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2.2 Electronic Configuration and the Periodic Table

GCSE Chemistry · AQA · Chapter 2: The Periodic Table

A free GCSE Chemistry lesson for AQA (8462) from OBA School: how electronic structure links to groups and periods, metals and non-metals, and the noble gases of Group 0. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Write the electronic structure of the first 20 elements and link it to group and period
  • Explain why elements in the same group have similar chemical properties
  • Identify metals and non-metals from their position and properties
  • Predict the charge on the ion an element forms from its group
  • Explain why the noble gases are unreactive and describe the trend in their boiling points

Key words

  • Electronic structure: How the electrons in an atom are arranged in shells (energy levels), written like 2,8,1.
  • Outer shell: The highest occupied energy level. Its electrons decide how the element reacts.
  • Group: A column. Elements in groups 1–7 have the same number of outer electrons as the group number.
  • Period: A row. The period number equals the number of occupied shells.
  • Noble gases: The Group 0 elements: helium, neon, argon, krypton, xenon and radon.
  • Monatomic: Made of single atoms, not molecules. The noble gases are monatomic.

Electronic structure and position

Electrons fill shells from the inside out. For the first 20 elements: the first shell holds up to 2 electrons, the second up to 8 and the third up to 8 before the fourth shell starts.

Worked example: finding an element’s place

An atom has 16 electrons. Fill the shells: 2, then 8, then the remaining 6 → 2,8,6.
6 outer electrons → Group 6. Three occupied shells → Period 3. Atomic number 16 → sulfur.

Answer: sulfur, Group 6, Period 3

Key fact: Elements in the same group have the same number of electrons in their outer shell, so they have similar chemical properties. Group 0 is the exception to the “group number = outer electrons” rule: their outer shells are full.

Metals and non-metals

Most elements are metals. They are found to the left of and below the stepped line. Non-metals are found at the top right.

  • Ions formed; Metals lose electrons to form positive ions; Non-metals do not form positive ions (many gain electrons to form negative ions)
  • Conduct electricity?; Metals yes, good conductors; Non-metals no (poor conductors, except graphite)
  • Melting point; Metals usually high; Non-metals usually low (many are gases)
  • Look and feel; Metals shiny; malleable and ductile; Non-metals dull; brittle when solid
  • Oxides; Metals basic (react with acids); Non-metals acidic (e.g. CO₂, SO₂)

Exam tip: Whether an element forms positive ions is the chemical test of “metal or non-metal”. Atoms with 1, 2 or 3 outer electrons lose them easily to form positive ions.

Ions and the periodic table

When metals react, their atoms lose their outer electrons. When non-metals react with metals, their atoms gain electrons. Either way, the ion ends up with the same stable electronic structure as a noble gas. So the group number tells you the charge on the ion.

  • potassium (K); Electrons 2,8,8,1; Change loses 1; Ion K⁺ [2,8,8]; Same structure as argon (end of the period above)
  • magnesium (Mg); Electrons 2,8,2; Change loses 2; Ion Mg²⁺ [2,8]; Same structure as neon (end of the period above)
  • sulfur (S); Electrons 2,8,6; Change gains 2; Ion S²− [2,8,8]; Same structure as argon (end of its own period)
  • fluorine (F); Electrons 2,7; Change gains 1; Ion F− [2,8]; Same structure as neon (end of its own period)

Worked example: predicting an ion

Nitrogen has atomic number 7. Predict the ion it forms with a metal.
Electronic structure 2,5: five outer electrons, so Group 5. Gaining 3 electrons is far easier than losing 5.
It gains 3 electrons → 2,8 (like neon), with 3 more electrons than protons.

Answer: the nitride ion, N³−

Group 0: the noble gases

The noble gases are unreactive because their atoms have a stable arrangement of electrons: a full outer shell. Helium has 2 electrons (its only shell is full); the others have 8 outer electrons. They have no tendency to lose, gain or share electrons, so they exist as single atoms (they are monatomic).

  • all are colourless gases at room temperature
  • non-flammable and very unreactive (only a few compounds of the heaviest ones, such as xenon with fluorine, have ever been made)
  • boiling points increase going down the group, as relative atomic mass increases

Worked example: using the trend

Krypton boils at −153 °C and radon at −62 °C. Xenon lies between them in the group.
So xenon’s boiling point should be between −153 °C and −62 °C (it is actually −108 °C).
At −120 °C: krypton is above its boiling point, so it is a gas; xenon is below its boiling point, so it is not a gas.

Answer: xenon boils between −153 °C and −62 °C; at −120 °C krypton is a gas and xenon is not

Using the noble gases

  • Helium; Use party and weather balloons, airships; Property that makes it suitable much less dense than air and does not burn
  • Neon; Use advertising signs; Property that makes it suitable glows brightly when electricity passes through it
  • Argon; Use filling filament lamps; shielding gas in welding; Property that makes it suitable unreactive, so hot metal does not react with oxygen

Common mistakes

  • Writing 2,8,8,1 as 2,9 or putting more than 2 electrons in the first shell. Fill 2, then 8, then 8.
  • Saying the noble gases are unreactive because they have “8 electrons”. Helium has 2; the point is a full outer shell.
  • Saying Group 0 has 0 outer electrons. Its outer shell is full.
  • Mixing up groups (columns) and periods (rows).
  • Giving a Group 6 element a 6+ charge. Non-metals in Groups 5–7 gain 3, 2 or 1 electrons: 3−, 2−, 1−.
  • Saying boiling points of the noble gases decrease down the group. They increase.

Revision questions

Try these, then check your answers below each one.

Question 1. Magnesium has atomic number 12. Write its electronic structure (for example 2,8,1).

Answer: 2,8,2

Working: 12 electrons: 2 in the first shell, 8 in the second and 2 in the third: 2,8,2.

Question 2. Why are the noble gases unreactive? (Choose from: Their atoms have a full outer shell of electrons, Their atoms have no electrons in their outer shell, They are gases at room temperature, Their atoms have eight protons.)

Answer: Their atoms have a full outer shell of electrons

Working: A full outer shell is a stable arrangement, so the atoms do not easily lose, gain or share electrons. (Helium has a full shell of 2.)

Question 3. Bromine is in Group 7. How many electrons are in the outer shell of a bromine atom?

Answer: 7

Working: Group 7 elements have 7 outer electrons.

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