The halogens are the most reactive elements as a family. Fluorine is the most reactive of all the halogens. The reactivity of the halogens decreases down the group. The high reactivity of halogens is due to the following reasons:

(i) Low dissociation energies

All the halogens have very low dissociation energies. As a result, they can readily dissociate into atoms and react with other substances. As shown below, the dissociation energies of halogens are quite low in comparison to common molecules such as H2, O2 and N2.

(ii) High electron affinity

Halogens have very high electron affinity values and therefore, have very strong tendency to gain an electron. Thus halogens are very reactive elements due to their low dissociation energies and high electron affinity values. As clear from the values of bond dissociation energies, fluorine has the lowest bond dissociation energy. This is due to weak F-F bond because of the repulsion between the non-bonding electrons in the small molecule. Therefore, it is most reactive among the halogens.

Some of the important chemical reactions of halogens are discussed ahead.

All matter can be broadly divided into two major groups “Pure” and “Impure”. The term ‘purity’ has quite a different meaning in chemistry than in our day-to-day life. Normally when we refer to pure water, pure milk, etc., what is implied is that the water, milk etc., are free from harmful substances such as bacteria, fungi, viruses, etc. ‘Purity’ in chemistry is entirely of a different nature. When we say a substance is pure, it means that the substance is made of only one type of constituent particles.

Example: In chemical terms, pure water means that it is made of only one type of molecules i.e., H2O.

As mentioned above matter can be divided into pure and impure substances. The pure substances can be further divided into “Elements” and “Compounds”. The impure substances, commonly called “Mixtures” can also be divided further into ‘Homogeneous’ and ‘Heterogeneous’ mixtures.

Vitamins are small molecules (where small is relative to proteins!) that a living organism cannot synthesize, but are nevertheless required. The word vitamin was coined by a Polish biochemist, Kazimierz Funk by sandwiching together “vital” and “amine”. Not all vitamins turned out to be amines (molecules with an NH2 group in them), however the name stuck.

One such non-amine “vital amine” has the structure shown below. It’s a carboxylic acid (the COOH group). Originally designated as vitamin PP, it is now better known as the third of the B vitamin complex or B3. PP stood for pellagra preventing factor. Pellagra is a nutritional deficiency, once common in Italy, that results in rough skin – pella is Italian for skin.

The original common chemical name for B3 was nicotinic acid. (The synthetic form can be made by oxidizing nicotine with nitric acid.) In the late 1930s, niacin (NIcotinic ACid vitamIN) was adopted as the preferred name, to avoid confusion with nicotine. (I’m unclear why this was undesirable; smoking was pervasive.)

Repackaging scientific terms to make them less frightening for the general public is not just a historical phenomenon. Much more recently the application of NMR (nuclear magnet resonance) to medical imaging saw its “nuclear” dropped (thus forestalling any potential association with nuclear radiation) to become MRI (magnetic resonance imaging). It should be made clear, that like nicotinic acid, which contains no nicotine, NMR does not require nuclear radiation.

Enzymes

Chemistry is the study of matter and the interactions between different types of matter and energy. The fundamental building block of matter is the atom. An atom consists of three main parts: protons, neutrons, and electrons. Protons have a positive electrical charge. Neutrons have no electrical charge. Electrons have a negative electrical charge. Protons and neutrons are found together in what is called the nucleus of the atom. Electrons circle around nucleus.

Chemical reactions involve interactions between the electrons of one atom and the electrons of another atom. Atoms which have different amounts of electrons and protons have a positive or negative electrical charge and are called ions. When atoms bond together, they can make larger building blocks of matter called molecules.