All Free Chemistry MCQs with Answers

381. The shape of the methane molecule and the carbon dioxide molecule are respectively

• A. tetrahedral and bent
• B. trigonal planar and linear
• C. tetrahedral and linear
• D. pyramidal and bent

Explanation: Methane has four bond pairs and no lone pairs, so it is tetrahedral, while the carbon in carbon dioxide has only two regions of electron density, since each double bond counts as one region, giving a linear molecule at 180 degrees. Treating a double bond as a single region is the step candidates most often miss. This is also why carbon dioxide is non polar despite its polar bonds.

382. The bond angle in a molecule of methane, ammonia and water decreases in the order 109.5, 107 and 104.5 degrees because

• A. the central atoms become larger in that order
• B. the number of lone pairs on the central atom increases from zero to one to two
• C. the bonds become more ionic
• D. the molecules become heavier

Explanation: Each additional lone pair exerts stronger repulsion than a bond pair, squeezing the remaining bonds closer together, so the angle falls by roughly two and a half degrees for each one added. The underlying arrangement of four electron regions is tetrahedral in all three cases. This trio is the classic demonstration that lone pairs must be counted but are not seen in the shape.

383. In the ethene molecule, the carbon to carbon double bond consists of

• A. two sigma bonds
• B. two pi bonds
• C. one sigma bond formed by sp2 overlap and one pi bond formed by sideways overlap of unhybridised p orbitals
• D. one ionic and one covalent bond

Explanation: Each carbon uses three sp2 hybrids for sigma bonds to two hydrogens and to the other carbon, and the leftover p orbital on each overlaps sideways above and below the plane to give the pi bond. This is why the molecule is flat and why rotation is blocked. Every multiple bond follows the same pattern of exactly one sigma plus the remainder as pi.

384. The principal quantum number n determines

• A. the shape of the orbital
• B. the main energy level and the average distance of the electron from the nucleus
• C. the orientation of the orbital in space
• D. the direction of electron spin

Explanation: The value of n fixes the shell, so a larger n means a higher energy and an electron held further out on average. Shape is set by the azimuthal quantum number l, orientation by the magnetic quantum number m, and spin by s. Knowing which of the four quantum numbers controls which property answers a large family of questions.

385. For n equals 3, the possible values of the azimuthal quantum number l are

• A. 0 only
• B. 0 and 1
• C. 0, 1 and 2
• D. 1, 2 and 3

Explanation: The azimuthal quantum number runs from 0 up to n minus 1, so for the third shell it takes the values 0, 1 and 2, corresponding to the 3s, 3p and 3d subshells. It never equals n itself, which rules out the last option. This is why the first shell has only an s subshell.

386. The maximum number of electrons that can occupy the third shell is

• A. 8
• B. 18
• C. 32
• D. 2

Explanation: Shell capacity is 2n squared, so for n equal to 3 the maximum is 2 multiplied by 9, that is 18, made up of 2 in 3s, 6 in 3p and 10 in 3d. The figure 8 is the octet that fills only the s and p subshells, which is why it is the tempting answer. The fourth shell holds 32 by the same formula.

387. The number of orbitals in a d subshell is

• A. 3
• B. 5
• C. 7
• D. 1

Explanation: The number of orbitals in a subshell is 2l plus 1, and for a d subshell l equals 2, giving five orbitals that together hold up to ten electrons. A p subshell has three orbitals and an f subshell seven. Each orbital holds a maximum of two electrons with opposite spins.

388. A node in an orbital is a region where

• A. the electron density is maximum
• B. the probability of finding the electron is zero
• C. the nucleus is located
• D. two orbitals overlap

Explanation: The wave function passes through zero at a node, so the electron is never found there, and the number of nodes increases with the principal quantum number. A 2s orbital has one radial node while a 1s has none. Nodes are a direct consequence of treating the electron as a wave.

389. Hund's rule states that within a subshell, electrons

• A. pair up in the lowest orbital first
• B. occupy separate orbitals singly with parallel spins before any pairing occurs
• C. always have opposite spins
• D. fill the highest energy orbital first

Explanation: Spreading out into empty orbitals minimises electron electron repulsion, so nitrogen's three 2p electrons occupy the three separate p orbitals rather than crowding two into one. This is why so many transition metal ions are paramagnetic. Pairing only begins once every orbital in the subshell holds one electron.

390. The Pauli exclusion principle states that

• A. no two electrons in an atom can have the same set of all four quantum numbers
• B. electrons always pair up
• C. orbitals fill in order of increasing energy
• D. electrons occupy the nucleus

Explanation: Because the first three quantum numbers define an orbital, the principle limits any orbital to two electrons, which must then differ in spin. This single rule is what gives every shell its fixed capacity and ultimately why the periodic table has the shape it does. The filling order is the separate aufbau principle.