The Periodic Table
Edexcel International GCSE ChemistryΒ· 1.18β1.24 (sub-topic 1(d))Β· 25 min read
1. Structure of the Modern Periodic Tableβ β ββββ± 5 min
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The modern Periodic Table orders all known elements by increasing atomic number (proton number). Vertical columns are called groups, and horizontal rows are called periods. For main group elements (Groups 1 to 7 and 0), the group number equals the number of electrons in the outer shell, and the period number equals the number of occupied electron shells.
Periodic Table Key Rules
- Elements ordered by increasing atomic number. 2. Group number (main groups) = number of outer shell electrons. 3. Period number = number of occupied electron shells.
State the group and period number of oxygen (atomic number 8), and give its electronic configuration.
- 1
Oxygen has 8 electrons, so filling shells following the 2,8 rule gives a configuration of 2,6.
- 2
The outer shell has 6 electrons, so oxygen is in Group 6.
- 3
There are 2 occupied electron shells, so oxygen is in Period 2.
2. Deducing Electronic Configurations for First 20 Elementsβ β β βββ± 6 min
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For the first 20 elements, electron shells fill following a fixed 2,8,8 rule: the first shell holds a maximum of 2 electrons, the second holds 8, the third holds 8, and the fourth holds the remaining electrons for elements 19 and 20. You can deduce configurations directly from group and period position without counting all electrons.
An element is in Group 1, Period 4. Deduce its electronic configuration and identify the element.
- 1
Period 4 means the element has 4 occupied electron shells.
- 2
Group 1 means the element has 1 electron in its outer (fourth) shell.
- 3
Fill inner shells following the 2,8,8 rule: first shell = 2, second = 8, third = 8, fourth = 1. Full configuration is 2,8,8,1.
- 4
Total electrons = 2+8+8+1 = 19, so atomic number 19 corresponds to potassium.
What is the electronic configuration of calcium (Group 2, Period 4)?
2,8,2
2,8,8,2
2,8,10
2,8,18,2
Reveal answer
2,8,8,2 βCalcium is in Period 4 so it has 4 shells, and Group 2 so it has 2 outer electrons. For first 20 elements, the third shell fills with 8 electrons before the fourth shell starts filling, so the correct configuration is 2,8,8,2.
3. Classifying Metals and Non-Metalsβ β ββββ± 5 min
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You can classify elements as metals or non-metals using two sets of evidence: 1) Physical and chemical properties: metals conduct electricity and form basic oxides; non-metals generally do not conduct electricity (except graphite) and form acidic oxides. 2) Periodic Table position: metals are found to the left and below the stepped dividing line, non-metals to the right and above the line. Hydrogen is a non-metal exception placed at the top of Group 1.
An element forms an acidic oxide and does not conduct electricity in its solid state. State if it is a metal or non-metal, and describe its position in the Periodic Table.
- 1
Acidic oxides are a characteristic property of non-metals, and most non-metals do not conduct electricity in their solid state.
- 2
Non-metals are located to the right of the stepped dividing line between metals and non-metals in the Periodic Table.
4. Group Chemical Similarityβ β β βββ± 4 min
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Elements in the same group have the same number of outer shell electrons. Since all chemical reactions involve the transfer or sharing of outer shell electrons, elements in the same group show very similar chemical reactivity patterns. For example, all Group 1 elements react vigorously with water to form an alkaline solution and hydrogen gas.
Group Similarity
Identical outer shell electron count for elements in the same group leads to identical chemical reactivity patterns, allowing you to predict the behaviour of unknown group members from known examples.
Lithium (Group 1) reacts with chlorine to form lithium chloride, with the formula LiCl. Predict the formula of the compound formed when rubidium (also Group 1) reacts with chlorine, and explain your answer.
- 1
Rubidium is in Group 1, so it has 1 outer electron, identical to lithium.
- 2
It will lose this 1 outer electron to form a 1+ ion, just like lithium.
- 3
Chlorine gains 1 electron to form a 1- ion, so the ratio of rubidium to chlorine ions is 1:1, giving the formula RbCl.
5. Reactivity of Group 0 Noble Gasesβ β ββββ± 3 min
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Group 0 elements, called noble gases, have full outer electron shells: helium has a full first shell with 2 electrons, and all other Group 0 elements have a full outer shell with 8 electrons. Full outer shells are extremely stable electronic configurations, so noble gases do not easily gain, lose or share electrons, making them almost completely unreactive (inert) under standard conditions.
