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.

John Dalton, a British school teacher, published his theory about atoms in the year 1808. His findings were based on experiments and also from laws of chemical combination.
Main assumptions or postulates of Dalton

* All matter consists of indivisible particles called atoms.

* Atoms of the same element are similar in shape and mass, but differ from the atoms of other elements.

* Atoms cannot be created or destroyed.

* Atoms of different elements may combine with each other in a fixed, simple, whole number ratio to form compound atoms.

* Atoms of same element can combine in more than one ratio to form two or more compounds.

* Atoms are the smallest unit of matter that can take part in a chemical reaction.

Drawbacks of Dalton’s atomic theory of matter

* The indivisibility of an atom was proved wrong, for, an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle, which takes part in chemical reactions.

* According to Dalton, the atoms of same element are similar in all respects. This is wrong because atoms of some elements vary in their mass and density. Such atoms of the same element having different masses are called isotopes. For example, chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.

* Dalton also said atoms of different elements are different in all respects. This has been proved wrong in certain cases like argon and calcium atoms, which have the same atomic mass of 40. Such atoms of different elements that have the same atomic mass are called isobar.

* According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.

* The theory completely fails to explain the existence of allotropes. The difference in properties of charcoal, graphite, diamond went unexplained in spite of being made up of same kind of atoms.

Merits of Dalton’s atomic theory

* It has enabled us to explain the laws of chemical combination.

* Dalton was the first person to recognize a workable distinction between the ultimate particle of an element (atom) and that of a compound (molecule).

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO₂(g) + H₂O(l) = C₆H₁₂O₆(aq) + 6O₂(g)

An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl₂(s) = NaCl(s)

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.