Page 40 - Chemistry ICSE Class IX

28 ICSE Chemistry – 9
(iii) The molecular formula of calcium phosphate is
(a) CaPO ˆ
4
(b) Ca (PO ) ˆ
3
4 2
(c) Ca PO ˆ
4
2
(d) Ca(PO ) ˆ
4 3
(iv) The number of molecules in 8 g of oxygen gas is
23
(a) 6.02 × 10 ˆ
23
(b) 3.01 × 10 ˆ
23
(c) 1.5 × 10 ˆ
12
(d) 3.01 × 10 ˆ
AŶƐǁĞƌƐ

(i) (c) (ii) (b) (iii) (b) (iv) (c)

Let us say it Again

— Dalton’s atomic theory. Dalton’s atomic theory considers matter to be made up of very small/indivisible particles called
atoms. Atoms cannot be created or destroyed by a simple chemical reaction.
— Atomic symbols.
 The modern atomic symbols are based on the scheme proposed by J. J. Berzelius.
 Symbol of an element is an abbreviation for the full name of the element.
 Symbol of an element consists of the first letter of its Common/Greek/Latin name. For elements having the same first
letter, the symbol consists of two letters. First letter followed by another letter not in common.
— Atom. Atom is the smallest particle of a substance which takes part in a chemical reaction.
— Ion. An electrically charged particle that is formed when an atom loses or gains electrons is called an ion.
 The ions that carry positive charge are called cations.
 The ions that carry negative charge are called anions.
— Valency. The combining capacity of an element is termed as its valency.
— Radical. An atom or a group of atoms of different elements carrying positive or negative charge and which exists in
compounds, is called a radical.
The negatively charged radical is called the acid radical, and the positively charged radical is called the basic radical.
Radicals retain their identity in chemical reactions.
— Molecule. The smallest unit of a substance which can exist independently and shows all the properties of the substance
is called a molecule.
— Molecular formula. A shorthand notation for the molecule of a substance in terms of symbols and numbers of atoms of
each element present in it, is called its molecular formula.
Covalent compounds which exist as discrete molecules can be described by their molecular formulae.
Ionic and giant network covalent compounds are described by their empirical or stoichiometric formulae.
— Empirical formula. The simplest formula which gives the lowest whole-number ratio between the number of atoms of
different elements present in the substance is called empirical formula.
— Chemical equation. A shorthand representation of a chemical reaction in terms of symbols and formulae of the
substances involved in a reaction is called a chemical equation.
During a chemical reaction, the law of conservation of matter holds good.
— Balanced chemical equation. A chemical equation in which the number of atoms of each element on both the sides of
the equation are equal, is called a balanced chemical equation.
Chemical equations are generally balanced by Hit-and-Trial method, and Partial Equation method.