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The modern periodic table: a calendar with uneven weeks

Your guide: Miss SanaCounts atoms the way you count rupees: carefully.

The problem

A calendar puts every Monday in one column, because Mondays behave alike: assembly, a fresh timetable, sleepy faces. The periodic table does the same with elements. Similar elements sit in one column.

But here is the twist: a calendar week always has 7 days. The rows of the periodic table have 2, 8, 8, 18, 18 and 32 elements. Why the rows grow is the whole story of this topic.

Groups are the vertical columns of elements with similar properties: eight groups, I to VIII, each split into A and B subgroups. Periods are the seven horizontal rows, numbered 1 to 7.

Step 1 / 7

Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

The problem

A calendar puts every Monday in one column, because Mondays behave alike: assembly, a fresh timetable, sleepy faces. The periodic table does the same with elements. Similar elements sit in one column.

But here is the twist: a calendar week always has 7 days. The rows of the periodic table have 2, 8, 8, 18, 18 and 32 elements. Why the rows grow is the whole story of this topic.

Groups are the vertical columns of elements with similar properties: eight groups, I to VIII, each split into A and B subgroups. Periods are the seven horizontal rows, numbered 1 to 7.

Key terms

A and B subgroups
A subgroups hold the representative, or normal, elements. B subgroups hold the less typical transition elements, placed in the centre of the table.
Short and long periods
Period 1 has only hydrogen and helium. Periods 2 and 3 are short, with 8 elements each. Periods 4 and 5 are long, with 18. Period 6 has 32, including the 14 lanthanides.
Families
Some A groups have family names. IA: alkali metals, as they form strong alkalies with water. IIA: alkaline earth metals. VIIA: halogens, the salt formers. VIIIA: noble gases, the least reactive.
Blocks
Blocks are named after the orbital that holds the valence electrons. IA and IIA: s-block. IIIA to VIIIA, except helium: p-block. Transition elements: d-block. Lanthanides and actinides: f-block.

Short questions with model answers

  1. Q1. Period 6 contains 32 elements. Show how the book breaks this number down.

    • Representative (A) elements: 8
    • Transition (B) elements: 10; lanthanides: 14
    • 8 + 10 + 14 = 32

    8 + 10 + 14 = 32

  2. Q2. Why is group IA called the alkali metals? Use sodium as your example.

    • 2Na + 2H2O → 2NaOH + H2
    • NaOH is a strong alkali.

    Because they form strong alkalies, such as NaOH, with water.

  3. Q3. Classify sodium (IA), silicon (IVA) and chlorine (VIIA) by block and by metal type.

    • Na: IA → s-block · metal
    • Si: IVA → p-block · metalloid
    • Cl: VIIA → p-block · non-metal

    Na: s, metal · Si: p, metalloid · Cl: p, non-metal

Common mistakes

  • ✗ Writing that the second and third periods contain eighteen elements each.

    ✓ Periods 2 and 3 are the short periods, with 8 elements each. Periods 4 and 5 have 18. This is a true/false item in the exercise.

  • ✗ Saying the metals are in the top right corner of the table.

    ✓ The top right corner holds the non-metals. Metals are on the left, in the centre and at the bottom.

  • ✗ Putting helium in the p-block because it sits in group VIIIA.

    ✓ The book says p-block is IIIA to VIIIA except helium. Helium's two electrons are in an s orbital, so it is s-block.

MCQs

  1. 1. Members of group VIIA are called:

    1. (a) alkali metals
    2. (b) halogens
    3. (c) noble gases
    4. (d) alkaline earth metals
    Show answer

    (b) Halogens, named for their salt-forming property.

  2. 2. Alkaline earth metals are present in group:

    1. (a) IA
    2. (b) IIA
    3. (c) IIB
    4. (d) VIIIA
    Show answer

    (b) Group IIA, named for their presence in the Earth's crust and their alkaline character.

  3. 3. How many elements do periods 4 and 5 each contain?

    1. (a) 2
    2. (b) 8
    3. (c) 18
    4. (d) 32
    Show answer

    (c) Eighteen: 8 representative and 10 transition elements. That is why potassium's partner, rubidium, comes 18 places later.

  4. 4. Elements whose valence electrons are in d orbitals are called:

    1. (a) s-block elements
    2. (b) p-block elements
    3. (c) d-block elements
    4. (d) f-block elements
    Show answer

    (c) The transition elements: their d electrons decide their valency, so they are the d-block.

  5. 5. Which of these is a metalloid?

    1. (a) Na
    2. (b) As
    3. (c) Cl
    4. (d) Ca
    Show answer

    (b) Arsenic sits just under the steps, like silicon and tellurium. Na and Ca are metals; Cl is a non-metal.

Quick revision

  • Eight groups, each split into A (representative) and B (transition); seven periods with 2, 8, 8, 18, 18 and 32 elements.
  • Families: alkali metals (IA), alkaline earth metals (IIA), halogens (VIIA), noble gases (VIIIA). Blocks: s, p, d, f.
  • Metals left, centre and bottom; non-metals top right; metalloids such as Si, As and Te just under the steps.