Explain why neon (atomic number 10) does not form compounds with other elements under standard conditions.
- 1
Neon has atomic number 10, so its electronic configuration is 2,8.
- 2
Its outer shell is full (the second shell holds a maximum of 8 electrons), giving it a very stable electronic structure.
- 3
It does not need to gain, lose or share electrons to achieve a stable outer shell, so it is unreactive.
6. Common Pitfalls
Wrong move:
Ordering elements by atomic mass instead of atomic number
Why:
Early periodic tables used atomic mass, but the modern Periodic Table uses atomic number to fix anomalies like argon and potassium ordering.
Correct move:
Always state elements are ordered by increasing atomic number (proton number) in the modern Periodic Table.
Wrong move:
Assigning group number from total electron count instead of outer shell electron count
Why:
Total electrons equal atomic number, but group number only corresponds to the number of electrons in the highest occupied shell.
Correct move:
Count only electrons in the outermost occupied shell to find the group number for main group elements.
Wrong move:
Using spdf sub-shell notation for electronic configurations
Why:
Edexcel IGCSE only requires shell notation (e.g. 2,8,8,2 for calcium) for the first 20 elements, sub-shell notation is A-level content and out of scope.
Correct move:
Write electronic configurations as comma-separated numbers of electrons per shell, following the 2,8,8 filling rule for first 20 elements.
Wrong move:
Classifying hydrogen as a metal because it is in Group 1
Why:
Hydrogen only shares the 1 outer electron property with Group 1 metals, it does not form basic oxides or conduct electricity, so it is a non-metal.
Correct move:
Recognise hydrogen as an exception: a non-metal placed at the top of Group 1, but not counted as an alkali metal.
Wrong move:
Stating all Group 0 elements have 8 outer electrons
Why:
Helium only has 2 total electrons, so its outer (first) shell is full with 2 electrons, not 8.
Correct move:
State Group 0 elements have full outer shells: helium has 2 outer electrons, all other Group 0 elements have 8 outer electrons.
Wrong move:
Predicting transition metal properties from group number
Why:
Transition metals are in the d-block, their group number does not directly correspond to outer electron count, and transition metal chemistry is out of scope for this topic.
Correct move:
Only apply the group number = outer electron rule to main group elements (Groups 1 to 7 and 0).
7. Quick Reference Cheatsheet
Key Rule | Exam Application |
|---|---|
Elements ordered by increasing atomic number | Use the given Periodic Table's atomic numbers to identify elements |
Period number = number of occupied electron shells | Find the period row to count the number of shells for any element |
Group number = number of outer electrons (main groups) | Find the group column to get outer electron count for Groups 1-7 |
First 20 elements: shells fill 2,8,8 | Deduce configuration: e.g. Ca (Group 2, Period 4) = 2,8,8,2 |
Metals: left/below stepped line, basic oxides, conduct electricity | Classify elements using position, oxide type or conductivity data |
Non-metals: right/above stepped line, acidic oxides, do not conduct (except graphite) | Cross-reference multiple properties if a neutral oxide is given |
Same group = same outer electrons = similar reactivity | Predict behaviour of unknown group elements from known group members |
Group 0: full outer shell = very low reactivity | Explain inertness by referring to the stable full outer electron shell |
8. Frequently Asked
How do I find the electronic configuration of an element using the Periodic Table?
The period number tells you the number of occupied electron shells, and the group number tells you the number of outer shell electrons. Fill shells from the inside out following the 2,8,8 rule for the first 20 elements. For example, magnesium (Period 3, Group 2) has a configuration of 2,8,2.
Why do elements in the same group have similar chemical properties?
Elements in the same vertical group have the same number of electrons in their outer shell. Chemical reactions only involve outer shell electrons, so elements with the same number of outer electrons react in similar ways.
Is hydrogen a metal because it is placed in Group 1?
No, hydrogen is a non-metal. It only shares the 1 outer electron property with Group 1 alkali metals, but does not form basic oxides or conduct electricity, so it is not classified as a metal.
Going deeper
What's Next
Now you have mastered core Periodic Table rules for Edexcel IGCSE Chemistry, you can move on to studying the specific chemical properties of Group 1 alkali metals and Group 7 halogens, which build directly on the group similarity principles you learned here. You will also use your electronic configuration knowledge to understand how ions form and ionic bonding works, a key foundation for the rest of the Principles of Chemistry unit. Make sure to practice past paper questions that ask you to deduce configurations and classify elements, as these are common low-mark questions that are easy to score full marks on if you remember the core rules.