Class 11 Chemistry · Original EZT practice
Classification of Elements and Periodicity (Class 11 Chapter 3): notes and worked practice
One table, four ideas: why elements needed classifying at all, how Mendeleev built his table on atomic mass (and boldly left gaps), how Moseley's atomic number fixed it into the modern law, and how position in the table lets you predict radius, ionization energy and electronegativity before you ever touch a lab. All examples are original EZT tutor practice, ordered to match the chapter's own section flow. Attempt each before opening the steps.
9 original worked examples, 9 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 18 October 2026:
1. Why classify, and the road to Mendeleev
With more than a hundred elements, studying each one alone is impossible - grouping by similar properties compresses the work. Early attempts (triads, octaves) worked for a few elements only. Mendeleev arranged elements by atomic mass and found properties repeat periodically; he left gaps and predicted unknown elements - eka-aluminium was later found as gallium, eka-silicon as germanium - the predictions that made his table famous.
2. Modern periodic law and the shape of the table
Moseley's work with atomic numbers fixed the flaws in the mass-based table. Modern periodic law: the physical and chemical properties of elements are periodic functions of their atomic numbers. The present table has 7 periods (horizontal rows; the period number equals the highest principal quantum number n in the atom's configuration) and 18 groups (vertical columns; same outer-shell configuration pattern, similar properties).
3. s, p, d, f blocks and names beyond 100
The block names come from the subshell being filled: s-block = groups 1-2, p-block = groups 13-18, d-block = groups 3-12 (the transition metals), f-block = the two rows set apart at the bottom. For elements with atomic numbers above 100, IUPAC uses systematic roots (0 = nil, 1 = un, 2 = bi, 3 = tri, 4 = quad...) until a permanent name is agreed: 101 = Unnilunium, 102 = Unnilbium, 103 = Unniltrium, 104 = Unnilquadium.
4. Periodic trends you can predict
Atomic radius decreases across a period (nuclear charge pulls harder on the same shell) and increases down a group (a new shell each step). Ionization enthalpy - the energy to remove an electron - generally increases across a period and decreases down a group. Electronegativity increases across a period (fluorine is the most electronegative) and decreases down a group. Metallic character runs opposite to non-metallic: metallic increases down a group; non-metallic character increases across a period.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Explain what made Mendeleev's table convincing
Choose the method: Look for evidence beyond tidy sorting - what did the table dare to claim?
EZT practice question
What made Mendeleev's table more convincing than earlier attempts?
Show the worked solution
- Earlier attempts (triads, octaves) worked for a few elements only.
- Mendeleev predicted not just patterns but missing elements - naming eka-aluminium and eka-silicon and describing their properties.
- Gallium and germanium were later discovered matching those predictions.
Answer: He predicted not just patterns but missing elements - eka-aluminium and eka-silicon - with their properties, before gallium and germanium were discovered. Correct predictions are stronger evidence than tidy sorting.
Check: The answer pairs each prediction with its later discovery.
Watch for this: Do not credit only the orderly arrangement; the gaps and predictions are the convincing part.
Now try it yourself
Which property did Mendeleev use as the basis of his table?
Check your answer
Atomic mass.
He found properties repeat periodically when elements are arranged by atomic mass.
Decode the "eka" names
Choose the method: Read eka- as a positional marker within a group, then map to the modern name.
EZT practice question
What does "eka" mean in eka-aluminium, and what is its modern name?
Show the worked solution
- Eka- marks the element predicted one place below the named element in the same group.
- Eka-aluminium is the element one place below aluminium - gallium.
Answer: It marks the element predicted one place below aluminium in the same group; eka-aluminium is gallium.
Check: The answer covers both the positional meaning and the modern name.
Watch for this: Do not treat eka-aluminium as a compound or alloy; it was a placeholder for an undiscovered element.
Now try it yourself
Name the element discovered that filled the eka-silicon gap.
Check your answer
Germanium.
Eka-silicon was later found and named germanium.
Find period and group from a configuration
Choose the method: Read the highest shell for the period, then the outer electrons for the group.
EZT practice question
An element has configuration 2, 8, 7. Find its period and group from the configuration alone.
Show the worked solution
- The highest shell occupied is n = 3, so the element is in period 3.
- Seven electrons in the outer shell place it in group 17 (the halogens).
- The element is chlorine, Z = 17.
Answer: Period 3 (highest n = 3) and group 17 (seven outer electrons) - the element is chlorine.
Check: Both answers come from the configuration alone, before naming the element.
Watch for this: Do not count total electrons for the period; the period is the highest occupied shell.
Now try it yourself
Place Z = 20 (2, 8, 8, 2) by period and group.
Check your answer
Period 4, group 2.
Highest shell n = 4 gives period 4; two outer electrons give group 2.
Place element Z = 12 in the table
Choose the method: Write the configuration first, then read off period, group and block.
EZT practice question
Element with Z = 12: write the configuration and place it.
Show the worked solution
- Configuration: 2, 8, 2.
- Highest n = 3, so period 3.
- Two outer electrons, so group 2 - an s-block element; it is magnesium.
Answer: 2, 8, 2 - period 3, group 2, s-block: magnesium.
Check: Period, group and block all follow from the configuration.
Watch for this: Do not stop at period and group; naming the element confirms the placement.
Now try it yourself
How many periods and groups does the present table have?
Check your answer
7 periods and 18 groups.
The modern table has 7 horizontal periods and 18 vertical groups.
Classify elements by block
Choose the method: Find the subshell that receives the last electron.
