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AFCAT Periodic Table

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This page covers AFCAT Periodic Table with complete concept notes, 8 graded practice MCQs, key points and exam-specific tips. Free to study.

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Concept Notes

Periodic Table— Rules & Concept

Core ConceptRead this first — the foundation of the topic

The Periodic Table is a systematic arrangement of all known elements based on their atomic numbers. In RRB Group D exams, this topic appears in almost every paper with 2-3 questions guaranteed. Understanding the periodic table helps solve questions on element properties, chemical behavior, and atomic structure. Core Concept: Elements are arranged in horizontal rows called PERIODS and vertical columns called GROUPS. The position of an element tells us its properties. There are 18 groups and 7 periods in the modern periodic table.

Key RulesCore rules you must know cold
1

Atomic size DECREASES from left to right in a period

2

Atomic size INCREASES from top to bottom in a group

3

Metallic character DECREASES from left to right

4

Non-metallic character INCREASES from left to right

5

Ionization energy INCREASES from left to right

6

Electronegativity INCREASES from left to right

Formula BlockMemorise — at least one formula appears in every paper
- Group Number = Number of valence electrons (for groups 1, 2, 13-18)
- Period Number = Number of electron shells
- Atomic Number = Number of protons = Number of electrons
- Mass Number = Protons + Neutrons
Exam PatternsWhat examiners ask — read before attempting PYQs

Questions focus on element positions, group properties, periodic trends, and identifying elements from given data. Common question types include finding group/period of elements, comparing atomic sizes, and predicting chemical behavior.

ShortcutsUse these to save 30–60 seconds per question

- Noble Gas Rule: Elements in Group 18 are noble gases with complete outer shells. They have 8 electrons in outer shell (except Helium with 2). Any element trying to achieve noble gas configuration will gain/lose electrons accordingly.

Worked ExampleSolve this step-by-step before moving on
1
Step 1

Write electronic configuration: 2, 8, 7

2
Step 2

Count electron shells = 3 shells = Period 3

3
Step 3

Count valence electrons = 7 electrons = Group 17 Answer: Chlorine is in Group 17, Period 3 Worked Example 2: Compare atomic sizes of Sodium (Na, Group 1) and Chlorine (Cl, Group 17) in Period 3.

1
Step 1

Both are in same period (Period 3)

2
Step 2

Apply periodic trend - atomic size decreases left to right

3
Step 3

Sodium is on left, Chlorine is on right Answer: Sodium has larger atomic size than Chlorine Shortcut for Valency: Group 1 = valency 1, Group 2 = valency 2, Group 13 = valency 3, Group 14 = valency 4, Group 15 = valency 3, Group 16 = valency 2, Group 17 = valency 1, Group 18 = valency 0 Most

Exam TrapsCommon mistakes students make — avoid these

Students confuse GROUP NUMBER with VALENCY for groups 15, 16, 17. Remember - Group 15 has 5 valence electrons but valency is 3 (8-5=3). Group 16 has 6 valence electrons but valency is 2 (8-6=2).

Group 17 has 7 valence electrons but valency is 1 (8-7=1). Always subtract from 8 for these groups to find valency.

Memory HookRemember this — never confuse the two again

Trick: 'LION' - Lithium is in group 1, period 1 (Li-1-1). This helps remember that period = number of shells, group = valence electrons. The modern periodic table was given by Henry Moseley based on atomic numbers, not atomic masses like Mendeleev's table.

Key Points to Remember

  • Modern periodic table has 118 elements arranged in 18 groups and 7 periods
  • Group number equals valence electrons for groups 1, 2, and 13-18
  • Period number equals number of electron shells in an atom
  • Atomic size decreases left to right in period, increases top to bottom in group
  • Metallic character decreases left to right, increases top to bottom
  • Valency formula: Groups 15,16,17 = 8 minus valence electrons
  • Noble gases (Group 18) have complete outer electron shells
  • Ionization energy increases left to right in a period
  • Electronegativity increases left to right, decreases top to bottom
  • Elements in same group have similar chemical properties

Exam-Specific Tips

  • Modern periodic table was proposed by Henry Moseley in 1913
  • Hydrogen is placed in Group 1 but behaves like Group 17 elements
  • Helium has only 2 electrons in outer shell but is a noble gas
  • Group 1 elements are called Alkali metals (except Hydrogen)
  • Group 2 elements are called Alkaline earth metals
  • Group 17 elements are called Halogens
  • Group 18 elements are called Noble gases or Inert gases
  • Mendeleev left gaps for Gallium (Eka-aluminum) and Germanium (Eka-silicon)
Practice MCQs

Periodic Table — Practice Questions

8graded MCQs · easy to hard · full solution & trap analysis

All MCQs →
Practice 1easy

Which of the following elements is located in Period 3, Group 13 of the periodic table?

Practice 2medium

An element has atomic number 17 and mass number 35. How many neutrons are present in an atom of this element?

Practice 3medium

Which block of the periodic table contains elements whose valence electrons are in d-orbitals?

Practice 4medium

The atomic radius of elements generally decreases across a period (left to right) in the periodic table. Which of the following best explains this trend?

Practice 5medium

An atom of element X has 8 protons and 8 neutrons. An atom of element Y has 8 protons and 10 neutrons. What is the relationship between X and Y?

Practice 6medium

Which of the following elements belongs to Group 17 (Halogens) of the periodic table?

Practice 7medium

An element has atomic number 17 and mass number 35. How many neutrons does this atom contain?

Practice 8hard

An element has an atomic number of 17 and a mass number of 35. How many neutrons does this atom contain, and in which block of the periodic table is this element located?

60-Second Revision — Periodic Table

  • Remember: Group = valence electrons, Period = electron shells
  • Formula: Valency of groups 15,16,17 = 8 minus group valence electrons
  • Trend: Atomic size decreases left to right, increases top to bottom
  • Trap: Don't confuse group number with valency for groups 15-17
  • Key: Noble gases have complete outer shells (8 electrons except He)
  • Fact: Modern periodic table arranged by atomic number, not atomic mass
  • Quick check: Same group elements have same valence electrons
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