Chemistry is the study of various kinds of matter and its properties. All around us, we see various kinds of substances and materials. It is therefore imperative to classify and categorize them so that their properties can be easily studied, analyzed, and understood. The matter around us can be categorized into the pure matter and impure matter. Pure substances are the ones that have a fixed chemical composition. Impure substances are the ones that can have varying compositions, and as such do not exhibit a fixed set of properties.
Pure substances can further be classified into elements and compounds. Impure substances are also called mixtures. Depending on their composition, they can further be classified into homogeneous and heterogeneous.
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In chemistry, an element is a substance that cannot be further broken down into a simpler substance by using common chemical methods. All matter is fundamentally composed of elements. To date, more than 119 elements have been discovered and many more are in the process of being discovered. Every element has a fixed place in the periodic table, depending on its properties.
The smallest entity of an element is called an atom. Atoms are highly unstable, therefore do not exist independently. They always tend to combine with other elements in order to attain stability. To represent an atom of an element, we use symbols. Every element has a fixed symbol. Some commonly used symbols of elements are listed below.
Depending on the properties, elements can be classified into metals, nonmetals, and metalloids.
Metals are the elements that have the tendency of losing electrons in order to gain stability, that is, they exhibit electropositivity. The physical properties of metals include hardness, high tensile strength, luster, conductivity, high melting and boiling points, etc.
Examples include sodium, potassium, iron, calcium, magnesium, lead, tin, etc.
Non-metals are the elements that have the tendency of gaining electrons in order to gain stability, that is, they exhibit electronegativity. The physical properties of non-metals include brittleness, comparatively lower tensile strength, non-luster, non-conductivity or insulation, lower melting and boiling points, etc.
Examples include hydrogen, helium, chlorine, iodine, sulfur, phosphorus oxygen, etc.
Metalloids are the elements that have properties that are in between those of metals and nonmetals.
Examples include boron, silicon, arsenic, antimony, germanium, etc.
A compound is a pure substance that is formed by the chemical combination of two or more elements in a fixed proportion by mass. Compound formation always takes place as a result of a chemical reaction. Therefore, a compound does not exhibit the properties of its constituent elements. On breaking down the compound, the smallest entity so obtained is called a molecule of the compound.
One molecule of a compound is represented by its chemical formula. The formula of a compound is the statement of the composition of that particular compound in which the symbols represent the elements that are present in it while the subscripts show the number of atoms of each element. A molecule is stable and has an independent existence.
The molecular formulae of some commonly used compounds are listed below:
On the basis of the formation, compounds can be classified as ionic compounds and covalent compounds.
Ionic compounds are formed between a metal and a non-metal. They are also called electrovalent compounds as they involve ions. The metal atom loses electrons to form a cation, and the non-metal atom gains the electron to form an anion.
Examples include sodium chloride, magnesium iodide, calcium oxide etc.
Covalent compounds are formed between two non-metals. They are also called molecular compounds. They are formed as a result of the sharing of electrons between two or more non-metals.
Examples include water, carbon dioxide, methane, sugar etc.
Question 1: What is the Difference Between Compounds and Mixtures?
Solution: Compounds are pure substances whereas mixtures are impure. A compound always has a fixed chemical composition by mass, which means that the elements that combine together to form a compound always do so in a fixed ratio. This is why compounds have a molecular formula. Compounds have fixed melting and boiling points. They do not show the properties of their constituent elements. A compound cannot be broken down into its elements by simple physical methods.
Example: Water is a compound. We cannot separate the hydrogen and oxygen present in it by using physical methods.
On the other hand, a mixture does not have a fixed chemical composition. The substances present in a mixture can be added in varying proportions. A mixture does not have a fixed chemical formula. It also does not have a fixed melting and boiling point. It will show the properties of its constituents and can be decomposed into its constituents by using simple methods. Dependent on the uniform or non-uniform distribution of the constituents, mixtures can be either homogeneous or heterogeneous.
Example: A salt solution is a mixture. The salt and water can be separated by heating.
Question 2: What are Some Exceptions to the Common Physical Properties of Metals and Non-Metals?
Metals are usually hard, but sodium and potassium are soft.
Metals have high melting and boiling points, but gallium and cesium can melt even with the heat of the human body.
Metals are usually solids, except mercury which is a liquid.
Metals are good conductors, but lead and bismuth are not.
Non-metals are usually brittle, but the diamond is the strongest substance known.
Both graphite and diamond have extremely high melting and boiling points.
Non-metals are also not good conductors, but graphite conducts electricity.
Non-metals are usually not lustrous but diamond, graphite, and iodine are.