The periodic table and electronic structure – GCSE Combined Science Chemistry

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The periodic table is a chart of all the known elements. An element is a substance made of only one kind of atom. The chart looks busy, but it follows clear rules.

It matters because the table works like a map. Once you know where an element sits, you can predict how it behaves and what ions it forms. Chemists use it every day.

By the end you will be able to describe how the table is laid out and compare metals with non-metals. You will explain how Mendeleev built his table. You will also write the electronic structure of the first 20 elements.

Order and layout

Every element has an atomic number. This is the number of protons in its atoms. A proton is a tiny positive particle in the nucleus.

The table lists the elements in order of atomic number. Each row is called a period. Each column is called a group. Think of a school seating plan, with rows and columns of desks.

Elements in the same group have similar chemical properties. The chart shows the number of outer shell electrons across period 2. It rises from 1 to 8. You will see why shortly.

Mendeleev’s table

Before protons were known, scientists ordered the elements by atomic mass. Atomic mass tells you how heavy an atom is. In 1869 Mendeleev did this too, but he made one clever change.

He put elements with similar properties in the same column, even when this broke the order of mass. For example, he swapped tellurium and iodine so that each sat in the right group.

He also left gaps for elements nobody had found yet, and he predicted their properties. Think of a jigsaw with missing pieces. You can still guess what the picture should show. When new elements were found with matching properties, his table was accepted.

Metals and non-metals

Most elements are metals. They sit to the left of the stepped line on the table. Metals are shiny and they conduct heat and electricity. They are malleable, which means they can be hammered into shape. They are also ductile, which means they can be pulled into wires.

Non-metals sit to the right of the line. Solid non-metals are dull and brittle, which means they break when hit. They are usually poor conductors. Graphite is an exception because it does conduct electricity.

In reactions, metals lose electrons to form positive ions. Non-metals gain electrons to form negative ions, or they share electrons. An ion is an atom with an electric charge.

Electron shells

An atom has a nucleus in the middle. Electrons move around it in layers called shells. Think of a block of flats. Electrons fill the lowest floor first.

Each shell can only hold so many electrons. The first shell holds up to 2. The second shell holds up to 8. The third shell also holds up to 8, for the first 20 elements.

Electrons always fill the lowest shell first. A shell must be full before the next one starts to fill. The last shell with electrons in it is called the outer shell. Potassium, number 19, is the first atom to start a fourth shell.

Writing an electronic structure

An electronic structure shows how many electrons are in each shell. Start with the atomic number. In a neutral atom, this is also the number of electrons. Then fill the shells in order: 2, then 8, then 8.

Take sodium, atomic number 11. Two go in the first shell, leaving 9. Eight go in the second shell, leaving 1. That last one goes in the third shell. So sodium is 2,8,1.

Chlorine has 17 electrons. Two plus 8 leaves 7 for the third shell, so chlorine is 2,8,7. Always check by adding the numbers. They must add up to the atomic number.

Group, period and ions

The last number in an electronic structure is the outer shell. It gives the group number. The number of shells gives the period number.

Sodium is 2,8,1. It has 1 outer electron, so it is in Group 1. It has 3 shells, so it is in Period 3. Helium has 2 outer electrons, but it is in Group 0 because its only shell is full.

Atoms tend to end up with a full outer shell. Group 1 metals lose 1 electron to form 1+ ions. Group 2 metals form 2+ ions. Group 6 non-metals gain 2 to form 2− ions. Group 7 gain 1 to form 1− ions. Group 0 atoms are already full, so they rarely form ions.

Recap

The table is in order of atomic number. Rows are periods and columns are groups.

Mendeleev ordered by atomic mass, swapped some elements, and left gaps. His predictions came true.

Metals lose electrons to form positive ions. Non-metals gain or share electrons.

Shells fill 2, 8, 8. The group number is the outer electrons. The period number is the shells.