Q1. Explain what valence electrons and valency are, using Sodium (Na), Oxygen (O) and Chlorine (Cl) as examples.
Answer:
Valence electrons are the electrons present in the outermost shell of an atom. These electrons decide how an atom will react or bond with other atoms. Valency is the number of electrons an atom will gain, lose or share to achieve a stable outer shell (usually an octet).
Sodium (Na) has one valence electron; it tends to lose 1 electron, so its valency is 1 and it forms Na⁺.
Oxygen (O) has six valence electrons; it needs 2 more to complete the octet, so its valency is 2 and it commonly forms O²⁻.
Chlorine (Cl) has seven valence electrons; it needs 1 more to complete the octet, so its valency is 1 and it commonly forms Cl⁻.
In short, valence electrons determine the valency, which predicts how atoms combine to form compounds.
Q2. Describe how the octet rule helps in calculating valency and apply this to Carbon (C), Sulphur (S) and Aluminium (Al).
Answer:
The octet rule states that atoms tend to attain eight electrons in their outer shell (or two for small atoms like H and He) by losing, gaining or sharing electrons. Valency is the number of electrons gained, lost or shared to reach this stable configuration.
Carbon (C) has 4 valence electrons. To reach an octet, it needs 4 more or can share four electrons; thus its valency is 4 (forms covalent bonds like in CH₄).
Sulphur (S) has 6 valence electrons. It needs 2 more to complete the octet, so its usual valency is 2 (e.g., H₂S). Sulphur can also expand octet in higher compounds but basic valency is 2.
Aluminium (Al) has 3 valence electrons and tends to lose 3, so its valency is 3 (forms Al³⁺).
Using the octet rule gives a simple way to predict bonding behavior and valency for many main-group elements.
Q3. Why are noble gases generally inert and said to have valency zero? Give Helium and Neon as examples.
Answer:
Noble gases (like Helium and Neon) have completely filled outer shells. A filled outer shell makes an atom stable, so it has little tendency to gain, lose, or share electrons. Because of this lack of tendency to react, their valency is zero.
Helium (He) has a full first shell with 2 electrons, which is the maximum for that shell; it is very stable and chemically inactive.
Neon (Ne) has 8 electrons in its outer shell (a complete octet), making it similarly unreactive.
Being inert means they rarely form bonds under normal conditions. This stability is the reason noble gases are used in applications where non-reactive environments are needed, such as neon signs or helium in balloons.
Q4. Explain three ways atoms can achieve an octet (sharing, gaining, losing) with examples: H₂, Cl₂, NaCl, and MgO.
Answer:
Atoms achieve an octet by sharing, gaining, or losing electrons.
Sharing (covalent bond): In H₂, each hydrogen shares one electron with the other to get a pair (duet for H). In Cl₂, each chlorine atom shares one electron, so both achieve an octet. These are covalent bonds formed by mutual sharing.
Gaining (forming anions): Chlorine or oxygen can gain electrons to complete their octet. In ionic compounds, a non‑metal (like Cl) gains an electron to become Cl⁻.
Losing (forming cations): Metals like Na lose electrons to achieve a stable configuration. In NaCl, Na loses one electron (Na⁺) and Cl gains one (Cl⁻), forming an ionic bond.
Example (MgO): Magnesium (Mg) loses two electrons to become Mg²⁺ and oxygen gains two to become O²⁻; they combine to form MgO, an ionic compound.
These three methods explain most common ways atoms attain stable outer shells.
Q5. Calculate the valency of Magnesium (Mg), Aluminium (Al), Phosphorus (P) and Chlorine (Cl) and explain the reasoning.
Answer:
Magnesium (Mg): Mg has 2 valence electrons (outer shell). It tends to lose 2 electrons to achieve a full shell, so its valency is 2 (forms Mg²⁺).
Aluminium (Al): Al has 3 valence electrons; it commonly loses 3 electrons, so its valency is 3 (forms Al³⁺).
Phosphorus (P): P has 5 valence electrons. It can gain 3 electrons to reach 8, so its typical valency is 3 (as in P³⁻) or it can share electrons in covalent bonds (e.g., PCl₃ or PCl₅ shows multiple bonding possibilities). For simple valency, we say 3.
Chlorine (Cl): Cl has 7 valence electrons and needs 1 more to complete its octet, so its valency is 1 (forms Cl⁻).
The reasoning follows the difference from 8 (or how many electrons to lose/gain) and whether the atom tends to lose (metals) or gain/share (non-metals) electrons.
