Molecules & Covalent Bonds (Multiple Choice) QP - 2 PDF | Download ... - Free Printable

Let's solve each question step by step with clear explanations.

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Question 1:

The table shows the electronic structure of four atoms.

| Atom | Electronic Structure |
|------|-----------------------|
| W | 2,8,1 |
| X | 2,8,4 |
| Y | 2,8,7 |
| Z | 2,8,8 |

Which two atoms combine to form a covalent compound?

Options:
A) W and X
B) W and Y
C) X and Y
D) X and Z

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#### Step-by-step Analysis:

- Covalent compounds are formed when non-metal atoms share electrons to achieve stable electron configurations (usually octets).
- Metals tend to lose electrons (form ionic bonds), while non-metals share electrons (covalent bonds).

Let’s identify the elements based on their electron configurations:

- W: 2,8,1 → 1 valence electron → Group 1 → Metal (e.g., Sodium)
- X: 2,8,4 → 4 valence electrons → Group 14 → Non-metal (e.g., Carbon)
- Y: 2,8,7 → 7 valence electrons → Group 17 → Non-metal (e.g., Chlorine)
- Z: 2,8,8 → 8 valence electrons → Noble gas (e.g., Argon) → Unreactive

Now, which pairs form covalent compounds?

- A) W and X: Metal + Non-metal → Likely ionic bond (e.g., Na and C → not typical, but Na forms ionic compounds)
- B) W and Y: Metal + Non-metal → Ionic (e.g., NaCl)
- C) X and Y: Both non-metals → Covalent (e.g., C and Cl → CCl₄)
- D) X and Z: X is non-metal, Z is noble gas → No reaction, inert

✔ So, only X and Y are both non-metals and likely to form a covalent compound.

Answer: C) X and Y

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Question 2:

The electronic structures of atoms X and Y are shown.

- X has 1 electron (dot) → valence = 1
- Y has 7 electrons in outer shell → valence = 7

They form a covalent compound.

What is its formula?

Options:
A) XY₅
B) XY₃
C) XY
D) X₃Y

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#### Analysis:

- X has 1 valence electron → needs 1 more to complete duet (or form one bond)
- Y has 7 valence electrons → needs 1 more to complete octet → can form one covalent bond

So, X and Y can share one pair of electrons → One X bonds to one Y

Thus, the formula should be XY

But wait — let’s double-check the diagrams.

From the description:
- X: small circle with one dot → 1 valence electron
- Y: larger circle with 7 dots → 7 valence electrons

So, X is like hydrogen or lithium, but since it's forming a covalent bond, it's probably hydrogen (H), and Y is a halogen (like Cl).

But hydrogen forms HCl → H and Cl → XY

However, hydrogen typically forms only one bond, and halogens also form one bond.

So, one X bonds to one Y → XY

But wait — could it be that X is carbon? No — carbon has 4 valence electrons.

Wait — from Question 1, we know:
- X: 2,8,4 → 4 valence electrons → so 4 valence electrons
- But here, diagram shows X with only 1 dot?

Hmm — contradiction?

Wait — this may be a different X?

No — it says “the electronic structures of atoms X and Y are shown” — so this is the same X and Y as in Question 1.

But in Question 1, X has 2,8,4 → 4 valence electrons

But here, diagram of X shows only one dot → that contradicts.

Unless the diagram is showing only the valence electrons — but still, if X has 4 valence electrons, it should have four dots, not one.

Ah! There might be an error in interpretation.

Let’s look carefully:

- The diagram for X shows a single dot → implies 1 valence electron
- But from Question 1, X is 2,8,4 → 4 valence electrons → should have four dots

So either:
- The diagram is wrong, or
- This is a different X?

But the question says: "The electronic structures of atoms X and Y are shown" — implying same atoms.

Wait — perhaps the diagrams are simplified?

But no — Y has 7 dots → matches 2,8,7 → correct.

So X must have 4 valence electrons → should show four dots, but it shows one dot.

This is inconsistent.

Wait — unless the diagram is not showing all valence electrons? That doesn't make sense.

Alternatively, maybe X is not the same?

But the label is X — same symbol.

Possibility: The diagram for X is incorrect, or it's a typo.

But let’s assume the diagram is accurate — then:

- X has 1 valence electron → like hydrogen
- Y has 7 valence electrons → like chlorine

Then they form HX → HCl → XY

So formula is XY

But earlier, from Q1, X has 4 valence electrons → so cannot be H.

So there’s a conflict.

Wait — perhaps the diagrams are not showing full electron shells, but just valence electrons?

