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GCSE Chemistry Revision

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GCSE Chemistry revision

Electronic structure

A simple model of the atom, symbols, relative atomic mass, electronic charge and isotopes

AQA 4.1.1.7 Foundation & Higher
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Revision summary

Key knowledge

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Introduction

  • Every shell is a different energy level.
  • The atomic number of an element tells us both the number of protons and the number of electrons in a neutral atom.
  • Shells are drawn as rings around the nucleus, and electrons are represented as dots or crosses on these rings.

Rules for Filling Electron Shells

  • The first shell, closest to the nucleus and with the lowest energy level, fills first and can hold a maximum of 2 electrons.
  • The second and third shells can each hold a maximum of 8 electrons.
  • When filling a shell, place one electron in the top, right, bottom, and left positions before adding a second electron to each position (filling clockwise).
  • Electrons always fill the innermost available shell before moving to the next one.

Writing Electron Configurations

  • The electron arrangement of an atom can be written as a series of numbers separated by commas, each number representing the electrons in successive shells.
  • For example, sodium (atomic number 11) has the electron configuration 2, 8, 1, meaning 2 electrons in the first shell, 8 in the second, and 1 in the third.
  • You need to be able to work out the electron arrangement of any of the first 20 elements in the periodic table for your GCSE exam.

Stability and the Full Outer Shell

  • An atom is considered stable when its outermost shell (the shell furthest from the nucleus) is completely full.
  • Most single atoms have incomplete outer shells and are therefore unstable, meaning they will react with other atoms to achieve a full outer shell.
  • Atoms react to form molecules or compounds in order to achieve a stable, full outer shell of electrons.

Noble Gases: The Stable Exception

  • The noble gases, found in Group 0 of the periodic table, are the exception because they already have completely full outer shells.
  • Because noble gases have full outer shells, they have no tendency to gain or lose electrons and so they do not readily react with other elements.
  • Neon is an example of a noble gas with a full outer shell, giving it its characteristic chemical inertness.

Worked Example: Argon

  • Argon has an atomic number of 18, so it has 18 electrons arranged as 2, 8, 8 across three shells.
  • Since 2 + 8 + 8 = 18, argon's third shell is completely full, making it a stable noble gas.
  • This electron arrangement can be written numerically as 2, 8, 8 rather than drawing out all the shells.

Worked Example: Calcium

  • Calcium has an atomic number of 20, giving it the electron arrangement 2, 8, 8, 2 across four shells.
  • Calcium is unstable because its outer (fourth) shell contains only 2 electrons instead of a full 8.
  • If calcium loses its 2 outer electrons, the third shell becomes the outermost shell and is completely full, making the atom stable.
  • After losing 2 electrons, calcium still has 20 protons but only 18 electrons, giving it an overall charge of 2+, making it a calcium ion written as Ca²⁺.

Ions and Ionic Charge

  • An ion is formed when an atom gains or loses electrons, resulting in an overall positive or negative charge.
  • Ions are represented with square brackets around the electron diagram and the overall charge written in the top right corner.
  • A positive ion (cation) is formed when an atom loses electrons, whilst a negative ion (anion) is formed when an atom gains electrons.

Worked Example: Fluorine

  • Fluorine has an atomic number of 9, giving it the electron arrangement 2, 7, meaning its outer shell has 7 out of a possible 8 electrons.
  • To become stable, fluorine needs to gain 1 electron from another atom, giving it a full outer shell of 8 electrons.
  • After gaining 1 electron, fluorine has 9 protons but 10 electrons, giving it an overall charge of 1−, making it the fluoride ion F with the electron arrangement 2, 8.