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Explain the following terms-1. Periodic Table 2. Elements 3. Compounds 4. Chemical Symbols 5. Chemical Formula
The Periodic Table provides the names, atomic numbers, symbols and atomic weights of known elements. It serves as a great tool for solving chemistry problems.
A periodic table is divided into groups (columns), where elements with each group behave similarly while bonding with other elements; and periods (rows), where elements in one period have same number of electron shells.
Earlier Mendeleev's Periodic Table was based on atomic mass but the Modern Periodic Table is based on atomic number.
An element is a pure substance, which is made up of only one kind of atoms. It cannot be further reduced to simpler substances by any physical or chemical methods. An atom is the smallest unit of an element.
Elements are classified into metals, non-metals, metalloids and noble gases.
A compound is a pure substance made up of two or more different elements combined chemically in a fixed proportion. Example of Atoms of different elements forming a compound.
Eg: atoms of Sodium and atoms of Chlorine combine together to form Sodium Chloride (compound)
Na +Cl --> NaCl
A compound has unique properties that are distinct from the properties of its elemental constituents. One familiar chemical compound is water, a liquid that is nonflammable and does not support combustion. It is composed of two elements: hydrogen, an extremely flammable gas, and oxygen, a gas that supports combustion.
A chemical symbol is a shorthand method of representing an element. Instead of writing out the whole name of an element, we represent an element name with one or two letters. Example Nitrogen as N or calcium as Ca.
A Chemical formula is a symbolic representation of a molecule’s composition.
It indicates the elements that are present in the compound and the ratio of these elements in the compound.
In order to write a chemical formula, the following steps should be taken into account.
Step 1- Write down the symbols of the elements/ions, which combine to form a molecule of the compound, side by side.(While writing the formula of a compound containing a metal and a non-metal, the symbol of the metal is written first followed by that of the non-metal.)
Step 2- Write the valency of each element /ion.
Step 3- Interchange the valencies of the elements /ions and write as the subscript. Write them close together and ignore ‘1’ to obtain the formula.
Step 4- The radical/polyatomic ions must be written in brackets before the subscript is written. In case the number of polyatomic ions is one, the bracket is not required. For example – NaOH.
Whenever applicable, write simple ratios of the valencies.
The final formula must be written without any sign of the charge.
Hydrogen sulphide contains the elements of hydrogen and sulphur.
(a) Write the symbol. H S
(b) Write the valency of each element 1 2
(c) Interchange the valencies of the elements and write as the subscript H2 S1
(d) Write them close together and ignore ‘1’ to obtain the formula. H2S
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