High Complexity (Analytical & Scenario-Based)
Q6. Determine the chemical formula for compounds formed between: (a) Sodium (Na) and Oxygen (O), (b) Magnesium (Mg) and Chlorine (Cl), (c) Aluminium (Al) and Oxygen (O). Explain how valency leads to these formulas.
Answer:
To find formulas, use the valencies and balance positive and negative charges so the compound is neutral.
(a) Na has valency 1 (Na⁺). O has valency 2 (O²⁻). To balance, two Na⁺ ions are needed for one O²⁻. Formula: Na₂O.
(b) Mg has valency 2 (Mg²⁺). Cl has valency 1 (Cl⁻). One Mg²⁺ pairs with two Cl⁻ ions. Formula: MgCl₂.
(c) Al has valency 3 (Al³⁺). O has valency 2 (O²⁻). To balance charges, two Al³⁺ (total +6) combine with three O²⁻ (total −6). Formula: Al₂O₃.
The cross-over method (swap valencies as subscripts) or charge balancing gives the simplest whole-number ratio that makes the compound neutral. This method is based directly on valency and charge balance.
Q7. An unknown element X has 5 valence electrons. Predict its likely valency, the type of bonds it will form, and write the formula of its compound with hydrogen. Explain your reasoning.
Answer:
An element with 5 valence electrons is three electrons short of an octet (8 − 5 = 3). Therefore its usual valency is 3 because it tends to gain 3 electrons or share 3 electrons to reach an octet.
Such elements are typically non-metals and form covalent bonds by sharing electrons. They can also gain electrons to form negative ions (e.g., P³⁻ for phosphorus).
With hydrogen (H), each hydrogen can provide 1 electron through sharing. To satisfy three missing electrons, the element X will bond with three H atoms, giving the formula XH₃ (for example, NH₃ where X = nitrogen).
The behavior fits common elements with 5 valence electrons (like Nitrogen), which form compounds such as NH₃ by covalent bonding and show valency 3.
Q8. Explain why some elements (especially transition metals) show variable valency, using iron (Fe) as an example. How does this differ from main-group elements?
Answer:
Variable valency means an element can form ions with different positive charges. Transition metals show this because they have d-orbitals whose electrons can participate in bonding along with s‑electrons. The number of electrons that are involved can vary, so they form multiple stable oxidation states.
Iron (Fe) commonly shows valencies 2 (Fe²⁺) and 3 (Fe³⁺). For example, FeO contains Fe²⁺, while Fe₂O₃ contains Fe³⁺. The presence of partially filled d-orbitals allows Fe to lose different numbers of electrons depending on chemical conditions.
In contrast, main-group elements (like Na, Mg, Al) usually follow predictable valencies based on achieving a noble gas configuration and thus show fixed valency.
The variable valency of transition metals leads to diverse compounds and colors, and is important in catalysis, redox reactions, and biological systems.
Q9. A compound has formula AO₂ (one atom of A combines with two oxygen atoms). Determine the valency of A and suggest which group of the periodic table A might belong to. Explain using valency and charge balance.
Answer:
Oxygen has valency 2 (forms O²⁻). In AO₂, there are two oxygen atoms, each with −2 charge, so total negative charge = −4.
For the compound to be neutral, the single A atom must have a total positive charge of +4, meaning A has valency 4 (A⁴⁺).
Elements that commonly show valency 4 include Carbon (C) and Silicon (Si) from Group 14 (also called group IV). These elements form oxides like CO₂ (carbon dioxide) and SiO₂ (silicon dioxide), which fit the AO₂ pattern.
Therefore, A likely belongs to Group 14 and has valency 4, forming covalent/ionic oxides depending on element nature. The reasoning is based purely on charge balance and the known valency of oxygen.
Q10. Scenario: Element P has valency 3 and element Q has valency 2. Predict the formula of the compound they will form and explain step-by-step how you arrive at that formula. Give a real example.
Answer:
To form a neutral compound, combine P and Q so that the total positive and negative charges cancel. P has valency 3 (P³⁺ if it loses electrons or needs 3 electrons to complete octet), and Q has valency 2 (Q²⁻).
Use the lowest common multiple of valencies (3 and 2), which is 6. Achieve +6 by taking two P atoms (2 × +3 = +6) and −6 by taking three Q atoms (3 × −2 = −6). Therefore the formula is P₂Q₃.
A real example: Aluminium (Al) has valency 3 and Oxygen (O) has valency 2. The compound formed is Al₂O₃ (aluminium oxide).
Steps: (1) write valencies Al = 3, O = 2; (2) swap valencies to become subscripts (Al₂O₃) or find LCM; (3) ensure charges balance to zer...
