Periodic Classification — Study Notes

Overview

The periodic table is chemistry's foundational tool, organizing all known elements by atomic number and electron configuration. For Railway Group D exams, questions test your knowledge of the modern periodic table's structure, element groups, and how properties change predictably across periods and down groups. Understanding periodic trends — atomic size, valency, metallic character, reactivity — lets you predict element behavior without memorizing every single element's properties.

This topic typically yields 1–2 direct questions in the General Science section. Expect questions on group names (alkali metals, halogens), period/group numbers for given elements, or identifying which property increases/decreases in a particular direction. Master the 18-group modern table layout, first 20 elements, and the four major periodic trends. This knowledge also supports other chemistry topics like chemical bonding, metals/non-metals, and reactivity series.

Candidates often confuse Mendeleev's original table with the modern one or mix up trend directions. Focus on the modern periodic law (arrangement by atomic number, not atomic mass) and visualize the table's geography — metals on the left, non-metals on the right, metalloids along the staircase.

Key Concepts

• **Modern Periodic Law**: Elements are arranged in order of increasing atomic number (number of protons). Properties repeat periodically when elements are arranged this way. This corrected Mendeleev's atomic mass-based arrangement.

• **Periods and Groups**: The table has 7 horizontal rows (periods) and 18 vertical columns (groups). Period number = number of electron shells in an atom. Group number often relates to valence electrons (Group 1 = 1 valence electron, Group 17 = 7 valence electrons).

• **Metals, Non-metals, Metalloids**: Metals occupy the left and center (about 78% of elements), non-metals the upper right corner, and metalloids form a diagonal staircase boundary (B, Si, Ge, As, Sb, Te, At).

• **Valency**: The combining capacity of an element. For Groups 1–2, valency equals group number. For Groups 13–17, valency = 18 – group number (e.g., Group 17 has valency 1). Group 18 noble gases have valency 0 (stable, unreactive).

• **Atomic Size (Radius)**: Decreases left to right across a period (increasing nuclear charge pulls electrons closer), increases top to bottom down a group (additional electron shells added).

• **Metallic Character**: Increases down a group (easier to lose electrons with more shells) and decreases across a period left to right (non-metallic character increases toward the right).

• **Reactivity Patterns**: For metals, reactivity increases down a group (Group 1: Li < Na < K). For non-metals, reactivity decreases down a group and increases across a period toward Group 17 (halogens are highly reactive).

• **Noble Gases (Group 18)**: Helium, Neon, Argon, Krypton, Xenon, Radon — chemically inert with completely filled outermost shells, making them stable and unreactive.

Formulas / Key Facts

• **Modern Periodic Law**: Properties of elements are periodic functions of their atomic numbers.

• **Total elements discovered**: 118 (as of current knowledge); first 20 are most important for this exam.

• **First 20 elements by atomic number**: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca.

• **Group names to remember**:
• Group 1 = Alkali metals (Li, Na, K, Rb, Cs, Fr) — highly reactive metals
• Group 2 = Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra)
• Group 17 = Halogens (F, Cl, Br, I, At) — highly reactive non-metals
• Group 18 = Noble gases (He, Ne, Ar, Kr, Xe, Rn) — inert gases

• **Valency formula**: Groups 1–2 valency = group number. Groups 13–17 valency = 18 – group number. Group 18 valency = 0.

• **Electronic configuration shorthand**: Period number = number of shells. Group number (for main groups) relates to valence electrons. Example: Sodium (Na, atomic number 11) is in Period 3, Group 1, so it has 3 shells and 1 valence electron.

• **Periodic trend directions**:
• Atomic size: decreases → across period, increases ↓ down group
• Metallic character: decreases → across period, increases ↓ down group
• Non-metallic character: increases → across period, decreases ↓ down group

• **Mendeleev vs Modern**: Mendeleev arranged by atomic mass and left gaps for undiscovered elements. Modern table arranges by atomic number, resolving anomalies (e.g., Co-Ni, Ar-K positions).

Worked Examples

**Example 1: Identify period and group** *Q: An element has atomic number 17. Which period and group does it belong to?*

**Solution**: Step 1: Atomic number 17 = Chlorine (Cl). Step 2: Electronic configuration = 2, 8, 7 (3 shells). Step 3: Number of shells = 3 → Period 3. Step 4: Valence electrons = 7 → Group 17. **Answer**: Period 3, Group 17 (Halogens).

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**Example 2: Predict valency** *Q: What is the valency of an element in Group 15?*

**Solution**: Step 1: Use formula valency = 18 – group number. Step 2: Valency = 18 – 15 = 3. **Answer**: Valency is 3 (e.g., Nitrogen, Phosphorus both have valency 3).

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**Example 3: Compare atomic sizes** *Q: Arrange Na, Mg, Al in order of decreasing atomic size.*

**Solution**: Step 1: All three are in Period 3. Step 2: Atomic size decreases left to right across a period. Step 3: Order in table: Na (Group 1) → Mg (Group 2) → Al (Group 13). **Answer**: Na > Mg > Al (sodium largest, aluminum smallest).

Common Mistakes

• **Confusing valency formula**: Students often use group number = valency for all groups. **Fix**: Remember valency = 18 – group number for Groups 13–17. Group 18 has valency 0.

• **Mixing atomic number with atomic mass**: Questions may give atomic mass and ask about position; you need atomic number for placement. **Fix**: Atomic number determines position in modern periodic table, not atomic mass.

• **Reversing trend directions**: Saying atomic size increases left to right is a frequent error. **Fix**: Visualize: moving right, nuclear charge increases, pulling electrons tighter, so size decreases. Moving down, new shells added, so size increases.

• **Assuming all metals are reactive**: Students think all metals behave like sodium or potassium. **Fix**: Reactivity varies. Group 1 metals are highly reactive; transition metals (like gold, silver) are less reactive.

• **Forgetting noble gas inertness**: Treating Group 18 like other groups in reactivity questions. **Fix**: Noble gases have complete outer shells; they do not readily form compounds. Valency = 0.

Quick Reference

• Modern periodic table has **7 periods, 18 groups**; arranged by **atomic number**.
• **Valency**: Groups 1–2 = group number; Groups 13–17 = (18 – group number); Group 18 = 0.
• **Atomic size**: decreases → across period, increases ↓ down group.
• **Metallic character**: decreases → across period, increases ↓ down group.
• **Group 1 = Alkali metals** (Li, Na, K); **Group 17 = Halogens** (F, Cl, Br, I); **Group 18 = Noble gases** (He, Ne, Ar).
• **First 20 elements**: H to Ca — know their symbols and approximate positions.