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ICSE Class 9 Chemistry Question 4 of 20

The Periodic Table — Question 19

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Question 18

Name or state the following with reference to the elements of the first three periods of the periodic table.

(a) The noble gas having duplet arrangement of electrons.

(b) The noble gas having an electronic configuration 2, 8, 8.

(c) A metalloid in period 2 and in period 3.

(d) The number of electron shells in elements of period 1, period 2 and period 3.

(e) The valency of elements in group 1 [IA].

(f) The group whose elements have zero valency.

(g) An alkaline earth metal in period 3.

(h) The non-metallic element present in period 3 other than sulphur and chlorine.

(i) A non-metal in period 2 having electronic configuration 2, 6.

(j) An electrovalent compound formed between an alkali metal and a halogen.

(k) A covalent compound formed between an element in period 1 and a halogen.

(l) An alkali metal in period 3 which dissolves in water giving a strong alkali.

(m) A metal in period 3 having valency 3.

(n) The bridge elements of period 3 of group 1 [IA], 2 [IIA] and 13 [IIIA].

(o) The periods which contain the inner transition elements.

(p) The formula of the hydroxide of the element having electronic configuration 2, 8, 2.

(q) The valency of the element in period 3 having atomic number 17.

(r) A non-metal in period 2 which is tetravalent.

Answer

(a) The noble gas having duplet arrangement of electrons — Helium

(b) The noble gas having an electronic configuration 2, 8, 8 — Argon

(c) A metalloid in period 2 and in period 3 — Metalloid in period 2 is B (Boron) and in period 3 is Si (Silicon)

(d) The number of electron shells in elements of period 1, period 2 and period 3 — Period 1 —1 shell, Period 2 — 2 shells, Period 3 — 3 shells

(e) The valency of elements in group 1 [IA] — +1

(f) The group whose elements have zero valency — Group 18 or 0 group

(g) An alkaline earth metal in period 3 — Magnesium [Mg]

(h) The non-metallic element present in period 3 other than sulphur and chlorine — Phosphorus [P]

(i) A non-metal in period 2 having electronic configuration 2, 6 — Oxygen [O]

(j) An electrovalent compound formed between an alkali metal and a halogen — Sodium chloride [NaCl]

(k) A covalent compound formed between an element in period 1 and a halogen — Hydrogen chloride [HCl]

(l) An alkali metal in period 3 which dissolves in water giving a strong alkali — Alkali metal is Na and it dissolves in water to give strong alkali - NaOH

(m) A metal in period 3 having valency 3 — Aluminium [Al]

(n) The bridge elements of period 3 of group 1 [IA], 2 [IIA] and 13 [IIIA] — Li & Mg, Be & Al

(o) The periods which contain the inner transition elements — Period 6 and 7

(p) The formula of the hydroxide of the element having electronic configuration 2, 8, 2 —
Since valency of element X is 2+ and valency of OH is 1-

Formula of the compound:

X2+↗OH1−X22 ↘↙ OH1⇒X11 ↘↙ OH2\text{X}^{2+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{1}{1}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{OH}} \\[0.5em]

So, we get the formula as X(OH)2\bold{X(OH)}\bold{_2}

As the element with electronic configuration 2,8,2 is Mg, Hence the compound is Mg(OH)2

(q) The valency of the element in period 3 having atomic number 17 — -1

(r) A non-metal in period 2 which is tetravalent — Carbon [C]

Chapter Overview: The Periodic Table

The Periodic Table organises all known elements based on their properties. Dobereiner grouped elements into triads, Newlands proposed the Law of Octaves, and Mendeleev arranged elements by increasing atomic mass, predicting undiscovered elements. The Modern Periodic Table (Moseley) arranges elements by increasing atomic number. It has 7 periods and 18 groups. Periods represent the number of electron shells, while groups contain elements with similar valence electron configurations and chemical properties. The Class IX syllabus introduces students to the basic structure of the periodic table, the distinction between metals (left side), non-metals (right side), and metalloids (along the staircase line). Students learn periodic properties at an introductory level: atomic size, metallic character, and their general trends. The table is divided into s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12, transition metals), and f-block (lanthanides and actinides). Understanding the periodic table helps predict element behaviour and forms the basis for Class X chemistry.

Key Definitions

Term Definition
Dobereiner's TriadsGroups of three elements where the middle element's atomic mass is the average of the other two
Newlands' OctavesEvery eighth element has properties similar to the first (like musical octaves)
Mendeleev's Periodic LawProperties of elements are periodic functions of their atomic masses
Modern Periodic LawProperties of elements are periodic functions of their atomic numbers
MetalsElements that are shiny, malleable, ductile, good conductors; tend to lose electrons
Non-metalsElements that are dull, brittle, poor conductors; tend to gain electrons
MetalloidsElements with properties intermediate between metals and non-metals (e.g., Si, Ge)

Must-Know Concepts

  • Alkali metals (Group 1): Li, Na, K - very reactive, 1 valence electron, stored in oil
  • Halogens (Group 17): F, Cl, Br, I - very reactive non-metals, 7 valence electrons
  • Noble gases (Group 18): He, Ne, Ar - inert, fully filled outer shells
  • Metals are on the left and centre; non-metals on the upper right of the table
  • Mendeleev left gaps for undiscovered elements: eka-boron (Scandium), eka-aluminium (Gallium), eka-silicon (Germanium)
  • Limitations of Mendeleev's table: position of hydrogen, no place for isotopes, some elements out of order by mass

Metals vs Non-metals

Property Metals Non-metals
StateMostly solids (except Hg)Solids, liquids, or gases
LustreShiny (lustrous)Dull (except diamond, iodine)
ConductivityGood conductorsPoor conductors (except graphite)
Electron tendencyLose electrons (form cations)Gain electrons (form anions)

Important Diagrams to Practice

  • Layout of the Modern Periodic Table showing s, p, d, f blocks
  • Position of metals, non-metals, and metalloids on the table
  • Staircase line separating metals and non-metals

Common Mistakes

  • Confusing Mendeleev's law (atomic mass) with the Modern law (atomic number)
  • Thinking noble gases have 8 electrons in outermost shell (He has only 2)
  • Placing hydrogen definitively in Group 1 or Group 17 (its position is debatable)
  • Forgetting exceptions: Hg is a liquid metal, graphite conducts electricity despite being non-metal

Scoring Tips

  • Know Mendeleev's predictions and how they were validated (eka-aluminium = Gallium)
  • For comparison questions, always present in tabular form for clarity
  • Remember the limitations of each classification attempt (triads, octaves, Mendeleev)
  • Group names to memorise: 1 = Alkali metals, 2 = Alkaline earth, 17 = Halogens, 18 = Noble gases

Frequently Asked Questions

Why is hydrogen's position in the periodic table controversial?

Hydrogen has 1 valence electron like alkali metals (Group 1), but it is a non-metal and can gain 1 electron like halogens (Group 17). It does not fit perfectly in either group, so its position remains special.

What were the limitations of Newlands' Law of Octaves?

It worked only up to calcium (Z=20). After that, elements did not fit the pattern. He assumed all elements had been discovered and left no gaps for new elements. Also, he sometimes placed two elements in the same slot.

Why are noble gases called inert gases?

Noble gases have completely filled outermost electron shells (2 for He, 8 for others), making them extremely stable. They have no tendency to gain, lose, or share electrons, so they rarely participate in chemical reactions. However, some compounds of heavier noble gases (Xe, Kr) have been synthesised.