Covalent bonding

Carbon has atomic number 6, so its electronic configuration is 2,4. It has four electrons in its valence shell and generally shares four electrons to complete a stable outer shell. Consequently, carbon forms four covalent bonds and has valency four. Nitrogen commonly shows valency three, while one and two are not the usual valencies tested for neutral carbon in this context. Hence, option D is correct.

Saturated hydrocarbons are compounds in which carbon atoms are joined only by single covalent bonds; the alkane family is the standard example. Each carbon uses shared electron pairs to complete its valence, and no carbon–carbon double or triple bond is present. A double bond characterises alkenes and a triple bond characterises alkynes, both of which are unsaturated. Therefore, option A is correct, while B, C, and D conflict with saturation.

The governing concept is covalent bonding and valency. Methane has the molecular formula CH₄. Carbon has valency four, meaning it needs four shared electron pairs to complete its outer shell. One carbon atom therefore forms four single covalent bonds, one with each of four hydrogen atoms. Hence option D is correct. A formula such as CH₃ would not represent the neutral methane molecule, so one, two or three are incorrect.

The governing concept is electrical conduction by mobile charged particles. In most covalent compounds, atoms share electrons to form neutral molecules, so the electrons are localized in bonds and free ions are absent. Without mobile charges, an electric current cannot pass easily. Therefore option A is correct. Option B is false because covalent substances are generally non-metals, while option C describes ionic behavior that is not typical of them.

Melting and boiling require separation of particles. Many covalent substances consist of separate molecules, and the intermolecular forces holding those molecules together are relatively weak. Consequently, comparatively little thermal energy is needed to overcome these forces, giving low melting and boiling points. Option A is correct. Option B refers to ionic lattices, which usually require much more energy, while options C and D are not general explanations.

The governing concept is the raw-material requirement of photosynthesis. Plants take in carbon dioxide from the atmosphere through stomata, while water comes mainly from the roots. Using light energy and chlorophyll, carbon dioxide contributes carbon to the carbohydrates formed. Thus option A is correct; oxygen is mainly released as a product, and nitrogen or hydrogen gas is not the principal gas taken in for this process.

Carbon has a valency of 4, so it can share its four outermost electrons and form four covalent bonds. Having four neutrons does not define tetravalency; the number of neutrons can vary among isotopes. Exam tip: ‘Tetra’ means four, so link tetravalency with four bonds.

Carbon has atomic number 6 and the electronic configuration 2, 4. Hence, its outermost shell contains 4 electrons, which are called valence electrons. The other 2 electrons are in the inner K shell, so 2 is not the correct answer. Exam tip: To find valence electrons, count the electrons in the outermost shell of the electronic configuration.