Periodic Table and p-Block Elements: Important Topics and Questions for NEET by PW

The Periodic Table and p-Block Elements can be challenging to revise because many questions test small differences in periodic trends, electronic configurations, and the stability of oxidation states. Simply memorising the general rules may not be enough, as exceptions involving atomic size, ionisation enthalpy, and electron gain enthalpy can change the correct answer.

PW’s Periodic Table + The p-Block Elements FULL CHAPTER || NEET 2026 video covers important concepts, notes, practice questions, and PYQs to help you revise both chapters. Use the key topics and questions below to strengthen your understanding, recall important exceptions, and identify areas that need more practice.

Periodic Table: Important Topics to Revise

The Periodic Table chapter covers the position of elements, their electronic configurations, and the trends that help explain their chemical properties. Focus on the following topics to understand the general patterns and remember the exceptions frequently tested in exam-style questions.

1. Modern Periodic Table and Element Location

The modern periodic table arranges elements in increasing order of atomic number. An element’s position depends on its electronic configuration, particularly its valence-shell configuration.

While revising this topic, focus on:

• The structure of the modern periodic table, including periods and groups.

• Locating elements using their electronic configurations.

• Identifying the block to which an element belongs.

• IUPAC nomenclature of elements with atomic numbers greater than 100.

• Remembering the systematic names and original names of elements covered in your notes.

2. Atomic Radius and Ionic Radius

Atomic radius generally decreases from left to right across a period because the effective nuclear charge increases while electrons enter the same principal shell. Down a group, atomic radius generally increases because new electron shells are added.

Ionic radius depends on the charge and electronic configuration of an ion.

• Cations: A positively charged ion is generally smaller than its parent atom.

• Anions: A negatively charged ion is generally larger than its parent atom.

• Isoelectronic species: When species have the same number of electrons, the one with the greater nuclear charge generally has the smaller radius.

These comparisons are important for questions that ask you to arrange atoms or ions in increasing or decreasing order of size.

3. Important Atomic-Size Exceptions in Group 13

Group 13 does not follow a perfectly regular increase in atomic size down the group. The order highlighted in the notes is:

Boron < Gallium < Aluminium < Indium < Thallium

This irregularity is associated with d-block contraction and lanthanide contraction.

For the d-block elements, atomic size initially decreases across the 3d series, remains nearly constant through the middle, and increases slightly towards the end. The sizes of iron, cobalt, and nickel are almost similar.

The 4d elements are generally larger than their corresponding 3d elements, while the corresponding 4d and 5d elements have nearly equal sizes because of lanthanide contraction.

Across the 4f series, atomic and ionic sizes generally decrease gradually. Europium is an important size anomaly mentioned in the notes.

4. Ionisation Enthalpy

Ionisation enthalpy is the energy required to remove an electron from an isolated gaseous atom. Removing an electron is an endothermic process.

Successive ionisation enthalpies increase because each electron is removed from an increasingly positively charged species.

First ionisation enthalpy < Second ionisation enthalpy < Third ionisation enthalpy

The general trends are:

• Ionisation enthalpy increases from left to right across a period.

• Ionisation enthalpy generally decreases down a group.

• Atomic size, effective nuclear charge, shielding effect, and electronic configuration influence ionisation enthalpy.

Important Exceptions

• Beryllium and boron: Boron has a lower first ionisation enthalpy because its electron is removed from a higher-energy 2p orbital, whereas beryllium has a filled 2s subshell.

• Nitrogen and oxygen: Oxygen has a lower first ionisation enthalpy than nitrogen. Nitrogen has a relatively stable half-filled 2p subshell, while oxygen has a paired electron in one of its 2p orbitals.

Ionisation enthalpy also helps determine the number of valence electrons. A large jump between successive ionisation enthalpies indicates that the next electron would be removed from an inner shell.

5. Electron Gain Enthalpy

Electron gain enthalpy is the energy change when an electron is added to an isolated gaseous atom. For most elements, this process is exothermic, giving a negative electron gain enthalpy.

Important exceptions include beryllium, magnesium, nitrogen, and noble gases, which do not readily accept an additional electron under the conditions considered.

For Group 16, the magnitude order given in the notes is:

Sulfur > Selenium > Tellurium > Polonium > Oxygen

For Group 17, the order is:

Chlorine > Fluorine > Bromine > Iodine

Chlorine has a greater magnitude of electron gain enthalpy than fluorine because fluorine’s compact 2p subshell experiences greater electron-electron repulsion when an electron is added.

Remember to distinguish between the magnitude of electron gain enthalpy and its actual signed value when solving numerical or order-based questions.

6. Electronegativity

Electronegativity describes an atom’s tendency to attract shared electrons in a chemical bond. The general trend is an increase across a period and a decrease down a group.

Important approximate Pauling-scale values from the notes include:

• Fluorine: 4.0

• Oxygen: 3.5

• Nitrogen: 3.0

• Chlorine: approximately 3.0

• Carbon: 2.5

• Sulfur: 2.5

• Iodine: 2.5

• Hydrogen: 2.1

Fluorine has the highest electronegativity. For comparison questions, focus on the positions of the given elements and the exceptions to general periodic trends.

7. Nature of Oxides

The nature of oxides generally changes across a period from basic to amphoteric and then acidic.

• Basic oxides: Generally formed by metallic elements, particularly those towards the left of the periodic table.

• Amphoteric oxides: Show both acidic and basic behaviour.

• Acidic oxides: Generally formed by non-metallic elements towards the right of the periodic table.

Boron forms boron oxide, B₂O₃, which is an acidic oxide. Questions may test the formula of an oxide as well as its chemical nature.

