Elements X, Y and Z have 4, 5 and 7 valence electrons respectively. (i) Write the molecular formula of the com

NCERT Exemplar · Q51

Q. Elements X, Y and Z have 4, 5 and 7 valence electrons respectively.

(i) Write the molecular formula of the compounds formed by these elements individually with hydrogen.

(ii) Which of these compounds will have the highest dipole moment?

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Concept understanding — Dipole Moment Applications

Dipole Moment Applications: From Intuition to Precision

Imagine you have a magnet. One end pulls, the other pushes. Now imagine a molecule that behaves like a tiny magnet — not because of iron, but because of how its electrons are distributed. That's the idea behind a dipole moment.

A dipole moment arises when there's a separation of charge inside a molecule. One end becomes slightly negative (δ−), the other slightly positive (δ+). This imbalance creates a tiny electric "arrow" pointing from positive to negative. The arrow has both a size (how much charge is separated) and a direction (which way the molecule is polar).

Note

The dipole moment is a vector quantity. Its magnitude is given by $\mu = q \times d$ , where $q$ is the magnitude of separated charge and $d$ is the distance between the charge centers. The unit is the Debye (D).

Now, why does this matter? Because this tiny electric arrow determines how a molecule behaves around other molecules, around electric fields, and even how it interacts with light. Let's see the key applications.

1. Predicting Molecular Shape (Symmetry Check)

This is the most common exam application. If a molecule has polar bonds (like C–Cl or O–H), does it have a net dipole moment? The answer depends on symmetry.

Consider carbon dioxide, CO₂. Each C=O bond is polar (oxygen pulls electrons). But the molecule is linear: O=C=O. The two dipole arrows point in opposite directions and cancel out. Net dipole moment = zero. The molecule is nonpolar.

Now consider water, H₂O. Each O–H bond is polar. But water is bent (104.5°). The two arrows do not cancel — they add up to a net dipole pointing upward through the oxygen. Net dipole moment = 1.85 D. Water is polar.

Important

Symmetry kills polarity. If a molecule has a center of symmetry or identical polar bonds arranged symmetrically, the net dipole moment is zero. This is how you distinguish between linear CO₂ (nonpolar) and bent SO₂ (polar, 1.63 D).

Exam tip: For molecules like CH₄ (tetrahedral, zero dipole) vs. CH₃Cl (tetrahedral but one C–Cl bond, dipole = 1.87 D), the key is whether the polar bonds are arranged so their vectors cancel.

2. Determining Bond Character (Ionic vs. Covalent)

The dipole moment tells you how "unequal" the sharing of electrons is in a bond. A pure covalent bond (like H–H) has zero dipole. A pure ionic bond (like Na⁺Cl⁻) would have a huge dipole — but in reality, ions are separate.

For a bond like H–Cl, the measured dipole moment is 1.08 D. If the bond were 100% ionic (one full electron transferred), the dipole would be much larger (about 6.1 D for the same bond length). The ratio gives you the percent ionic character:

\% \text{ ionic character} = \frac{\mu_{\text{observed}}}{\mu_{\text{calculated for 100\% ionic}}} \times 100

For HCl: $\frac{1.08}{6.1} \times 100 \approx 17.7\%$ . So the H–Cl bond is about 18% ionic, 82% covalent.

Tip

This is a standard numerical problem. Remember: $\mu_{\text{ionic}} = e \times d$ , where $e = 4.8 \times 10^{-10}$ esu (or $1.6 \times 10^{-19}$ C in SI). Convert bond length to cm or m accordingly.

3. Intermolecular Forces and Physical Properties

Polar molecules (with a nonzero dipole) experience dipole-dipole interactions — the positive end of one molecule attracts the negative end of another. This is stronger than the London dispersion forces in nonpolar molecules of similar size.

Consequences:

Boiling points: Polar molecules have higher boiling points than nonpolar ones of similar molar mass. Example: HCl (polar, bp −85°C) vs. F₂ (nonpolar, bp −188°C). Both have about 38 g/mol, but HCl's dipole adds extra attraction.
Solubility: "Like dissolves like." Polar solutes dissolve in polar solvents (water, ethanol). Nonpolar solutes dissolve in nonpolar solvents (hexane, CCl₄). The dipole moment explains why NaCl dissolves in water but not in oil.
Dielectric constant: Polar liquids have high dielectric constants (water = 80), meaning they can weaken the electric field between charges. This is why water is such a good solvent for ionic compounds.

Watch out

Don't confuse dipole moment with boiling point directly. A molecule can have a large dipole but low boiling point if it's very small (like HF, bp 19.5°C, dipole 1.91 D). Hydrogen bonding (a special case of dipole interaction) is even stronger.

