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Chemistry · Class 11 Science

Ch 3Classification of Elements and Periodicity in Properties — Class 11 Chemistry, concept-first.

By the mid-19th century chemists knew of over sixty elements, each with its own set of physical and chemical properties, and simply memorising every fact about every element one at a time was becoming unworkable.

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Key concepts

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Modern Periodic Law

Henry Moseley (1913) studied the characteristic X-rays emitted by elements bombarded with high-energy electrons and found a strict linear relationship between the square root of the emitted X-ray frequency and the elemen…

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Chapter contents

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3.1

Why Do We Need to Classify Elements?

By the mid-19th century chemists knew of over sixty elements, each with its own set of physical and chemical properties, and simply memorising every fact about every element one at a time was becoming…

3.2

Modern Periodic Law and the Present Form of the Periodic Table

Moseley's modern periodic law states: the physical and chemical properties of the elements are periodic functions of their atomic number.

3.3

Periodic Trends in Atomic Radius, Ionic Radius and van der Waals' Radius

"Atomic size" is not a single fixed number — an atom has no sharp outer boundary, since the probability of finding an electron never quite reaches zero even far from the nucleus.

3.4

Periodic Trends in Ionization Enthalpy

The ionization enthalpy (, also written ) of an element is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom in its ground state, forming a gaseous ca…

3.5

Periodic Trends in Electron Gain Enthalpy

Electron gain enthalpy () is the enthalpy change that occurs when one mole of isolated gaseous atoms of an element each gains one electron to form one mole of gaseous anions: Most atoms release energy…

3.6

Electronegativity

Electronegativity is the relative tendency of an atom, when it is part of a covalent bond, to attract the shared pair of bonding electrons toward itself.

3.7

Periodic Trends in Valency

Valency is an element's combining capacity — a measure of how many bonds (or how many univalent atoms such as hydrogen or chlorine) one atom of the element can typically combine with.

3.8

Nomenclature of Elements with Atomic Number Greater than 100

When a new, superheavy element (atomic number greater than 100) is first claimed to have been synthesised, it does not yet have a permanent name — under IUPAC rules, the discovering team is invited to…

3.9

Summary

This chapter traced the classification of elements from Dobereiner's triads and Newlands' octaves through Mendeleev's mass-based periodic table to Moseley's modern periodic law — properties are a peri…

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

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  1. Q1(i) The outermost electronic configuration of the atom of an element is 3s²3p³. Mention the position of the element in the long periodic tab…Preview

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  1. Example 1State the modern periodic law and explain how it differs from Mendeleev's original periodic law.Free
  2. Example 4Arrange $Na$, $Mg$, $Al$, $Si$ (Period 3) in order of decreasing atomic radius and explain the trend.Free
  3. Example 6Which has the larger radius, $Na$ or $Na^{+}$? Explain your answer and generalise the rule that relates the radius of a cation or an anion t…Free
  4. Example 8Define van der Waals' radius and explain why it is always larger than the covalent (atomic) radius of the same atom.Preview
  5. Example 10The first ionization enthalpy of boron ($801\ kJ\,mol^{-1}$) is lower than that of beryllium ($899\ kJ\,mol^{-1}$), even though boron has on…Preview
  6. Example 12For magnesium ($1s^{2}\,2s^{2}\,2p^{6}\,3s^{2}$), the successive ionization enthalpies are $IE_1 = 738$, $IE_2 = 1451$ and $IE_3 = 7733\ kJ\…Preview
  7. Example 14Define electron gain enthalpy. Why do the noble gases have positive (unfavourable) electron gain enthalpies?Preview
  8. Example 16Explain why nitrogen has a much less negative electron gain enthalpy (in fact essentially zero) compared with its Period 2 neighbours carbon…Preview
  9. Example 18Explain why electronegativity increases across a period from left to right but decreases on descending a group.Preview
  10. Example 20State the valency shown by elements of Group 1, Group 16 and Group 17 of the periodic table, and explain how the group valency of a represen…Preview
  11. Example 22Using the IUPAC numerical roots (0 = nil, 1 = un, 2 = bi, 3 = tri, 4 = quad, 5 = pent, 6 = hex, 7 = sept, 8 = oct, 9 = enn), derive the syst…Preview
  12. Example 24Derive the systematic IUPAC name and three-letter symbol for a hypothetical element with atomic number 120.Preview
+Show 12 questions12 questions
  1. Q2In the long form of the periodic table, how many periods and how many groups are there, and what determines an element's period number?Free
  2. Q3The periodic table is divided into four blocks: $s$, $p$, $d$ and $f$. State the rule used to assign an element to a block, and name the two…Free
  3. Q5Explain why atomic radius increases on descending a group, using $Li$, $Na$, $K$ as an example.Free
  4. Q7The isoelectronic species $N^{3-}$, $O^{2-}$, $F^{-}$, $Na^{+}$ and $Mg^{2+}$ all have 10 electrons. Arrange them in order of decreasing ion…Preview
  5. Q9Define the first ionization enthalpy of an element. Why does it generally increase across Period 3 from $Na$ to $Ar$?Preview
  6. Q11Explain why the first ionization enthalpy of oxygen ($1314\ kJ\,mol^{-1}$) is lower than that of nitrogen ($1402\ kJ\,mol^{-1}$), even thoug…Preview
  7. Q13Why does the first ionization enthalpy generally decrease on descending a group, using $Li$, $Na$, $K$ as an example?Preview
  8. Q15Among the halogens, chlorine has a more negative electron gain enthalpy ($-349\ kJ\,mol^{-1}$) than fluorine ($-328\ kJ\,mol^{-1}$), even th…Preview
  9. Q17Define electronegativity on the Pauling scale. Which element has the highest electronegativity, and what value is assigned to it on this sca…Preview
  10. Q19How is an element's electronegativity related to its metallic or non-metallic character?Preview
  11. Q21Why do the noble gases (Group 18) generally show a valency of zero?Preview
  12. Q23Derive the systematic IUPAC name and symbol for the element with atomic number 118, and name the element it was later officially given after…Preview