But even so, if X has 4 valence electrons, it should have four dots.

But it has one dot.

So unless it's a mistake, we must reconcile.

Alternative idea: Maybe X in Q2 is different?

But the question says "atoms X and Y", referring back to the table.

So likely, the diagram is inaccurate, or we misread.

Wait — look at the diagram again:

- X: a small circle with one dot → possibly hydrogen, but hydrogen has configuration 1 → not 2,8,4

So inconsistency.

But perhaps the diagrams are not the same atoms?

Wait — the question says: "The electronic structures of atoms X and Y are shown." — and the table defines them.

So we must trust the table.

Therefore, X has 4 valence electrons, Y has 7 valence electrons.

So X (e.g., carbon) and Y (e.g., chlorine)

Carbon has 4 valence electrons → needs 4 more → forms 4 covalent bonds

Chlorine has 7 → needs 1 → forms 1 bond

So to satisfy both, one carbon atom bonds with four chlorine atoms → CCl₄

So formula is X Y₄ → but that's not among options.

Wait — options are:
A) XY₅
B) XY₃
C) XY
D) X₃Y

None is XY₄.

But we have X with 4 valence electrons, Y with 7

So to balance:

- Each Y needs 1 electron → can accept 1
- X can donate 4 electrons → can bond with 4 Y atoms

So X Y₄

But not listed.

But option A is XY₅ — too many Y

Option B: XY₃ → only 3 Y atoms → X would have one electron left → not stable

Option D: X₃Y → three X atoms bonding to one Y → Y can only form one bond → impossible

Option C: XY → one bond → X has 3 electrons unpaired → unstable

So none seem correct?

But wait — perhaps X is not carbon?

Wait — X: 2,8,4 → atomic number = 2+8+4 = 14 → Silicon (Si)

Y: 2,8,7 → atomic number = 17 → Chlorine

Si and Cl form SiCl₄ → silicon tetrachloride → SiCl₄ → XY₄

Again, not listed.

But option A is XY₅ — not matching.

Wait — perhaps X is nitrogen? No — nitrogen is 2,5 → not 2,8,4

Wait — maybe the diagrams are misleading?

Look again at the diagram:

- X: small circle with one dot → suggests 1 valence electron → like hydrogen or alkali metal
- Y: larger circle with seven dots → 7 valence electrons → halogen

But from table, X has 4 valence electrons → so diagram is wrong?

Or perhaps the diagram is showing only the shared electrons?

No — it says "electronic structures"

Wait — perhaps X in diagram is not the same as in table?

But the question says: "The electronic structures of atoms X and Y are shown" — and refers to the table.

So likely, there's a mistake in the diagram.

But let's suppose the diagram is correct, and ignore the table?

But that would contradict Q1.

Alternatively, perhaps the diagram shows the valence electrons, and X has only one dot — so valence = 1

Then X is like hydrogen, Y is like chlorine

Then compound is HCl → XY

So answer C) XY

But from table, X has 2,8,4 → not hydrogen.

So contradiction.

Wait — unless X in Q2 is a different element?

But labeled same.

Perhaps the diagram is for a different atom?

But it says "atoms X and Y".

I think the most plausible explanation is that the diagram is inaccurate, or we're supposed to use the table, not the diagram.

But the diagram shows:

- X: one dot → valence = 1
- Y: seven dots → valence = 7

So regardless of table, if diagram is correct, then:

- X needs 1 electron
- Y needs 1 electron
- They share one pair → XY

So formula is XY

And option C is XY

But from table, X has 4 valence electrons → so it should form XY₄

But that’s not an option.

Wait — unless X in the diagram is not the same as in the table?

But the question says "the electronic structures of atoms X and Y are shown" — so it should match.

Perhaps the diagram is for a different X?

But it's labeled X.

Another possibility: The diagram shows only the valence electrons, and X has 4 electrons, but only one is shown? That doesn’t make sense.

Wait — perhaps X is boron? Boron is 2,6 → not 2,8,4

No.

Wait — 2,8,4 is silicon, atomic number 14

2,8,7 is chlorine, atomic number 17

SiCl₄ is the compound → SiCl₄ → XY₄

But not listed.

But option A is XY₅ — close, but not correct.

Option B: XY₃ — e.g., AlCl₃, but Al is metal.

But X is non-metal.

Wait — perhaps X is carbon, Y is chlorine → CCl₄ → XY₄

Still not listed.

But none of the options are XY₄

So what’s going on?

Wait — perhaps X is nitrogen? No — nitrogen is 2,5

Wait — maybe X is phosphorus? 2,8,5 — not 2,8,4

No.