4. Reactivity and Orientation in Electric Fields

In an external electric field, polar molecules align themselves with the field. This is the principle behind microwave heating — water molecules in food rotate to align with the alternating microwave field, generating heat through friction.

In organic chemistry, the dipole moment helps predict reaction sites. The negative end of a dipole (where electrons are concentrated) is where electrophiles attack. The positive end (electron-deficient) is where nucleophiles attack. …

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The molecular formulas are $XH_4$ , $YH_3$ , and $ZH$ . Among these, $ZH$ will have the highest dipole moment because Z is the most electronegative element, leading to the largest charge separation in the $Z-H$ bond, which is not cancelled by molecular symmetry.

To determine the molecular formulas and compare dipole moments, we need to understand how elements form bonds based on their valence electrons and how molecular geometry and electronegativity differences contribute to a molecule's overall polarity.

A dipole moment arises from the separation of positive and negative charges within a molecule. It is a vector quantity, meaning it has both magnitude and direction.

Bond Polarity: This depends on the electronegativity difference between the bonded atoms. A larger difference leads to a more polar bond.
Molecular Geometry: The overall dipole moment of a molecule is the vector sum of all individual bond dipoles and any dipoles arising from lone pairs. If a molecule is symmetrical, even if it has polar bonds, the individual bond dipoles can cancel out, resulting in a net dipole moment of zero. If the molecule is asymmetrical, the dipoles will add up to give a net dipole moment.

Let's apply these concepts to the given elements.

Part (i): Molecular formula of compounds with hydrogen

Determine Valency:
- Element X has 4 valence electrons. To achieve a stable octet, it needs 4 more electrons. It will form 4 covalent bonds.
- Element Y has 5 valence electrons. To achieve a stable octet, it needs 3 more electrons. It will form 3 covalent bonds and will have 1 lone pair ( $5 - 3 = 2$ electrons, or 1 lone pair).
- Element Z has 7 valence electrons. To achieve a stable octet, it needs 1 more electron. It will form 1 covalent bond and will have 3 lone pairs ( $7 - 1 = 6$ electrons, or 3 lone pairs).
Write Molecular Formulas with Hydrogen:
- Since hydrogen (H) has 1 valence electron and forms 1 covalent bond:
  - For X (valency 4), it will bond with 4 hydrogen atoms. The molecular formula is $\mathbf{XH_4}$ .
  - For Y (valency 3), it will bond with 3 hydrogen atoms. The molecular formula is $\mathbf{YH_3}$ .
  - For Z (valency 1), it will bond with 1 hydrogen atom. The molecular formula is $\mathbf{ZH}$ .

Part (ii): Which compound will have the highest dipole moment?

To determine the highest dipole moment, we need to consider both the polarity of the bonds and the molecular geometry of each compound. We assume X, Y, and Z are typical elements from their respective groups (Group 14, 15, and 17, respectively, if they are in the same period). Electronegativity generally increases across a period. Therefore, $EN_Z > EN_Y > EN_X$ .

Analyze $XH_4$ :
- Geometry: With 4 bond pairs and 0 lone pairs around the central atom X, the molecule will have a tetrahedral geometry (e.g., methane, $CH_4$ ).
- Bond Polarity: The X-H bonds will be polar, but the electronegativity difference between X and H is relatively small (e.g., C-H bond).
- Net Dipole Moment: Due to the highly symmetrical tetrahedral arrangement, the individual bond dipoles cancel each other out.
- Therefore, $XH_4$ will have a net dipole moment of zero.
Analyze $YH_3$ :
- Geometry: With 3 bond pairs and 1 lone pair around the central atom Y, the molecule will have a trigonal pyramidal geometry (e.g., ammonia, $NH_3$ ).
- Bond Polarity: The Y-H bonds will be polar, with Y being more electronegative than H (e.g., N-H bond).
- Net Dipole Moment: The trigonal pyramidal geometry is asymmetrical. The individual Y-H bond dipoles (pointing towards Y) and the dipole from the lone pair (pointing away from Y) add up vectorially, resulting in a significant net dipole moment.
- Therefore, $YH_3$ will have a non-zero dipole moment.
Analyze $ZH$ :
- Geometry: This is a diatomic molecule with a single Z-H bond. Its geometry is linear (e.g., hydrogen fluoride, $HF$ ).
- Bond Polarity: Z has 7 valence electrons, placing it in Group 17 (halogens). Halogens are highly electronegative. Therefore, the electronegativity difference between Z and H will be very large, making the Z-H bond highly polar. …

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Q.Elements X, Y and Z have 4, 5 and 7 valence electrons respectively.

Part (i): Molecular formula of compounds with hydrogen

Part (ii): Which compound will have the highest dipole moment?

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