Figure 3.1 shows Bohr's Model for two atoms A and B. Can you identify three similarities and two differences in these atoms?
You will find,
Since both the atoms have the same atomic number, they must be the atoms of same element and are called isotopes. The word isotope was first used by Soddy. It is a Greek word "isos" means same and "tope" means place.
Isotopes are atoms of an element whose nuclei have the same atomic number but different mass number. This is because atoms of an element can differ in the number of neutrons. Isotopes are chemically alike and differ in their physical properties.
Hydrogen has three isotopes. Hydrogen 1 (Protium) has no neutron. Almost all the hydrogen is Hydrogen-1. Its symbol is ¹₁H. Hydrogen - 2 (deuterium) has one neutron and hydrogen-3 (Tritium) has two neutrons. Their symbols are ²₁H and ³₁H respectively. Because hydrogen -1 also known as protium has only one proton, adding a neutron doubles it mass.
Protium / Hydrogen is a colourless, odourless, and tasteless gas. It is insoluble in water and is highly inflammable gas. Water that contain hydrogen-2 atoms in place of hydrogen-1 is called heavy water.
| Property | Ordinary water | Heavy water |
|---|---|---|
| Melting Point | 0.00°C | 3.81°C |
| Boiling point | 100°C | 101.2°C |
| Density at 25⁰C | 0.99701 g/cm³ | 1.1044 g/cm³ |
Isotopes affect molecular mass of a substance, can change physical properties but do not change chemical properties.
Naturally occurring hydrogen contains 99.99% protium, 0.0015% Deuterium. Tritium is radioactive and is rare. Tritium is not found in naturally occurring hydrogen because its nucleus is highly unstable.
Carbon has three isotopes. Carbon-12, carbon-13 and carbon-14. Almost all the carbon is carbon-12. Its symbol is ¹²₆C. It has six neutrons and six protons. Carbon-13 has symbol ¹³₆C. It has seven neutrons and six protons. Carbon-14 has eight neutrons and six protons. Different forms of carbon are black or greyish black solids except diamond. They are odourless and tasteless. They have high melting and boiling points and are insoluble in water.
Carbon has three Isotopes ¹²₆C, ¹³₆C, ¹⁴₆C. Natural abundance of isotopes of carbon is as follows:
There are two natural isotopes of chlorine, chlorine-35 and chlorine-37. An atom of chlorine-35 has 17 protons and 18 neutrons. An atom of chlorine-37 has 17 protons and 20 neutrons. Chlorine-35 occurs in nature about 75% and chlorine-37 about 25%. Chlorine is a greyish yellow gas with sharp pungent irritating smell. It is fairly soluble in water.
Chlorine has two isotopes. Natural abundance:
Uranium has three isotopes with mass number 234, 235 and 238 respectively. ²³⁴₉₂U, ²³⁵₉₂U, ²³⁸₉₂U
The ²³⁵₉₂U isotope is used in nuclear reactors and atomic bombs, whereas the ²³⁸₉₂U isotope lacks the properties necessary for these applications. ²³⁴₉₂U is rare. Natural abundance of Uranium isotopes is as follows:
²³⁴₉₂U has ____ protons, ____ electrons and ____ neutrons
²³⁵₉₂U has ____ protons, ____ electrons and ____ neutrons
²³⁸₉₂U has ____ protons, ____ electrons and ____ neutrons
When uranium-238 decays into thorium-234, it emits alpha particle. An alpha particle is doubly positively charged helium nucleus.
The fission of uranium-235 yields neutron and energy. The nuclear energy released by the fission of one kilogram of uranium-235 is equivalent to chemical energy produced by burning more than 17000 kg of coal. Chemical properties of an element depend upon the number of protons and electrons. Neutrons do not take part in ordinary chemical reactions. Therefore, isotopes of an element have similar chemical properties.
The relative atomic mass of an element can be calculated from the relative masses of its isotopes and their relative abundance. Natural abundance of isotopes of carbon is as follows:
The relative atomic mass is a weighed average of all the naturally occurring isotopes of an element, taking into consideration of their natural abundance. Use general formula:
An element has two isotopes A and B. The relative atomic mass of element is 35.5 amu. Relative abundance of isotope A is 75.77% and its isotopic mass is 35. Find the isotopic mass of B if its relative abundance is 24.23%.
Stable and radioactive isotopes have many applications in science and medicines. Some of these are as follows:
Carbon-14 is used to estimate the age of carbon-containing substances. Carbon atoms circulate between the oceans, and living organism at a rate very much faster than they decay. As a result the concentration of C-14 in all living things, keep on increasing. After death organisms no longer pick up C-14. By comparing the activity of a sample of skull or jaw bones, with the activity of living tissues, we can estimate how long it has been since the organism died. This process is called dating.