Periodic Table: Important Questions with Answers

Attempt these questions after revising the trends and exceptions above.

1. How does atomic radius generally change across a period and down a group?

Answer: Atomic radius generally decreases across a period because effective nuclear charge increases while electrons are added to the same shell. It generally increases down a group because additional electron shells are added.

2. Why is the first ionisation enthalpy of boron lower than that of beryllium?

Answer: Beryllium has a filled 2s subshell, while boron has one electron in the higher-energy 2p subshell. The 2p electron of boron is easier to remove, so boron has a lower first ionisation enthalpy.

3. Why is the magnitude of electron gain enthalpy greater for chlorine than for fluorine?

Answer: Fluorine’s small atomic size causes stronger electron-electron repulsion in its compact 2p subshell. Chlorine’s larger 3p subshell accommodates the incoming electron with less repulsion, giving chlorine a greater magnitude of electron gain enthalpy.

4. What does a large jump in successive ionisation enthalpies indicate?

Answer: It indicates that the next electron would be removed from a more stable inner shell after the valence electrons have been removed. The number of electrons removed before the large jump helps identify the number of valence electrons, particularly in s-block and p-block elements.

5. What is the general change in oxide character across a period?

Answer: Oxide character generally changes from basic on the metallic side to amphoteric in intermediate cases and acidic towards the non-metallic side.

The p-Block Elements: Important Topics to Revise

The p-Block Elements chapter includes important periodic trends, oxidation states, and differences in the behaviour of elements across groups. Pay attention to exceptions in atomic size, ionization enthalpy, and oxidation-state stability, as these can help you solve comparison-based and conceptual questions.

1. Group 13 and Atomic-Size Anomalies

Group 13 includes boron, aluminium, gallium, indium, and thallium. Their atomic sizes do not follow a perfectly regular increase down the group.

The order highlighted in the notes is:

Boron < Gallium < Aluminium < Indium < Thallium

The irregularity is linked to d-block contraction and lanthanide contraction. These effects are important when comparing the sizes and properties of elements that appear to contradict the general trend.

2. Group 14 Elements

Group 14 consists of carbon, silicon, germanium, tin, and lead.

Their atomic numbers are:

• Carbon: 6

• Silicon: 14

• Germanium: 32

• Tin: 50

• Lead: 82

Revise their order in the periodic table and the trends discussed in your notes. The supplied material also identifies tin as having the lowest melting point in the comparison covered.

3. Inert-Pair Effect

The inert-pair effect becomes increasingly important in heavier p-block elements. The outermost s-electron pair becomes less readily involved in bonding, making the lower oxidation state more stable down the group.

Important examples include:

• Thallium: The +1 oxidation state is more stable than +3.

• Lead: The +2 oxidation state is more stable than +4.

Thallium in the +3 oxidation state tends to gain electrons and convert to the more stable +1 state. This explains why Tl³⁺ acts as an oxidising agent.

4. Group 15 and Ionization Enthalpy

The Group 15 elements discussed in the notes include nitrogen, phosphorus, arsenic, and antimony. Ionization enthalpy decreases down the group.

The decreasing order is:

Nitrogen > Phosphorus > Arsenic > Antimony

Learn the order and understand the general reason behind it: atomic size and shielding increase down the group, making the removal of an electron easier.

The notes also highlight the relative stability of bismuth in the +3 oxidation state and its resistance to disproportionation in the general situation discussed.

5. Oxidation States and Oxide Nature

Oxidation states help explain the chemical behaviour of p-block elements. In heavier groups, the lower oxidation state often becomes more stable down the group because of the inert-pair effect.

You should also revise how the nature of oxides changes across a period. Boron oxide, B₂O₃, is an important example of an acidic oxide.

The p-Block Elements: Important Questions with Answers

Use these questions to revise Group 13, Group 14, Group 15, and the inert-pair effect.

1. What is the atomic-size order of Group 13 elements given in the notes?

Answer: The order is:

Boron < Gallium < Aluminium < Indium < Thallium

This irregular order is associated with d-block contraction and lanthanide contraction.

2. What is the inert-pair effect? Give two examples.

Answer: The inert-pair effect refers to the increasing stability of the lower oxidation state in heavier p-block elements because the outermost s-electron pair is less readily involved in bonding.

Examples include Tl⁺ being more stable than Tl³⁺ and Pb²⁺ being more stable than Pb⁴⁺.

3. Why does thallium in the +3 oxidation state act as an oxidising agent?

Answer: Tl³⁺ tends to gain electrons and convert to the more stable Tl⁺ state. By accepting electrons, Tl³⁺ acts as an oxidising agent.

4. Arrange nitrogen, phosphorus, arsenic, and antimony in decreasing order of ionisation enthalpy.

Answer:

Nitrogen > Phosphorus > Arsenic > Antimony

Ionisation enthalpy generally decreases down Group 15 as atomic size and shielding increase.

5. What is the formula and nature of boron oxide?

Answer: Boron oxide has the formula B₂O₃ and is acidic in nature.

Watch Periodic Table + The p-Block Elements FULL CHAPTER || NEET 2026 by PW to revise both chapters in one session. The video covers important previous-year questions (PYQs) alongside concepts, notes, and regular practice questions.

Follow the explanations, attempt the questions discussed in the lecture, and review the solutions to understand how the concepts are applied. Pay particular attention to the exceptions and comparison-based questions, as these require you to connect the general trends with the behaviour of individual elements.

The Periodic Table and p-Block Elements become easier to revise when you understand the general trends and learn why important exceptions occur. Practise the questions, review your mistakes, and revisit concepts that you find difficult. PW’s one-shot revision video, notes, PYQs, and practice questions can help you revise both chapters and strengthen your preparation.

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