Cambridge IGCSE0620

Arrangement of elements

Chemistry 0620 Chapter Notes

What this chapter covers

Arrangement of elementsGroup I propertiesGroup VII propertiesTransition elementsNoble gases
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In this chapter

  1. 1The Architecture of the Periodic Table
  2. 2Decoding Elements: Groups and Periods
  3. 3The Transition Elements
  4. 4Lanthanoids and Actinoids

1. The Architecture of the Periodic Table

The Periodic Table is a systematic chart of all known chemical elements, arranged in order of increasing proton number (also called atomic number). This arrangement reveals a repeating pattern of properties, known as periodicity. The vertical columns are called Groups, and the horizontal rows are called Periods. A heavy zig-zag line separates the metals (on the left) from the non-metals (on the right), though hydrogen is a non-metal typically placed on the left.

Key term

Proton Number: The number of protons in the nucleus of an atom, which uniquely identifies a chemical element.

Examiner insight

Examiners expect you to know that the Periodic Table is ordered by proton number, not nucleon number or relative atomic mass.

Common pitfall

Forgetting that Hydrogen, although in Group I, is a non-metal and has unique properties different from the alkali metals below it.

Worked example 12 marks

An element has a proton number of 13. Use the Periodic Table to identify:a) the element,b) whether it is a metal or a non-metal.

  1. 1

    Step 1: The proton number is 13. The element with 13 protons is Aluminium.

  2. 2

    Step 2: Aluminium (Al) is located to the left of the zig-zag line in the Periodic Table.

  3. 3

    Step 3: Therefore, Aluminium is a metal.

Worked example 22 marks

Locate the element Chlorine (Cl), with proton number 17, on the Periodic Table. State its Group and Period.

  1. 1

    Step 1: Find the element with proton number 17, which is Chlorine (Cl).

  2. 2

    Step 2: Identify the horizontal row it is in. Chlorine is in the 3rd row, so it is in Period 3.

  3. 3

    Step 3: Identify the vertical column it is in. Chlorine is in the 17th column, which is also known as Group VII.

  4. 4

    Answer: Period 3, Group VII.

Recap

  • Elements are arranged in order of increasing proton number.
  • Vertical columns are called Groups.
  • Horizontal rows are called Periods.
  • Metals are on the left and non-metals are on the right of the zig-zag line.
  • Elements in the same group show similar chemical properties.

Quick check

  1. What property is used to order the elements in the modern Periodic Table?1 mark
  2. Are elements in the same Period or the same Group more chemically similar?1 mark

2. Decoding Elements: Groups and Periods

An element's position in the Periodic Table directly relates to its electronic configuration. The Period number tells you the number of occupied electron shells in an atom. The Group number (for Groups I to VIII/0) tells you the number of electrons in the outermost shell. These outer electrons are called valence electrons, and they are responsible for an element's chemical reactions. This is why elements in the same Group have similar chemical properties – they have the same number of valence electrons to lose, gain, or share.

Key term

Valence Electrons: The electrons located in the outermost shell of an atom, which are involved in forming chemical bonds.

Examiner insight

Marks are consistently awarded for clearly linking Period number to the number of shells and Group number to the number of outer electrons.

Common pitfall

Forgetting the electron capacity of the first few shells. Remember the rule: the first shell holds 2 electrons, the second holds 8, and the third also holds 8 for the first 20 elements.

Worked example 13 marks

An element has the electronic configuration 2.8.5.a) In which Period is it found?b) In which Group is it found?c) Identify the element.

  1. 1

    a) The configuration 2.8.5 shows electrons in three shells. The number of shells equals the Period number, so it is in Period 3.

  2. 2

    b) The outermost shell has 5 electrons. The number of valence electrons equals the Group number, so it is in Group V.

  3. 3

    c) The total number of electrons is 2 + 8 + 5 = 15. For a neutral atom, proton number = electron number. The element with proton number 15 is Phosphorus (P).

Worked example 22 marks

Magnesium (Mg) is in Period 3 and Group II. Deduce its electronic configuration.

  1. 1

    Step 1: Period 3 indicates the atom has three electron shells.

  2. 2

    Step 2: Group II indicates the atom has two valence electrons (electrons in the outer shell).

  3. 3

    Step 3: Fill the shells from the inside out. The first shell holds a maximum of 2 electrons. The second holds a maximum of 8.

  4. 4

    Step 4: The configuration is therefore 2.8.2.

Recap

  • The Period number equals the number of occupied electron shells.
  • The Group number (I-VIII) equals the number of valence electrons.
  • Valence electrons determine an element's chemical reactivity.
  • Elements in the same Group have the same number of valence electrons and similar chemical properties.

Quick check

  1. How many electron shells does an atom of an element in Period 4 have?1 mark
  2. An element is in Group VII. How many electrons does it need to gain to achieve a stable outer shell?1 mark

3. The Transition Elements

The transition elements are the large block of metals found between Group II and Group III in the Periodic Table, including familiar metals like iron, copper, and zinc. They have typical metallic properties but with some key distinctions. They are generally harder, denser, and have higher melting points than Group I metals. Chemically, they are known for forming coloured compounds, having variable oxidation states (meaning they can form ions with different charges, e.g., Fe²⁺ and Fe³⁺), and acting as excellent catalysts in industrial processes.

