# Chapter 3 ATOMS AND MOLECULES

## MOLE CONCEPT

Take an example of the reaction of hydrogen and oxygen to form water: 2H2 + O2 → 2H2O.The above reaction indicates that(i) two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water, or(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u …

## Q. Calculate the formula unit mass of CaCl2 .

Solution:Atomic mass of Ca + (2 × atomic mass of Cl)= 40 + 2 × 35.5 = 40 + 71 = 111 u

## FORMULA UNIT MASS

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass iscalculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for thosesubstances whose constituent particles are …

## Q (a) Calculate the relative molecular mass of water (H2O). (b) Calculate the molecular mass of HNO3

Solution:(a) Atomic mass of hydrogen = 1u, oxygen = 16 uSo the molecular mass of water, which contains two atoms of hydrogen and oneatom of oxygen is = 2 × 1+ 1×16 = 18 u(b) The molecular mass of HNO3= the atomic mass of H + the atomic mass of N+ 3 × the atomic …

## MOLECULAR MASS

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the Substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u). For example, the molecular mass of water, which has two atoms of hydrogen and one atom of …

## Writing Chemical Formulae

The chemical formula of a compound is a symbolic representation of its composition. The chemical formulae of different compounds can be written easily. For this exercise, we need to learn the symbols and combining capacity of the elements. The combining power (or capacity) of an element is known as its valency. Valency can be used …

## WHAT IS AN ION ?

Compounds composed of metals and nonmetals contain charged species. The charged species are known as ions. Ions may consist of a single charged atom or a group of atoms that have a net charge on them. An ion can be negatively or positively charged. A negatively charged ion is called an ‘anion’ and the positively …

## MOLECULES OF COMPOUNDS

Atoms of different elements join together in definite proportions to form molecules of compounds. Few examples are given in Thus, the ratio by number of atoms for water is H:O = 2:1.

## MOLECULES OF ELEMENTS

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc.are made up of only one atom of that element.But this is not the case with most of the nonmetals. For example, a molecule of oxygen consists of two atoms of oxygen …

## Relative Sizes of Atom of Hydrogen , water molecule , molecule of haemoglobin ,Grain of sand , Ant , water melon

Atomic radius is measured in nanometres. 1/10 9 m = 1 nm 1 m = 109 nm