C2.2 Electronic Structures and The Periodic Table

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how electron shells link to groups and periods, metals and non-metals, ions, 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 use it to find the group and period
• Explain why elements in the same group have similar chemical properties
• Identify metals and non-metals from their position in the table and their 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 2,8,3.
• Outer shell: The highest occupied energy level. Its electrons decide how the element reacts.
• Group: A column. For Groups 1–7 the group number equals the number of outer-shell electrons.
• 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 in the table

Electrons fill the 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; then the fourth shell starts. The elements are in order of atomic number, so each element has one more electron than the one before it. At the end of every period the outer shell is full, and the next period starts a new shell.

Worked example: finding an element’s place

An atom has 15 electrons. Fill the shells: 2, then 8, then the remaining 5 → 2,8,5.
5 outer electrons → Group 5. Three occupied shells → Period 3. Atomic number 15 → phosphorus.

Answer: phosphorus, Group 5, 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. That is why the properties repeat regularly – the table is periodic. Group 0 atoms have full outer shells (8 electrons, or 2 for helium).

Metals and non-metals

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

• 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 (graphite is an exception)
• Melting point; Metals usually high; Non-metals usually low (many are gases)
• Look and feel; Metals shiny; malleable (can be hammered into shape); Non-metals dull; brittle when solid
• Oxides; Metals basic (react with acids); Non-metals acidic (e.g. CO₂, SO₂)

Exam tip: Forming positive ions is the chemical test of a metal. Atoms with 1, 2 or 3 outer electrons lose them when they react.

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.

• lithium (Li); Electrons 2,1; Change loses 1; Ion Li⁺ [2]; Same structure as helium
• calcium (Ca); Electrons 2,8,8,2; Change loses 2; Ion Ca²⁺ [2,8,8]; Same structure as argon (end of the period above)
• oxygen (O); Electrons 2,6; Change gains 2; Ion O²− [2,8]; Same structure as neon (end of its own period)
• chlorine (Cl); Electrons 2,8,7; Change gains 1; Ion Cl− [2,8,8]; Same structure as argon (end of its own period)

Worked example: predicting an ion

Aluminium has atomic number 13. Predict the ion it forms.
Electronic structure 2,8,3: three outer electrons, so Group 3. Losing 3 electrons is far easier than gaining 5.
It loses 3 electrons → 2,8 (like neon), with 13 protons but only 10 electrons.

Answer: the aluminium ion, Al³⁺

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 and are non-metals
• they do not burn and are very unreactive
• boiling points increase going down the group, as relative atomic mass increases

Worked example: using the trend

Argon boils at −186 °C and xenon at −108 °C. Krypton lies between them in the group.
So krypton’s boiling point should be between −186 °C and −108 °C (it is actually −153 °C).
At −170 °C: argon is above its boiling point, so it is a gas; xenon is below its boiling point, so it is not a gas.

Answer: krypton boils between −186 °C and −108 °C; at −170 °C argon is a gas and xenon is not

• Helium; Use balloons and 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 it

Common mistakes

• Putting more than 2 electrons in the first shell, or writing 2,8,8,1 as 2,9. 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. What is the electronic structure of potassium (atomic number 19)? (Choose from: 2,8,8,1, 2,8,9, 2,9,8, 2,8,7,2.)

Answer: 2,8,8,1

Working: 19 = 2 + 8 + 8 + 1, so 2,8,8,1. Potassium has one outer electron, so it is in Group 1, Period 4.

Question 2. An element is in Group 3 and Period 3. What is its atomic number?

Answer: 13

Working: 2,8,3 → 2 + 8 + 3 = 13 electrons, so atomic number 13 (aluminium).

Question 3. True or false? The boiling points of the noble gases decrease going down Group 0.

Answer: False

Working: False. Boiling points increase down the group as the relative atomic mass increases (helium −269 °C up to radon −62 °C).

Related lessons

• Previous lesson: C2.1 Development of The Periodic Table
• Next lesson: C2.3 Group 1 – The Alkali Metals
• In this chapter: C2.4 Group 7 – The Halogens
• In this chapter: C2.5 Explaining Trends

Continue in the full course

This lesson is section C2.2 (AGCSC1902) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.