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Isotopes
It is interesting to note that atoms of a given atomic number can have different number of neutrons. Some examples are listed below:
Hydrogen
Hydrogen atom (Z = 1) has no neutrons.
Number of protons = 1Number of electrons = 1Number of neutrons = 0It has been reported that the hydrogen element has atoms with mass number 2 and 3 also i.e.,
Atoms of elements having the same atomic number with different mass numbers are called isotopes.
Nuclear composition of isotopes of chlorine:
Characteristics of Isotopes
- All isotopes of an element have the same number of valence electrons thus have identical chemical properties.
- The physical properties of the isotopes are different due to the difference in the number of neutrons in their nuclei. The densities, melting points and boiling points etc., are slightly different.
Reason for Fractional Atomic Masses of Elements
Atomic masses of many elements are in fractions not in whole numbers.
Example:
Cl - 35.5Cu - 63.5The fractional atomic masses of elements are due to the existence of isotopes having different masses.Example:1
Natural chlorine consists of two isotopes:
Calculate the average atomic mass of chlorine.
Example: 2
A naturally occurring sample of Lithium contains 7.42% of 6Li and 92.58% of 7Li. The relative mass of 6Li is 6.015 and that of 7Li is 7.016. Calculate the atomic mass of a naturally occurring sample of lithium.
Solution:
Example: 3
Which of the following two nuclei are isotopes of each other?
Solution:
The two isotopes are:
Radioactivity
Isobars
The atoms of different elements, which have the same mass number but different atomic numbers are called isobars. These have different number of protons but equal sum of number of protons and neutrons.
Some Typical Isobars
| Isobars | Atomic number(Z) | MassNumber(A) | Electrons(e) | Proton(p) | Neutron(n) |
|---|---|---|---|---|---|
40 Ar 18 |
18 | 40 | 18 | 18 | 22(40-18) |
40 K 19 |
19 | 40 | 19 | 19 | 21(40-19) |
40
Ca
20
|
20 | 40 | 20 | 20 | 20(40-20) |
Isotones
The atoms of different elements, which have the same number of neutrons but different atomic numbers, are called isotones.
Some Typical Isotones
| Isotones | Atomic number(Z) | MassNumber(A) | Electrons(e) | Proton(p) | Neutron(n) |
|---|---|---|---|---|---|
14 C 6 |
6 | 14 | 6 | 6 | 8(14-6) |
15 N 7 |
7 | 15 | 7 | 7 | 8(15-7) |
16
O
8
|
8 | 16 | 8 | 8 | 8(16-8) |
Comparison of Dalton's Atomic Theory and Modern Atomic Theory
| Dalton's Atomic Theory | Modern Atomic Theory |
|---|---|
| All matter is made up of extremely small particles called atoms | An element is made up of extremely small particles called atoms |
| Atoms are indivisible and indestructible | Atoms are divisible and are no more indestructible |
| Atom is the smallest particle of an element | Atom consists of still smaller subatomic particles like proton, electron and neutron |
| All atoms of an element are alike in all respects | All atoms of an element may not be alike; they can have different masses (e.g. isotopes) |
| Atoms take part in all chemical reactions; the relative number and kinds of atoms are constant in a given compound | Atoms take part in all chemical reactions; the relative numbers and kinds of atoms are constant in a given compound |
