Page 32 - Chemistry ICSE Class IX

20 ICSE Chemistry – 9

What is the atomic mass unit
Atomic masses are very small. To express atomic masses, a unit called
atomic mass unit (abbreviation, amu; symbol, u) is commonly used. The
atomic mass unit is de ned as follows:
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The mass equal to th of the mass of a C atom is called one atomic
1
6
mass unit. 12
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Mass of a C atom
1 atomic mass unit = 6
12
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Absolute mass of a C atom is 1.9924 × 10 –23 g. Therefore,
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1.9924 × 10 –23 g
1 atomic mass unit = = 1.66 × 10 –24 g = 1.66 × 10 –27 kg
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What is the atomic mass (A) of an element
The average mass of an atom of an element is known as its atomic mass.
Atomic mass of an element is designated as # Since, atomic mass is actual
mass, hence it has the unit of mass, XK\ gram, kilogram, atomic mass unit,
etc. The internationally accepted unit for describing atomic mass is atomic
mass unit. The average mass of an atom of an element in atomic mass
unit is called its atomic mass.

What is relative atomic mass (A ) of an element
dŚĞ ĂƚŽŵŝĐ ŵĂƐƐ ƵŶŝƚ ŝƐ ŶŽƚ Ă r
ƉƌĂĐƟĐĂů ƵŶŝƚ ďĞĐĂƵƐĞ ŝƚ ŝƐ ĞƋƵŝǀĂůĞŶƚ The relative atomic mass (A QH CP GNGOGPV KU FGſPGF CU VJG CXGTCIG
r
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ƚŽ Ă ǀĞƌLJ ƐŵĂůů ŵĂƐƐ͘ KŶĞ ĂƚŽŵŝĐ mass of an atom of the element compared with an atom of C taken as
ŵĂƐƐ ƵŶŝƚ ŝƐ ĞƋƵĂů ƚŽ ϭ͘ϲϲ п ϭϬ ʹϮϳ ŬŐ͕ 12 u. Thus, 6
Žƌ ϭ͘ϲϲ п ϭϬ ʹϮϰ Ő͘ ^ƵĐŚ ƐŵĂůů ŵĂƐƐĞƐ Average mass of 1 atom of the element
ĐĂŶŶŽƚ ďĞ ŵĞĂƐƵƌĞĚ ŽŶ ĂŶLJ ďĂůĂŶĐĞ͘ Relative atomic mass of an element (A ) =
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ͻ r (Mass of one C atom)/12
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WůĞĂƐĞ ŶŽƚĞ ƚŚĂƚ ƚŚĞ ŵĂŐŶŝƚƵĚĞ ŽĨ The relative atomic mass is denoted by A The relative atomic mass
ƚŚĞ ƌĞůĂƟǀĞ ĂƚŽŵŝĐ ŵĂƐƐ ĂŶĚ ƚŚĞ is a pure number, and hence it has no unit. T
ĂƚŽŵŝĐ ŵĂƐƐ ŽĨ ĂŶ ĞůĞŵĞŶƚ ĂƌĞ ƚŚĞ
ƐĂŵĞ͕ ďƵƚ ƚŚĞ ƚǁŽ ĚŝīĞƌ ŝŶ ƵŶŝƚƐ͘ The relative atomic mass of an element indicates the number of
ͻഩ dŚĞ ƌĞůĂƟǀĞ ĂƚŽŵŝĐ ŵĂƐƐ ŚĂƐ ŶŽ times one atom of that element is heavier than 1 th of a C atom. For
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ƵŶŝƚ͘ 12 6
ͻഩ dŚĞ ĂƚŽŵŝĐ ŵĂƐƐ ŝƐ ĞdžƉƌĞƐƐĞĚ ŝŶ example, the average relative mass of chlorine is 35.45. This means that an
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ƚŚĞ ĂƚŽŵŝĐ ŵĂƐƐ ƵŶŝƚƐ͘ atom of chlorine on average is 35.45/12 times heavier than one atom of C.
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The names, symbols and relative atomic masses of some common
elements are presented in Table 1.8.
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Table 1.8 Relative atomic masses of some common elements (base C = 12.000 u)
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Element Relative atomic mass (A )
r
Name Symbol Exact Common value*
Hydrogen H 1.008 1.0
Carbon C 12.010 12.0
Oxygen O 15.999 16.0
Nitrogen N 14.007 14.0
Chlorine Cl 35.45 35.5
Sulphur S 32.06 32.0
Sodium Na 22.99 23.0
Silver Ag 107.87 108.0
Copper Cu 63.54 63.5
* These values are for use in numerical problems.