Wait — perhaps the diagram is correct, and X has 1 valence electron, so it's hydrogen, and Y has 7, so chlorine

Then compound is HCl → XY

And option C is XY

But from table, X is 2,8,4 → not hydrogen

So conflict.

Unless the table and diagram are for different questions?

But the question says "the electronic structures of atoms X and Y are shown" — and refers to the table.

So I think there's a mistake in the diagram or in our understanding.

But let’s look at Q3 — it mentions X and Y again — so likely consistent.

Wait — in Q3, it says: "In the following diagrams, X and Y are atoms of different elements. Which diagram correctly shows the arrangement of outer electrons in a molecule of methane?"

Methane is CH₄ → carbon bonded to four hydrogen atoms.

So X is carbon, Y is hydrogen

So in methane, carbon is central, bonded to four hydrogens.

So X is carbon, Y is hydrogen

Then in Q2, X is carbon (4 valence electrons), Y is hydrogen (1 valence electron)? But diagram shows Y with 7 dots → that can't be.

Wait — in Q2, Y has 7 dots → 7 valence electrons → halogen

But in Q3, Y is hydrogen → 1 valence electron

Contradiction.

So X and Y are not consistent across questions?

That seems odd.

But in Q3, it says: "X and Y are atoms of different elements" — not necessarily the same as before.

So perhaps Q2 and Q3 use X and Y as labels for new atoms, not the same as in Q1.

That makes sense.

So Q1 uses X and Y from the table.

Q2 shows diagrams of X and Y — but not necessarily the same atoms.

Similarly, Q3 introduces new X and Y.

So we should treat them independently.

So for Q2, the diagram shows:

- X: one dot → 1 valence electron → e.g., hydrogen
- Y: seven dots → 7 valence electrons → e.g., chlorine

They form a covalent compound → HCl → XY

So formula is XY

Answer: C) XY

Even though it conflicts with Q1, it's a separate context.

So Answer: C) XY

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Question 3:

Which diagram correctly shows the arrangement of outer electrons in a molecule of methane?

Methane: CH₄ → carbon bonded to four hydrogen atoms.

- Carbon has 4 valence electrons → forms 4 covalent bonds
- Hydrogen has 1 valence electron → forms 1 bond

Each bond is a shared pair.

In Lewis structure, carbon is central, surrounded by four H atoms, each sharing one electron with carbon.

So, carbon shares one electron with each H, and each H shares its one electron.

Result: four single bonds, no lone pairs on carbon; each H has 2 electrons (duet).

Now look at the diagrams:

A: Central X (carbon), surrounded by four Y (hydrogen). Each bond shows two electrons shared between X and Y. Also, X has no lone pairs. Y has no extra electrons.

→ Looks good.

B: Shows only two Y atoms, and only one bond shown. Not correct.

C: Central X, four Y around. But each Y has three lone pairs and one bond → that would mean Y has 7 electrons → not hydrogen.

Hydrogen can only have 2 electrons max.

Also, X has lone pairs? No — carbon has no lone pairs in methane.

So C is wrong.

D: Central X, four Y. Bonds show shared pairs. But each Y has two dots — that’s two electrons, but hydrogen already has one from bond, so two dots would be 2 electrons — but in Lewis diagrams, we usually show only the shared pair, or sometimes lone pairs.

But if Y has two dots, that means two electrons — but hydrogen has only one electron to share. If it has two dots, that implies it has two electrons, but in covalent bond, it shares one.

But in Lewis structure, each hydrogen should have only two electrons total — the shared pair.

So the dots represent unshared electrons.

So if Y has two dots, that means two unshared electrons → but hydrogen can't have unshared electrons — it has only one electron.

So if Y has two dots, that’s two electrons, plus one in bond → three electrons → impossible.

So D is wrong.

A: Each Y has no dots → no lone pairs → correct for hydrogen

X has no dots → no lone pairs → correct for carbon

Each bond has two electrons → correct

So A is correct

B: Only two Y atoms → incomplete

C: Y has three lone pairs → 6 electrons + 1 bond → 7 electrons → impossible for H

D: Y has two dots → 2 unshared electrons → plus bond → 3 electrons → impossible

So only A is correct.

Answer: A

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✔ Final Answers:

1. C) X and Y
2. C) XY
3. A

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Summary:

1. X and Y are both non-metals → form covalent compound.
2. From diagram, X has 1 valence electron, Y has 7 → form XY (like HCl).
3. Methane has carbon bonded to four hydrogens, each sharing one electron → diagram A shows correct electron sharing.

✔ Answers: 1-C, 2-C, 3-A