Electronic structure

The electronic structure of an atom is the way its electrons are arranged in shells (energy levels) around the nucleus. Electrons fill the lowest energy level first. For the first 20 elements the shells hold 2, 8 and 8 electrons, in that order, before a fourth shell starts. Sodium, with 11 electrons, has the electronic structure 2,8,1.

How electrons fill the shells

AQA's GCSE specification states that the electrons in an atom occupy the lowest available energy levels, meaning the innermost available shells. So you fill from the inside out:

1. Find the number of electrons. In a neutral atom it equals the atomic number (the number of protons).
2. Put up to 2 electrons in the first shell.
3. Put up to 8 in the second shell.
4. Put up to 8 in the third shell, then start the fourth.
5. Write the numbers in order from the inside, separated by commas.

For phosphorus, atomic number 15: 2 in the first shell leaves 13, 8 in the second leaves 5, so the electronic structure is 2,8,5. The numbers always add up to the total number of electrons, which is a quick check.

You can show the same thing as a diagram, with each shell drawn as a circle and each electron as a dot or cross. AQA expects you to be able to do both for the first 20 elements.

Electronic structure of the first 20 elements

Notice the pattern: each period starts a new shell, and each element has one more electron than the one before. At potassium the fourth shell starts even though the third holds only 8. The third shell can hold more, but at GCSE the rule for the first 20 elements is simply 2, 8, 8, then the fourth shell. The reason is explained at A-level with subshells, in the electron configuration guide.

Try one: fill the shells for an element, then press Mark to check it in the Atomic Structure course.

Electronic structure and the periodic table

The periodic table is in order of atomic number, and the electronic structure explains its shape:

• The number of occupied shells is the period number. Sodium (2,8,1) and chlorine (2,8,7) both have three shells, so they are in period 3.
• The number of outer electrons is the group number for groups 1 to 7. Sodium has 1 outer electron (group 1) and chlorine has 7 (group 7).
• Group 0 atoms have full outer shells. Helium has 2 and the others have 8, which AQA describes as stable arrangements of electrons. That is why the noble gases are so unreactive.

Every element in a group shares the same count of outer-shell electrons, so they react in similar ways. Look down group 1 in the table above: each of its elements ends with a single outer electron, which is why they all react vigorously with water. More on groups in the periodic table guide.

Electronic structure of ions

When atoms form ions, they lose or gain electrons until they have a full outer shell, the same electronic structure as a noble gas. Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.

For an ion's electronic structure, adjust the number of electrons first: subtract the charge for a positive ion and add it for a negative ion. Then fill the shells as normal. In a diagram, put the ion in square brackets with its charge outside.

Worked examples

Example 1: from the atomic number

Silicon has atomic number 14. Write its electronic structure and find its group and period. 14 electrons: 2, then 8, leaves 4. Electronic structure 2,8,4; three shells means period 3, and 4 outer electrons means group 4.

Example 2: from the electronic structure

An element has the electronic structure 2,8,6. Which element is it? Total electrons = 2 + 8 + 6 = 16, so the atomic number is 16: sulfur.

Example 3: an ion

Write the electronic structure of a sulfide ion, S²⁻. Sulfur has 16 electrons and the 2− charge means it has gained 2, so 18 electrons: 2,8,8, the same as argon.

Common mistakes

• Putting 8 in the first shell. The first shell holds only 2.
• Using the mass number. The number of electrons in a neutral atom equals the atomic number, not the mass number.
• Going 2,8,9 for potassium. For the first 20 elements, the fourth shell starts after 2,8,8: potassium is 2,8,8,1.
• Changing the protons in an ion. Only the electrons change when an ion forms; the atomic number stays the same.
• Adding the charge the wrong way. A positive ion has lost electrons, so it has fewer than the atom.

Find the electronic structure of any element in the electron configuration tool, or practise in today's GCSE Chemistry Higher or GCSE Combined Science Higher short timed paper.

Read next

Chemistry · Atoms and the periodic tableThe periodic table: how many elements, groups and periodsHow many elements are on the periodic table, how it is arranged into groups and periods, what it tells you about atoms, and who created it.