Periodic Table & Periodicity — Chemistry

Q.1

WBCS Prelims 2023

According to Newlands law of octaves, beryllium resembles which of the following elements?

A. Sodium

B. Magnesium

C. Calcium

D. Aluminium

Explanation

Why Correct: Newlands' Law of Octaves states that every eighth element has similar properties when arranged by atomic weight. Beryllium (4th) resembles magnesium (4+8=12th) in the arrangement.
Distractor Analysis: Sodium is the first element of the next period and resembles lithium. Calcium resembles strontium in the next octave. Aluminium resembles boron in the previous octave.
Takeaway: Newlands' arrangement was limited to lighter elements and was superseded by Mendeleev's periodic table which accounted for all elements.

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Q.2

WBCS Prelims 2017

The strongest electropositive element is

A. Cs

B. Li

C. Mg

D. K

Explanation

Why Correct: Cesium (Cs) has the lowest ionization energy and largest atomic radius among alkali metals, making it the most electropositive element, meaning it loses electrons most readily to form positive ions.
Distractor Analysis: Lithium (Li) has the highest ionization energy among alkali metals, making it the least electropositive. Magnesium (Mg) is an alkaline earth metal with higher ionization energy than alkali metals. Potassium (K) is electropositive but less than cesium due to smaller atomic size.
Takeaway: Electropositivity increases down a group in the periodic table due to increasing atomic radius and decreasing ionization energy, with francium being theoretically the most electropositive but cesium being the most electropositive naturally occurring element.

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Q.3

WBCS Prelims 2006

Which among the following is paramagnetic?

A. H2O

B. NO (Nitric oxide)

C. CO2

D. SO2

Explanation

Why Correct: Nitric oxide (NO) has an unpaired electron in its molecular orbital configuration, making it paramagnetic and attracted to magnetic fields.
Distractor Analysis: Water (H2O) has all electrons paired in covalent bonds, making it diamagnetic. Carbon dioxide (CO2) has a linear structure with all electrons paired. Sulfur dioxide (SO2) has a bent structure with all electrons paired in molecular orbitals.
Takeaway: Paramagnetism requires unpaired electrons, while diamagnetism occurs when all electrons are paired; O2 is another common paramagnetic molecule with two unpaired electrons.

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Q.4

WBCS Prelims 2002

Which one of the following is an element?

A. Table salt

B. Water

C. Graphite

D. Glass

Explanation

Why Correct: Graphite is an allotrope of carbon, a pure substance that cannot be broken down into simpler substances by chemical means.
Distractor Analysis: Table salt is sodium chloride, a compound of sodium and chlorine. Water is H2O, a compound of hydrogen and oxygen. Glass is an amorphous solid mixture of silica, soda, and lime.
Takeaway: Elements are listed on the periodic table; compounds contain two or more elements chemically combined in fixed ratios.

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Q.5

WBCS Prelims 2000

Which of the following are called Noble gases?

A. Oxygen and Hydrogen

B. Oxygen and Argon

C. Argon and Neon

D. Hydrogen and NeonWBCS Syllabus Guide

Explanation

Why Correct: Argon and Neon belong to Group 18 elements with full valence electron shells, making them chemically inert.
Distractor Analysis: Oxygen and hydrogen are reactive nonmetals that form water and numerous compounds. Oxygen combines readily with most elements. Hydrogen, though the lightest element, participates in acid-base and reduction-oxidation reactions.
Takeaway: The complete noble gas series includes helium, neon, argon, krypton, xenon, and radon, all characterized by extremely low chemical reactivity.

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Q.6

WBCS Prelims 2012

Gold has an extraordinarily high density than silver, the reason is :

A. Higher mass number of gold

B. Higher atomic number of gold

C. High screening effect

D. Lanthanide contraction

Explanation

Why Correct: Lanthanide contraction causes gold (atomic number 79) to have a smaller atomic radius than expected for its period, packing atoms more densely and increasing density to 19.3 g/cc compared to silver's 10.49 g/cc.
Distractor Analysis: Higher mass number refers to total nucleons but doesn't directly determine density in solids. Higher atomic number indicates more protons but doesn't explain the density difference between gold and silver. High screening effect reduces effective nuclear charge and would increase atomic size, which would decrease density.
Takeaway: Lanthanide contraction affects elements from period 6 onward, making gold, platinum, and mercury denser than their period 5 counterparts (silver, palladium, cadmium).

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