Page 39 - Understanding NCERT Science 09

observations are in agreement We might think that if atoms are so
with the law of conservation of insignificant in size, why should we care about
mass. them? This is because our entire world is made
sodium carbonate + acetic acid up of atoms. We may not be able to see them,
→ sodium acetate + carbon but they are there, and constantly affecting
dioxide + water whatever we do. Through modern techniques,
2. Hydrogen and oxygen combine in we can now produce magnified images of
the ratio of 1:8 by mass to form
surfaces of elements showing atoms.
water. What mass of oxygen gas
would be required to react
completely with 3 g of hydrogen
gas?
3. Which postulate of Dalton’s
atomic theory is the result of the
law of conservation of mass?
4. Which postulate of Dalton’s
atomic theory can explain the law
of definite proportions?

3.2 What is an Atom?

Have you ever observed a mason building
walls, from these walls a room and then a Fig. 3.2: An image of the surface of silicon
collection of rooms to form a building? What
is the building block of the huge building?
What about the building block of an ant-hill? 3.2.1 WHAT ARE THE MODERN DAY
It is a small grain of sand. Similarly, the SYMBOLS OF ATOMS OF DIFFERENT
building blocks of all matter are atoms.
ELEMENTS?
How big are atoms?
Dalton was the first scientist to use the
Atoms are very small, they are smaller than symbols for elements in a very specific sense.
anything that we can imagine or compare When he used a symbol for an element he
with. More than millions of atoms when
also meant a definite quantity of that element,
stacked would make a layer barely as thick that is, one atom of that element. Berzilius
as this sheet of paper.
suggested that the symbols of elements be
Atomic radius is measured in nanometres. made from one or two letters of the name of
9
1/10 m = 1 nm the element.
1 m = 10 nm
9
Relative Sizes

Radii (in m) Example
10 –10 Atom of hydrogen
10 –9 Molecule of water
10 –8 Molecule of haemoglobin
10 –4 Grain of sand

10 –3 Ant
Fig. 3.3: Symbols for some elements as proposed by
10 –1 Apple
Dalton

28 SCIENCE

Rationalised 2023-24