How to write Chemical Formula

In order to write a chemical formula, it is important to know the symbol of the elements present in the compound, formula of the radicals and the valency of the elements in that compound. Following points should be kept in mind while writing a chemical formula.

• Most of the compounds are binary compounds, i.e. they have two elements. Compounds with more than two elements are also known
• An atom with a positive charge is called a cation whereas an atom with a negative charge is called an anion
• For a compound containing a metal and a non-metal, the metal is named first followed by the non-metal. For example: NaCl which consists of Na⁺ (metal ion) and Cl^– (non-metal ion)
• Anions having -1 negative charge usually have a suffix as –ide. For example: F^– – Floride
• Anions having oxyanions (oxygen + another element) usually have a suffix as –ate. For example – SO₄^2- (Sulphate)
• When a polyatomic anion has H^– ion, bi- or hydrogen is used as a suffix. For example – HCO₃^–-Bicarbonate or hydrogen carbonate
• Some polyatomic anions can be named as:

Problem 1: In one molecule of the compound, determine how many atoms of every element are present for each one of these chemical formulas.

1.       HCN – hydrogen cyanide –It’s a toxic gas
2.       C₁₈H₂₁NO₃ – codeine, a painkilling drug
3.       Ca₁₀(PO₄)₆(OH)₂ – Hydroxyapatite, that is present in the enamel of the tooth

Answer

1. One atom is present in each of the elements hydrogen, carbon and nitrogen, respectively. Remember that the subscript 1 is understood when no subscript is mentioned.
2. This formula points out that in one molecule of the compound, 18 carbon atoms, 21 hydrogen atoms, one nitrogen atom and three oxygen atoms are existent.
3. There are ten calcium atoms. The amounts of phosphorous, hydrogen and oxygen are affected by the subscripts outside the parentheses. There are six phosphorous atoms and two hydrogen atoms existent. Oxygen atoms exist in two positions in the formula. There are overall 26 oxygen atoms: two from the OH subunit (2×1) and 24 from the PO₄ subunits (6×4)