EZT practice question
Classify by block: (i) Z = 11 (ii) Z = 17 (iii) Z = 26.
Show the worked solution
- Z = 11 is 2,8,1 - the last electron enters 3s: s-block.
- Z = 17 is 2,8,7 - the last electron enters 3p: p-block.
- Z = 26 is iron, a transition metal whose d subshell fills: d-block.
Answer: (i) s-block, (ii) p-block, (iii) d-block.
Check: Each classification cites the subshell being filled.
Watch for this: Do not read the block from the group number alone; the block is the subshell receiving the last electron.
Now try it yourself
Which groups form the p-block?
Check your answer
Groups 13-18.
The p-block spans groups 13 to 18; s-block is groups 1-2 and d-block groups 3-12.
Build an IUPAC systematic name
Choose the method: Convert each digit to its root, then add -ium.
EZT practice question
Write the IUPAC systematic name for element 105.
Show the worked solution
- Roots: 1 = un, 0 = nil, 5 = pent.
- Join the roots and add -ium: Un-nil-pent-ium.
Answer: Unnilpentium.
Check: The roots appear in digit order: 1, 0, 5.
Watch for this: Do not mix the root order; the name follows the digits of the atomic number.
Now try it yourself
Name element 104 in the IUPAC systematic scheme.
Check your answer
Unnilquadium.
1 = un, 0 = nil, 4 = quad, then -ium.
Arrange period-3 elements by radius
Choose the method: Apply the across-a-period trend: increasing nuclear charge, same shell.
EZT practice question
Arrange Na, Mg, Cl (all period 3) in increasing atomic radius.
Show the worked solution
- Atomic radius decreases across a period.
- Across period 3 the order is Na -> Mg -> Cl.
- So increasing radius runs Cl < Mg < Na.
Answer: Cl < Mg < Na.
Check: The order matches the periodic position: further right, smaller radius.
Watch for this: Do not reverse the inequality; increasing radius starts from the smallest, chlorine.
Now try it yourself
Does atomic radius increase or decrease from Na to Cl?
Check your answer
Decrease.
Nuclear charge pulls harder on the same shell across the period.
Compare ionization enthalpy down a group
Choose the method: Track the outer electron's distance and shielding down the group.
EZT practice question
Between Li and Cs (group 1), which loses its outer electron more easily, and why?
Show the worked solution
- Down the group each element adds a shell.
- The outer electron sits farther from the nucleus and is better shielded.
- Ionization enthalpy therefore decreases down the group, so Cs loses its electron more easily.
Answer: Cs - ionization enthalpy decreases down the group as the outer electron sits farther from the nucleus and is better shielded.
Check: The reason cites both distance and shielding.
Watch for this: Do not answer by reactivity memory alone; the explanation must use the ionization trend.
Now try it yourself
Which has the higher ionization enthalpy, Li or Cs?
Check your answer
Li.
Ionization enthalpy decreases down a group, so the smaller atom holds its electron more tightly.
Pick the more metallic element in a pair
Choose the method: Use the two metallic-character trends: down a group up, across a period down.
EZT practice question
Pick the more metallic in each pair: (i) Na or K (ii) Mg or Cl.
Show the worked solution
- Metallic character increases down a group - K beats Na.
- Non-metallic character increases across a period - so in Mg vs Cl, the metal is Mg.
Answer: (i) K - metallic character increases down a group. (ii) Mg - non-metallic character rises across the period, leaving Mg as the metal of the pair.
Check: Each pick is justified by the trend that applies to the pair's relative positions.
Watch for this: Do not apply the group trend to a same-period pair; check whether the pair shares a group or a period first.
Now try it yourself
Which element is the most electronegative?
Check your answer
Fluorine.
Electronegativity increases across a period and decreases down a group, peaking at fluorine.
Try a mixed set
Choose a method or explanation before opening each answer.
1. What basis did Mendeleev use for his table?
Show answer and reason
Atomic mass.
He found properties repeat periodically when elements are arranged by mass.
2. State the modern periodic law.
Show answer and reason
The physical and chemical properties of elements are periodic functions of their atomic numbers.
Moseley's atomic number replaced Mendeleev's atomic mass as the ordering basis.
3. Which block holds the transition metals?
Show answer and reason
The d-block (groups 3-12).
Their d subshells are being filled.
4. How does atomic radius change across a period?
Show answer and reason
It decreases.
Nuclear charge pulls harder on the same shell.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- Place Z = 19 (2, 8, 8, 1) by period, group and block.
- Why did Moseley's atomic number fix the flaws in Mendeleev's mass-based table?
- Arrange F, Cl, Br in decreasing electronegativity and explain the order.
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Questions about this practice page
- What is the modern periodic law?
- The physical and chemical properties of elements are periodic functions of their atomic numbers - Moseley's atomic number replaced Mendeleev's atomic mass as the ordering basis.
- How do I find an element's period from its configuration?
- The period number is the highest principal quantum number (shell) occupied - 2,8,1 means period 3.
- Why does ionization enthalpy decrease down a group?
- Each step down adds a shell, so the outer electron is farther from the nucleus and more shielded - easier to remove.
- What do the block letters s, p, d, f stand for?
- The subshell receiving the last electron: s-block groups 1-2, p-block 13-18, d-block 3-12, f-block the two separated bottom rows.
- Are these the NCERT exercise questions for Classification of Elements and Periodicity in Properties?
- No. EZT pages are original practice written by our tutors, aligned to the chapter's syllabus - use them to prepare for the textbook exercises, not as a substitute for your book.