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fluorine to gain one electron instead of losing 6 neutrons, 6 u + 6 u = 12 u. Similarly, the
seven electrons. Hence, its valency is determined mass of aluminium is 27 u (13 protons+14
by subtracting seven electrons from the octet neutrons). The mass number is defined as the
and this gives you a valency of one for fluorine. sum of the total number of protons and
Valency can be calculated in a similar manner neutrons present in the nucleus of an atom. It
for oxygen. What is the valency of oxygen that is denoted by ‘A’. In the notation for an atom,
you get from this calculation? the atomic number, mass number and symbol
Therefore, an atom of each element has a of the element are to be written as:
definite combining capacity, called its valency.
Mass Number
Valency of the first eighteen elements is given A
in the last column of Table 4.1. Symbol of
element Z X
Q 1. How will you find the valency For example, nitrogen is written as 7 N .
uestion
Atomic Number
14
of chlorine, sulphur and
magnesium?
uestions
is 8 and number of protons is also
4.5 Atomic Number and Mass Q 1. If number of electrons in an atom
8, then (i) what is the atomic
Number number of the atom? and (ii) what
is the charge on the atom?
4.5.1 ATOMIC NUMBER 2. With the help of Table 4.1, find
out the mass number of oxygen
and sulphur atom.
We know that protons are present in the
nucleus of an atom. It is the number of 4.6 Isotopes
protons of an atom, which determines its
atomic number. It is denoted by ‘Z’. All atoms
In nature, a number of atoms of some
of an element have the same atomic number, elements have been identified, which have the
Z. In fact, elements are defined by the number same atomic number but different mass
of protons they possess. For hydrogen, Z = 1, numbers. For example, take the case of
because in hydrogen atom, only one proton
hydrogen atom, it has three atomic species,
is present in the nucleus. Similarly, for
2
1
carbon, Z = 6. Therefore, the atomic number namely protium ( H), deuterium ( H or D)
1 1
is defined as the total number of protons 3
present in the nucleus of an atom. and tritium ( H or T). The atomic number of
1
each one is 1, but the mass number is 1, 2
4.5.2 MASS NUMBER and 3, respectively. Other such examples are
12 14 35
After studying the properties of the sub- (i) carbon, 6 C and 6 C, (ii) chlorine, 17 Cl
atomic particles of an atom, we can conclude 37
that mass of an atom is practically due to and 17 Cl, etc.
protons and neutrons alone. These are On the basis of these examples, isotopes
present in the nucleus of an atom. Hence are defined as the atoms of the same element,
protons and neutrons are also called having the same atomic number but different
nucleons. Therefore, the mass of an atom mass numbers. Therefore, we can say that
resides in its nucleus. For example, mass of there are three isotopes of hydrogen atom,
carbon is 12 u because it has 6 protons and namely protium, deuterium and tritium.
44 SCIENCE
Rationalised 2023-24
