| Characteristics | Compounds | Mixtures |
|---|---|---|
| Composition | Made up of atoms of elements in a fixed proportion | Made up of elements, or compounds, or both in any proportion |
| Nature | Particles are of the same kind | Particles are of different kinds |
| Structure | Always homogeneous | May or may not be homogeneous |
| Appearance | Components cannot be seen separately | Components may or may not be seen separately |
| Preparation | Always involves a chemical change | Involves only physical change |
| Properties | Entirely different from those of the constituents | No property of their own Show the average properties of all the constituents |
| Separation | Components can be separated only by chemical means | Components can be separated by physical means |
| Energy changes | Energy is always evolved or absorbed | Generally no energy is evolved or absorbed |
The following example of iron, sulphur and iron sulphide will make us understand the difference between mixtures and compounds.
| Properties | Iron | Sulphur |
|---|---|---|
| Colour | Greyish black | Yellow |
| Action of magnet | Attracted | Not attracted |
| On stirring the mixture with water | Sinks, and forms the lower layer of iron | Sinks and forms a layer over the iron |
| Action of dilute acids | Dissolves, producing hydrogen | Does not dissolve |
| Action with carbon disulphide | Does not dissolve | Dissolves |
The following experiment shows the difference in properties between the elements iron and sulphur (Fig.4.1).
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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:
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.
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.
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.
two “sp” hybrid orbitals
