Arrangement of Elements — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

Section 1 of 11Layout of the Periodic Table

• Order
  • elements are arranged by increasing atomic number, with each having one more proton than the one before.
• Reason
  • elements with similar properties fall in the same column.
  • Periods
    • horizontal rows numbered 1 to 7 showing the number of electron shells.
  • Groups
    • vertical columns I to VII and 0 showing the outer electrons.
• Examples
  • Period 2 elements have two shells, Group IV atoms have four outer electrons.

Section 2 of 11Groups and ion charges

• Metals
  • the group number is the number of electrons lost and the charge of the ion.
• Sodium
  • Group I, loses 1 electron, forms Na⁺.
  • Magnesium
    • Group II, forms Mg²⁺.
• Non-metals
  • Group VII gains one electron and Group VI gains two.
• Group VII
  • ions with 1- charge.
  • Group VI
    • ions with 2- charge.

Section 3 of 11Valency

• Definition
  • valency, or combining power, is how many bonds an atom can make or how many electrons it loses, gains or shares.
• Carbon
  • valency 4, so four single bonds or two double bonds.
• Valency by group
  • see the table below.

Group	I	II	III	IV	V	VI	VII	0
Valency	1	2	3	4	3	2	1	0

Section 4 of 11Metallic character trend

• Across a period
  • metallic character decreases from left to right.
• Down a group
  • metallic character increases.
• Reason
  • atoms to the right gain electrons more readily to fill the shell, while lower atoms lose outer electrons more easily.
  • Layout
    • metals on the left, non-metals on the right, with metalloids (semi-metals) between them.
  • Dividing line
    • a zig-zag line separates metals from non-metals.

Section 5 of 11Electronic configuration and position

• Period
  • the number of numbers in the configuration is the number of shells.
• Chlorine
  • 2,8,7 has three numbers, so Period 3.
• Group
  • the final number is the outer electrons.
• Chlorine
  • seven outer electrons, so Group VII.
• Same group
  • same outer electrons give similar chemical properties.
• Example
  • lithium and sodium both donate one electron to Group VII elements to form ionic compounds (1+ and 1- ions).
• Down a group
  • each element adds a full shell.
• Group I
  • Li 2,1; Na 2,8,1; K 2,8,8,1.

Section 6 of 11Predicting properties

• Patterns
  • trends in groups and across periods let us predict melting point, boiling point, density and reactivity.
• Group I
  • react quickly with water and melting point decreases down the group.
  • Noble gases
    • unreactive.
  • Transition elements
    • denser than Group I metals.
  • Group VII
    • reactivity decreases down the group.

Section 7 of 11Identifying trends from data

• Group I and water
  • lithium, sodium and potassium show reactivity increasing down the group.
• Prediction
  • rubidium, caesium and francium will react even more vigorously with air and water.
  • Francium
    • rare and radioactive, so predictions are hard to confirm.
• Method for data questions
  • put the elements in order down the group, then look for a pattern.
• Rule
  • list ascending or descending by position before judging a trend.

Section 8 of 11Using the table to answer questions

1. Find the element in the table and read its group and period.
2. Work out its outer electrons from the group.
3. Predict its ion charge from the number of electrons lost or gained.
4. Compare it with neighbours in its group to predict properties.

• Example
  • sulfur is in Group VI and Period 3, so it has six outer electrons and forms an S^2- ion.
• Example 2
  • calcium is in Group II and Period 4, so it forms Ca²⁺.

Section 9 of 11Worked examples

1. An element has configuration 2,8,3. Give its group, period and the charge on its ion. [3]3 marks

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2. Predict how rubidium reacts with water compared with potassium. [2]2 marks

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3. State the trend in metallic character across Period 3 and explain it. [2]2 marks

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4. State the valency of nitrogen, a Group V element. [1]1 mark

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Section 10 of 11Key terms

Term	Meaning
Period	A horizontal row of the Periodic Table, showing electron shells.
Group	A vertical column, showing outer electrons.
Valency	The combining power of an atom.
Metalloid	An element with properties of both metals and non-metals.
Metallic character	How metal-like an element is.
Outer electrons	Electrons in the outermost shell, also called valency electrons.

Section 11 of 11Exam technique

1. Confusing groups (columns) and periods (rows).
2. Saying group number is the number of shells — it is the outer electrons; period is shells.
3. Predicting trends without checking order in the group.
4. Saying Group 0 has valency 8 — noble gases have a full shell and valency 0.