Page 41 - Understanding NCERT Science 09

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whole numbers or as near to a whole numbers mass of the atom, as compared to 1/12 the
as possible. While searching for various mass of one carbon-12 atom.
atomic mass units, scientists initially took 1/
16 of the mass of an atom of naturally Table 3.2: Atomic masses of
occurring oxygen as the unit. This was a few elements
considered relevant due to two reasons:
• oxygen reacted with a large number of Element Atomic Mass (u)
elements and formed compounds.
• this atomic mass unit gave masses of Hydrogen 1
most of the elements as whole numbers. Carbon 12
However, in 1961 for a universally Nitrogen 14
accepted atomic mass unit, carbon-12 isotope
was chosen as the standard reference for Oxygen 16
measuring atomic masses. One atomic mass Sodium 23
unit is a mass unit equal to exactly one-twelfth Magnesium 24
th
(1/12 ) the mass of one atom of carbon-12.
Sulphur 32
The relative atomic masses of all elements
have been found with respect to an atom of Chlorine 35.5
carbon-12. Calcium 40
Imagine a fruit seller selling fruits without
any standard weight with him. He takes a
watermelon and says, “this has a mass equal 3.2.3 HOW DO ATOMS EXIST?
to 12 units” (12 watermelon units or 12 fruit
mass units). He makes twelve equal pieces of Atoms of most elements are not able to exist
the watermelon and finds the mass of each fruit independently. Atoms form molecules and
he is selling, relative to the mass of one piece ions. These molecules or ions aggregate in
of the watermelon. Now he sells his fruits by large numbers to form the matter that we can
relative fruit mass unit (fmu), as in Fig. 3.4. see, feel or touch.

Q uestions

1. Define the atomic mass unit.
2. Why is it not possible to see an
atom with naked eyes?

3.3 What is a Molecule?

A molecule is in general a group of two or
more atoms that are chemically bonded
together, that is, tightly held together by
attractive forces. A molecule can be defined
Fig. 3.4 : (a) Watermelon, (b) 12 pieces, (c) 1/12 of as the smallest particle of an element or a
watermelon, (d) how the fruit seller can compound that is capable of an independent
weigh the fruits using pieces of watermelon existence and shows all the properties of that
substance. Atoms of the same element or of
Similarly, the relative atomic mass of the different elements can join together to form
atom of an element is defined as the average molecules.

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