How to Solve: Valency Problems

How to Solve: Valency Problems

(For Students Who Want to Ace Their Exam & Teachers Who Need a Ready-to-Record Script)

Introduction

"If you can’t balance a chemical formula or predict an ion’s charge, you’re losing easy marks on every chemistry exam—let’s fix that in 10 minutes."

What You Need To Know First

Before tackling valency problems, you must understand: 1. Atomic structure – Protons, neutrons, electrons, and electron shells. 2. Periodic table basics – Groups (columns) and periods (rows), especially Groups 1, 2, 13–18. 3. Ions and charges – How atoms gain/lose electrons to form cations (+) or anions (–).

If any of these are unclear, pause and review them first.

Key Vocabulary

Formulas To Know

1. Valency from Group Number (Main-Group Elements)

Formula: - Group 1 (Alkali metals): Valency = +1 - Group 2 (Alkaline earth metals): Valency = +2 - Group 13 (Boron group): Valency = +3 - Group 14 (Carbon group): Valency = ±4 (can be +4 or –4) - Group 15 (Nitrogen group): Valency = –3 (or +3, +5 in some compounds) - Group 16 (Oxygen group): Valency = –2 - Group 17 (Halogens): Valency = –1 - Group 18 (Noble gases): Valency = 0 (do not form ions)

MEMORISE THIS – Examiners expect you to recall these instantly.

2. Charge of an Ion

Formula: Charge = (Number of protons) – (Number of electrons)

Variables: - Protons = Atomic number (from periodic table). - Electrons = Atomic number ± electrons gained/lost.

Example: - Sodium (Na) has 11 protons. If it loses 1 electron, charge = +1.

MEMORISE THIS – Critical for predicting ion charges.

3. Criss-Cross Method (for Writing Formulas)

Formula: 1. Write the symbols of the two elements/ions. 2. Write their valencies as superscripts. 3. Criss-cross the valencies (ignore signs). 4. Simplify the ratio if possible.

Example: - Aluminum (Al³⁺) + Oxygen (O²⁻) → Al₂O₃

MEMORISE THIS – The fastest way to write correct formulas.

Step-by-Step Method

How to Solve Any Valency Problem

Follow these steps in order. Do not skip.

1. Identify the elements/ions involved.
2. Write their symbols (e.g., Na, Cl, SO₄).

3. If it’s a polyatomic ion, use its formula (e.g., NO₃⁻, NH₄⁺).

4. Determine the valency of each.

5. Use the group number rules (see above).

6. For polyatomic ions, memorise common charges (e.g., SO₄²⁻, CO₃²⁻, OH⁻).

7. Apply the criss-cross method (if writing a formula).

8. Write the valencies as superscripts.
9. Swap and drop the numbers (ignore signs).

10. Simplify the ratio (e.g., Ca²⁺ + O²⁻ → CaO, not Ca₂O₂).

11. Check for neutrality (if forming a compound).

12. The total positive charge must equal the total negative charge.

13. Example: In Al₂O₃, (2 × +3) + (3 × –2) = 0.

14. Adjust for polyatomic ions (if needed).

15. Treat the polyatomic ion as a single unit.

16. Example: Ca²⁺ + NO₃⁻ → Ca(NO₃)₂ (brackets show two nitrate ions).

17. Verify with oxidation states (for advanced problems).

18. The sum of oxidation states in a neutral compound = 0.
19. Example: In H₂SO₄, H = +1, O = –2 → S must be +6 to balance.

Worked Example Using the Steps

Problem: Write the formula for calcium chloride.

1. Identify elements:

2. Calcium (Ca), Chlorine (Cl).

3. Determine valencies:

4. Ca is in Group 2 → Valency = +2.

5. Cl is in Group 17 → Valency = –1.

6. Criss-cross:

7. Ca²⁺ + Cl⁻ → CaCl₂ (swap 2 and 1, drop signs).

8. Check neutrality:

9. (1 × +2) + (2 × –1) = 0. ✔️

10. No polyatomic ions here.

11. Final formula: CaCl₂.

Worked Examples

Example 1 – Basic: Sodium Oxide

Problem: Write the formula for sodium oxide.

1. Elements: Na, O.
2. Valencies:
3. Na (Group 1) = +1.
4. O (Group 16) = –2.
5. Criss-cross:
6. Na⁺ + O²⁻ → Na₂O.
7. Neutrality check:
8. (2 × +1) + (1 × –2) = 0. ✔️
9. Final formula: Na₂O.

What we did and why: - Used group numbers to find valencies. - Criss-crossed to balance charges. - Verified neutrality to ensure correctness.

Example 2 – Medium: Aluminum Sulfate

Problem: Write the formula for aluminum sulfate.

1. Elements/ions: Al, SO₄ (sulfate is polyatomic).
2. Valencies:
3. Al (Group 13) = +3.
4. SO₄ (memorised) = –2.
5. Criss-cross:
6. Al³⁺ + SO₄²⁻ → Al₂(SO₄)₃.
7. Neutrality check:
8. (2 × +3) + (3 × –2) = 0. ✔️
9. Brackets for polyatomic ion:
10. Shows 3 sulfate ions.
11. Final formula: Al₂(SO₄)₃.

What we did and why: - Treated sulfate as a single unit. - Used brackets to avoid confusion. - Balanced charges for a neutral compound.

Example 3 – Exam Style: Disguised Problem

Problem: A compound contains potassium (K) and an unknown element X. The formula is K₂X. What is the valency of X?

1. Identify known valency:
2. K (Group 1) = +1.
3. Set up neutrality equation:
4. Total positive charge = (2 × +1) = +2.
5. Total negative charge must = –2 to balance.
6. Solve for X:
7. If K₂X is neutral, X must be –2.
8. Conclusion:
9. Valency of X = –2.

What we did and why: - Used the neutrality rule to work backward. - Recognised that the subscript (2) applies to potassium’s charge. - Deduced X’s valency without extra information.

Common Mistakes

Exam Traps

1-Minute Recap

"Okay, let’s lock this in. Valency is just how many electrons an atom gains, loses, or shares. For main-group elements, it’s the group number—Group 1 is +1, Group 2 is +2, Group 17 is –1, and so on. Polyatomic ions? Memorise the big ones: sulfate is –2, nitrate is –1, ammonium is +1.

To write a formula, criss-cross the valencies, simplify the ratio, and check that the charges cancel out. If the question gives you a formula like K₂X, work backward—potassium is +1, so X must be –2 to balance.

Watch out for traps: don’t ignore signs, don’t forget to simplify, and don’t assume all metals are +2. Group 1 is +1, Group 2 is +2—no exceptions.

Spend 5 minutes tonight writing formulas for 10 random compounds. If you can do that, you’ve got this. Good luck!