a) 

Define homologous series of carbon compounds. Mention two characteristics of homologous series. Give the name and formula of the next higher homologous of HCOOH

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b) Give an explanation of why carbon can only form covalent compounds and cannot produce C⁺⁴ cations and C^-4 anions. Why are covalent compounds are bad compounds, bad conductors of electricity, and have low melting and boiling points?

a) 

A homologous series is a series of hydrocarbons that have similar chemical properties and share the same general formula. They are organic compounds having similar structures and functional groups. The constituents of the homologous series show a gradation in physical properties.
(i) They contain the same functional group throughout the series. Like a homologous series of alcohols will contain only OH groups.

(ii) All the members of a homologous series can be represented by the same general formula, like the homologous series of alkanes, which can be represented by the same formula C_n+2C_nH_(2n+2) where n is an integer starting from 1 .

HCOOH is methanoic acid as it has one carbon atom, and the carboxylic acid functional group is present over here (COOH)

o its next homologous series would be ethanoic acid CH3COOH

                                                                                              OR

b)

Carbon cannot form a C⁺⁴  cation because removing 4 electrons require a large amount of energy. Furthermore, carbon cannot form C^-4 an anion as it would be difficult for its nucleus with 6 protons to hold onto 10 electrons. Therefore Carbon atoms share electrons and form covalent compounds.
The atomic number of carbon is six. so the electronic configuration of carbon is 2,4; hene it has four electrons in its valence cell. So it requires a lot of energy if it loses its four electrons  C⁺⁴

It cannot form  C^-4 because carbon cannot form C^-4 an anion, as its nucleus with six electrons will not be able to hold ten electrons. Thus carbon achieves noble gas electronic configuration by sharing its electron with other atoms.

1. Due to the lack of electrons in covalent compounds, they are bad conductors of electricity.
2. The intermolecular force of attraction in covalent compounds is weak. So they have low melting and boiling point.