Page 46 - Understanding NCERT Science 09

we use the word formula unit for those uestions
substances whose constituent particles are
ions. For example, sodium chloride as 1. Calculate the molecular masses
discussed above, has a formula unit NaCl. Its of H , O , Cl , CO , CH , C H ,
2
2
2
2
6
2
4
formula unit mass can be calculated as– Q C H , NH , CH OH.
2
4
3
3
1 × 23 + 1 × 35.5 = 58.5 u 2. Calculate the formula unit
2
2
3
Example 3.2 Calculate the formula unit masses of ZnO, Na O, K CO ,
given atomic masses of Zn = 65 u,
mass of CaCl .
2 Na = 23 u, K = 39 u, C = 12 u,
Solution: and O = 16 u.
Atomic mass of Ca
+ (2 × atomic mass of Cl)
= 40 + 2 × 35.5 = 40 + 71 = 111 u
What
you have
learnt
• During a chemical reaction, the sum of the masses of the
reactants and products remains unchanged. This is known
as the Law of Conservation of Mass.
• In a pure chemical compound, elements are always present
in a definite proportion by mass. This is known as the Law
of Definite Proportions.
• An atom is the smallest particle of the element that cannot
usually exist independently and retain all its chemical
properties.
• A molecule is the smallest particle of an element or a
compound capable of independent existence under ordinary
conditions. It shows all the properties of the substance.
• A chemical formula of a compound shows its constituent
elements and the number of atoms of each combining
element.
• Clusters of atoms that act as an ion are called polyatomic
ions. They carry a fixed charge on them.
• The chemical formula of a molecular compound is
determined by the valency of each element.
• In ionic compounds, the charge on each ion is used to
determine the chemical formula of the compound.

ATOMS AND MOLECULES 35

Rationalised 2023-24