Valency – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain how valence electrons decide the chemical activity of an element. Compare noble gases with oxygen.
Answer:
- The electrons in the outermost shell are called valence electrons.
- They control how an atom reacts with other atoms.
- If the outer shell is full (2 or 8 electrons), the atom is stable.
- Noble gases like Helium and Neon have full shells. They are least reactive.
- Oxygen has 6 valence electrons. It is reactive because it wants 2 more.
- So, more the need to reach an octet, more the reactivity.
- Valence electrons thus decide valency and chemical behavior.
Q2. Using the octet rule, explain why Sodium reacts with Chlorine to form NaCl.
Answer:
- Atoms try to achieve a full outer shell (the octet rule).
- Sodium (Na) has 1 valence electron. It tends to lose this one.
- Chlorine (Cl) has 7 valence electrons. It tends to gain one.
- Sodium loses 1 electron and becomes Na+. Chlorine gains 1 electron and becomes Cl-.
- They attract each other and form an ionic bond.
- The product NaCl is stable because both ions reach stable configurations.
- This transfer shows valency: Na is 1, Cl is 1.
Q3. Calculate the valency of Oxygen, Magnesium, Aluminium, Chlorine, and Sulphur. Explain each case.
Answer:
- Oxygen has 6 valence electrons. It needs 2 more to reach 8. So, valency is 2.
- Magnesium has 2 valence electrons. It loses 2 to become stable. Valency is 2.
- Aluminium has 3 valence electrons. It loses 3 easily. Valency is 3.
- Chlorine has 7 valence electrons. It gains 1 to complete 8. Valency is 1.
- Sulphur has 6 valence electrons. It generally gains or shares 2. Valency is 2.
- These choices follow the octet goal: reach 8 in the outer shell.
- So, valency equals electrons gained, lost, or shared to reach stability.
Q4. Why do noble gases have valency zero? Explain with Helium and Neon.
Answer:
- Noble gases already have a full outer shell.
- Helium has 2 electrons in its only shell. This is full for the first shell.
- Neon has 8 electrons in its outer shell. This is a complete octet.
- Atoms with full shells are stable. They do not need to gain, lose, or share.
- So, they show little chemical activity and are unreactive.
- As they do not combine easily, their valency is zero.
- This explains why noble gases rarely form compounds.
Q5. Explain how atoms choose between losing, gaining, or sharing electrons. Use Hydrogen and Fluorine as examples.
Answer:
- Atoms act to reach a stable shell (2 or 8 electrons).
- If an atom has 1, 2, or 3 valence electrons, it tends to lose them.
- If it has 5, 6, or 7, it tends to gain electrons.
- Hydrogen can become stable by losing 1 electron, or by sharing to reach a duplet.
- Fluorine has 7 valence electrons. It prefers to gain 1 electron.
- When atoms share, they form covalent bonds. When they transfer, they form ionic bonds.
- The path chosen depends on which way gives easy stability.
High Complexity (Analysis & Scenario-Based)
Q6. A student finds that element X has 7 valence electrons. Predict its valency and bonding with Sodium. Explain fully.
Answer:
- With 7 valence electrons, X needs 1 more to reach octet.
- So, X will likely gain 1 electron. Its valency is 1.
- Sodium (Na) has 1 valence electron. It tends to lose 1 electron.
- Sodium can transfer its electron to X. They form an ionic bond.
- Sodium becomes Na+. X becomes X-. Both become stable.
- The compound formed will have the formula NaX.
- This is similar to NaCl, where Cl also has 7 valence electrons.
Q7. Compare the reactivity of Magnesium, Aluminium, and Neon using valence electrons. Rank them with reasons.
Answer:
- Neon has a full outer shell (8). It is least reactive.
- Magnesium has 2 valence electrons. It can lose 2 easily.
- Aluminium has 3 valence electrons. It can lose 3, but needs more energy than Mg.
- So, among metals here, Magnesium reacts more readily than Aluminium.
- Neon does not react because its valency is zero.
- The order of reactivity is: Magnesium > Aluminium > Neon.
- This order follows the drive to reach stability quickly.
Q8. You want to write formulas using valency. Derive the formulas of compounds formed by Magnesium with Chlorine and with Oxygen.
Answer:
- Magnesium (Mg) has valency 2. It tends to lose 2 electrons.
- Chlorine (Cl) has valency 1. It tends to gain 1 electron.
- To balance charges, Mg needs two Cl atoms. So the formula is MgCl2.
- Oxygen (O) has valency 2. It needs 2 electrons.
- One Mg atom can give both electrons to one O atom.
- So the formula for magnesium oxide is MgO.
- These formulas come from balancing valencies to reach stability.
Q9. Water has the formula H2O. Explain how Oxygen and Hydrogen achieve stability in this molecule.
Answer:
- Oxygen has 6 valence electrons. It needs 2 more to reach octet.
- Hydrogen has 1 electron. It needs 1 more to reach a duplet.
- Oxygen shares one electron with each Hydrogen.
- Two O–H covalent bonds are formed by sharing.
- Oxygen reaches 8 electrons in its outer shell after sharing.
- Each Hydrogen reaches 2 electrons after sharing.
- So, both Oxygen and Hydrogen become stable in H2O.
Q10. An unknown element Y forms the oxide Y2O3. Using oxygen’s valency, find Y’s valency and explain its electron change.
Answer:
- In oxides, Oxygen usually has valency 2.
- The formula Y2O3 means 2 atoms of Y combine with 3 atoms of O.
- Total valency on oxygen side is 3 × 2 = 6.
- So, total valency on Y side must also be 6 for balance.
- With 2 Y atoms, each Y must have valency 3.
- Therefore, each Y likely loses 3 electrons to become stable.
- This is similar to Aluminium (valency 3) forming Al2O3.