Page 40 - Understanding NCERT Science 09

In the beginning, the names of elements passage of time and repeated usage you will
were derived from the name of the place automatically be able to reproduce
where they were found for the first time. For the symbols).
example, the name copper was taken from
Cyprus. Some names were taken from
specific colours. For example, gold was taken 3.2.2 ATOMIC MASS
from the English word meaning yellow. The most remarkable concept that Dalton’s
Now-a-days, IUPAC (International Union of atomic theory proposed was that of the atomic
Pure and Applied Chemistry) is an
international scientific organisation which mass. According to him, each element had a
approves names of elements, symbols and characteristic atomic mass. The theory could
units. Many of the symbols are the first one explain the law of constant proportions so well
or two letters of the element’s name in that scientists were prompted to measure the
English. The first letter of a symbol is always atomic mass of an atom. Since determining the
written as a capital letter (uppercase) and the mass of an individual atom was a relatively
second letter as a small letter (lowercase).
difficult task, relative atomic masses were
For example
determined using the laws of chemical
(i) hydrogen, H combinations and the compounds formed.
(ii) aluminium, Al and not AL Let us take the example of a compound,
(iii) cobalt, Co and not CO.
carbon monoxide (CO) formed by carbon and
Symbols of some elements are formed from oxygen. It was observed experimentally that 3
the first letter of the name and a letter, g of carbon combines with 4 g of oxygen to
appearing later in the name. Examples are: (i) form CO. In other words, carbon combines
chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken from the with 4/3 times its mass of oxygen. Suppose
names of elements in Latin, German or Greek. we define the atomic mass unit (earlier
For example, the symbol of iron is Fe from its abbreviated as ‘amu’, but according to the
Latin name ferrum, sodium is Na from natrium, latest IUPAC recommendations, it is now
potassium is K from kalium. Therefore, each written as ‘u’ – unified mass) as equal to the
element has a name and a unique mass of one carbon atom, then we would
chemical symbol.

Table 3.1: Symbols for some elements

Element Symbol Element Symbol Element Symbol

Aluminium Al Copper Cu Nitrogen N
Argon Ar Fluorine F Oxygen O
Barium Ba Gold Au Potassium K
Boron B Hydrogen H Silicon Si
Bromine Br Iodine I Silver Ag
Calcium Ca Iron Fe Sodium Na
Carbon C Lead Pb Sulphur S
Chlorine Cl Magnesium Mg Uranium U
Cobalt Co Neon Ne Zinc Zn

(The above table is given for you to refer to assign carbon an atomic mass of 1.0 u and
whenever you study about elements. Do not oxygen an atomic mass of 1.33 u. However, it
bother to memorise all in one go. With the is more convenient to have these numbers as

ATOMS AND MOLECULES 29

Rationalised 2023-24