomes something you can read.

The idea in plain words

Every element is made of atoms, and every atom has a nucleus of protons and neutrons, with electrons arranged in shells around it.

The atomic number is the number of protons. It is what makes an element that element. Carbon always has 6 protons, oxygen always has 8.

The modern periodic table lists the elements in order of atomic number, in rows and columns:

• Periods are the horizontal rows. There are 7.
• Groups are the vertical columns. There are 18 (numbered 1 to 18). Some GCSE and IGCSE courses number the main groups 1 to 7 and call the last column Group 0.

The clever part: elements in the same group have the same number of electrons in their outer shell, so they react in similar ways.

Reading the table

What you look at	What it tells you
Period number	Number of electron shells in use
Group 1 and 2	Number of outer electrons is 1 or 2
Groups 13 to 18	Number of outer electrons is the group number minus 10 (helium is the exception, with 2)
Left side and centre	Metals
Right side	Non-metals
Along the zig-zag line	Metalloids, such as boron, silicon and germanium

Some groups have names:

Group	Name	Notes
1	Alkali metals	Soft, very reactive, react with water to make alkalis
2	Alkaline earth metals	Reactive, but less than Group 1
3 to 12	Transition metals	Hard, often form coloured compounds
17	Halogens	Reactive non-metals, exist as pairs of atoms (Cl₂, Br₂)
18	Noble gases	Full outer shells, very unreactive

Electron arrangements for the first 20 elements

Electrons fill shells in order. For school level, the first shell holds up to 2, and for the first 20 elements the next shells hold up to 8 each.

So:

• Sodium (atomic number 11): 2, 8, 1
• Calcium (atomic number 20): 2, 8, 8, 2

Worked examples

Example 1: placing sodium

Sodium has atomic number 11. Find its position and predict how it reacts.

1. Electron arrangement: 2 + 8 = 10, then 1 left over, so 2, 8, 1
2. Three shells in use, so Period 3
3. One outer electron, so Group 1
4. Group 1 elements lose their one outer electron easily, forming ions with a charge of +1 (Na⁺). Sodium is a very reactive metal.

Example 2: placing chlorine

Chlorine has atomic number 17.

1. Electron arrangement: 2, 8, 7
2. Three shells, so Period 3
3. Seven outer electrons, so Group 17 (Group 7 in GCSE numbering)
4. It needs one more electron for a full shell, so it tends to gain one, forming Cl⁻. Chlorine is a reactive non-metal.

Example 3: working backwards

An element has the electron arrangement 2, 8, 6. Identify it.

1. Total electrons: 2 + 8 + 6 = 16, so the atomic number is 16
2. Element 16 is sulfur
3. Three shells, so Period 3. Six outer electrons, so Group 16 (Group 6 in GCSE numbering)

Example 4: protons, neutrons and electrons

Sodium-23 has atomic number 11 and mass number 23. How many of each particle does an atom have?

1. Protons = atomic number = 11
2. Electrons = protons in a neutral atom = 11
3. Neutrons = mass number - atomic number = 23 - 11 = 12

Trends you need to know

Trend	Across a period (left to right)	Down a group
Atomic size (radius)	Decreases	Increases
Metallic character	Decreases	Increases
Reactivity of Group 1 metals		Increases (Li, Na, K: potassium is the most reactive of the three)
Reactivity of Group 17 halogens		Decreases (fluorine is the most reactive)

Why atoms shrink across a period: more protons pull the electrons closer, while the number of shells stays the same.

Why Group 1 gets more reactive down the group: the outer electron is further from the nucleus and more shielded by inner shells, so it is lost more easily.

Why halogens get less reactive down the group: the outer shell is further from the nucleus, so the attraction for an extra electron is weaker.

Common mistakes and how to avoid them

Saying the table is ordered by atomic mass. The modern table uses atomic number.

Mixing up groups and periods. Groups go down (like columns in a building), periods go across. One way to remember: a full stop (period) ends a line of text, which runs across.

Mixing up outer electrons and valency. Oxygen has 6 outer electrons but a valency of 2, because it needs 2 more to fill its shell.

Getting the reactivity trends the wrong way round. Metals get more reactive down a group, halogens get less reactive. They react in opposite ways (losing versus gaining electrons), so the trends run in opposite directions.

Calling hydrogen an alkali metal. It sits near Group 1 but is a non-metal.

Putting more than 2 electrons in the first shell. The first shell only holds 2.

Try these

1. Magnesium has atomic number 12. Write its electron arrangement and give its group and period.
2. Which is more reactive, sodium or potassium? Explain briefly.
3. An element has the arrangement 2, 8, 8. Name it and say how reactive it is.
4. How many neutrons are there in an atom of chlorine-35 (atomic number 17)?
5. Which atom is larger, sodium or chlorine?

Answers

1. 2, 8, 2. Group 2, Period 3.
2. Potassium. Its outer electron is further from the nucleus, so it is lost more easily.
3. Argon (atomic number 18). It has a full outer shell, so it is very unreactive.
4. 35 - 17 = 18 neutrons.
5. Sodium. Both are in Period 3, and atomic size decreases from left to right.

Still unsure?

If the periodic table still feels like a list to memorise, a Qlive teacher can work through it with you one-to-one, starting with a free assessment that finds the exact gap, often in atomic structure. You can read more about how we teach on our chemistry page, and put it to use with our guide to balancing chemical equations.