Key term

Catalyst: A substance that increases the rate of a chemical reaction without being used up in the process.

Examiner insight

Be specific when giving properties. Instead of just 'they are catalysts', give an example like 'Iron is used as a catalyst in the Haber process'.

Fun fact

The vibrant colours in stained glass windows and gemstones like rubies and emeralds are often due to the presence of transition metal ions.

Worked example 14 marks

Iron is a transition element. State two physical and two chemical properties characteristic of iron.

  1. 1

    Physical Property 1: High density (or hard/strong).

  2. 2

    Physical Property 2: High melting point.

  3. 3

    Chemical Property 1: Forms coloured compounds (e.g., iron(II) salts are pale green, iron(III) salts are yellow/brown).

  4. 4

    Chemical Property 2: Acts as a catalyst (e.g., in the Haber process for making ammonia).

Worked example 21 mark

The name of the compound copper(II) oxide tells you the charge on the copper ion. What is the charge on the copper ion in this compound?

  1. 1

    Step 1: Transition element compounds often use Roman numerals in their names.

  2. 2

    Step 2: The Roman numeral indicates the oxidation state, which corresponds to the positive charge on the metal ion.

  3. 3

    Step 3: The numeral (II) means the oxidation state is +2. Therefore, the charge on the copper ion is 2+.

Recap

  • Transition elements are the block of metals between Groups II and III.
  • They are hard, dense, and have high melting points.
  • They form brightly coloured compounds.
  • They have variable oxidation states, shown by Roman numerals in names (e.g., iron(II)).
  • Many transition elements and their compounds are important catalysts.

Quick check

  1. What does the (III) in the name iron(III) chloride signify?1 mark
  2. Give one use of a transition element as a catalyst.1 mark

4. Lanthanoids and Actinoids

The Lanthanoids and Actinoids are two series of elements that are part of the main Periodic Table but are usually shown as two separate rows at the bottom to make the table less wide. They are collectively known as the 'f-block' elements. The Lanthanoids follow the element Lanthanum (La) and are relatively reactive metals. The Actinoids follow Actinium (Ac) and are all radioactive. This series includes well-known elements like Uranium (U) and Plutonium (Pu), which are used in nuclear reactors and weapons.

Key term

Radioactive: Describes an unstable atomic nucleus that spontaneously decays, emitting radiation.

Fun fact

Europium, a lanthanoid, is used as a red phosphor in television screens and fluorescent lamps, helping to create a full colour spectrum.

Worked example 12 marks

State the location of the Lanthanoids and Actinoids on a standard Periodic Table and give one key property of the Actinoid series.

  1. 1

    Step 1: Location: The Lanthanoids and Actinoids are typically displayed as two separate rows at the bottom of the Periodic Table.

  2. 2

    Step 2: Property of Actinoids: All elements in the Actinoid series are radioactive.

Recap

  • Lanthanoids and Actinoids are the two rows at the bottom of the Periodic Table.
  • They fit into Period 6 (Lanthanoids) and Period 7 (Actinoids) after Lanthanum and Actinium respectively.
  • All Actinoids are radioactive elements.
  • Uranium and Plutonium are important members of the Actinoid series.

Quick check

  1. Which series of elements includes Uranium and Plutonium?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Element X has a proton number of 16. a) Write its electronic configuration. b) State its group and period. c) Is element X a metal or a non-metal? Explain your answer.4 marks
  2. Sodium (Na) is in Group I and Iron (Fe) is a transition element. Compare sodium and iron in terms of two physical properties and one chemical property.6 marks
  3. The element Silicon (Si) has proton number 14. It lies on the 'zig-zag' line between metals and non-metals. Predict its electronic configuration and its character (metallic or non-metallic).3 marks
  4. An atom has 3 electron shells and 7 valence electrons. Identify the element and write the formula of the compound it forms with sodium (Na, proton number 11).3 marks
  5. Explain, in terms of electronic configuration, why elements in Group 0 (the Noble Gases) are almost completely unreactive.2 marks
  6. Vanadium is a transition element that can form several oxides, including VO, V₂O₃, VO₂, and V₂O₅. What chemical property of transition elements does this demonstrate? Explain what this property means.2 marks
  7. An element is in Period 3 of the Periodic Table. Describe how the properties of the elements change as you move from left to right across this period.3 marks
  8. Astatine (At) is at the bottom of Group VII. Predict its physical state at room temperature and its reactivity compared to Iodine (I), which is above it in the group.3 marks
  9. An ion has a charge of 2+, an electronic configuration of 2.8.8, and contains 20 neutrons. Identify the element, its proton number, and its nucleon number.4 marks
  10. Explain why elements in Group I (Alkali Metals) all react in a similar way with water, but the reactivity increases down the group.4 marks

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