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.

In graphite, each carbon atom forms three covalent bonds with neighboring carbon atoms in a planar layer. Its fourth valence electron is delocalized over the layer rather than fixed in one bond. These delocalized electrons can move and carry charge, so graphite conducts electricity, especially along its layers. Option A is correct; graphite is not an ionic crystal, and its conductivity does not depend on forming ions in water.

Graphite has a layered covalent structure. Strong covalent bonds hold carbon atoms within each sheet, but the forces between adjacent sheets are weak. When graphite is rubbed between surfaces, these sheets slide over one another and reduce friction. Thus option A correctly explains its lubricating action. Option B describes diamond more closely, while graphite is neither rapidly water-soluble nor an ionic liquid, so C and D are incorrect.

Carbon has four valence electrons and therefore commonly shows valency four. In methane, its formula is CH₄: one carbon atom shares one electron with each of four hydrogen atoms. These shared pairs form four C–H single covalent bonds, giving carbon a stable octet and each hydrogen a stable duplet. The alternatives describe other or incomplete bonding, so option A is correct.

The governing concept is dominance in Mendelian inheritance. In a heterozygous individual, a dominant allele can determine the observable phenotype while the recessive allele remains present in the genotype. The recessive trait is usually expressed when suitable recessive alleles occur together, such as in a homozygous condition. Therefore A is correct. It does not appear in every condition, is not automatically lost, and is not defined as an acquired trait.

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.