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 the same element and are called isotopes. The word isotope was first used by Soddy. It is a Greek word "isos" meaning same and "tope" meaning place.
Isotopes are atoms of an element whose nuclei have the same atomic number but different mass numbers. This is because atoms of an element can differ in the number of neutrons. Isotopes are chemically alike and differ in their physical properties.
The discovery of isotopes contradicted Dalton's atomic theory, which proposed that atoms of a given element are identical in mass and properties. The existence of isotopes shows that atoms of the same element can have different masses due to varying numbers of neutrons.
Hydrogen has three isotopes: Hydrogen-1 (Protium) with no neutrons, Hydrogen-2 (Deuterium) with one neutron, and Hydrogen-3 (Tritium) with two neutrons. Their symbols are H, D, and T, respectively. Because Hydrogen-1 (Protium) has only one proton, adding a neutron doubles its mass.
Protium / Hydrogen is a colorless, odorless, and tasteless gas. It is insoluble in water and is highly inflammable. Water that contains Hydrogen-2 atoms in place of Hydrogen-1 is called heavy water.
Isotopes affect the molecular mass of a substance and can change physical properties but do not change chemical properties.
Naturally occurring hydrogen contains 99.99% Protium, 0.0015% Deuterium, and Tritium is radioactive and rare. Tritium is not found in naturally occurring hydrogen because its nucleus is highly unstable.
Table 3.2 shows some physical properties of ordinary water and heavy water.
| Property | Ordinary Water | Heavy Water |
|---|---|---|
| Melting Point | 0.00°C | 3.81°C |
| Boiling Point | 100°C | 101.2°C |
| Density at 25°C | 0.99701 g/cm³ | 1.1044 g/cm³ |
Carbon has three isotopes: Carbon-12, Carbon-13, and Carbon-14. Almost all carbon is Carbon-12. Its symbol is ¹²C and it has six neutrons and six protons. Carbon-13 has seven neutrons and six protons, while Carbon-14 has eight neutrons and six protons. Their symbols are ¹³C and ¹⁴C, respectively. Different forms of carbon are black or grayish-black solids except diamond. They are odorless and tasteless, have high melting and boiling points, and are insoluble in water.
Chlorine has two natural isotopes: Chlorine-35 and Chlorine-37. An atom of Chlorine-35 has 17 protons and 18 neutrons, while 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 grayish-yellow gas with a sharp, pungent, irritating smell and is fairly soluble in water.
Uranium has three isotopes with mass numbers 234, 235, and 238. The natural abundance of Uranium isotopes is as follows: Uranium-234 (0.006%), Uranium-235 (0.72%), and Uranium-238 (99.27%). Uranium-235 is used in nuclear reactors and atomic bombs, whereas Uranium-238 lacks the properties necessary for these applications. Uranium-238 decays into Thorium-234 by emitting an alpha particle, which is a doubly positively charged helium nucleus.
The fission of Uranium-235 yields smaller nuclei, neutrons, and energy. The nuclear energy released by the fission of one kilogram of Uranium-235 is equivalent to the chemical energy produced by burning more than 17,000 kg of coal.
Chemical properties of an element depend on the number of protons and electrons. Neutrons do not take part in ordinary chemical reactions, so isotopes of an element have similar chemical properties.
The relative atomic mass of an element is calculated from the relative masses of its isotopes and their relative abundance. For carbon, the relative atomic mass is calculated as follows:
Relative atomic mass of C = (98.8% × 12) + (1.1% × 13) + (0.009% × 14) / 100 = 12.00026 amu
An element has two isotopes A and B. The relative atomic mass of the element is 35.5 amu. The 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 medicine. Some of these include:
Carbon-14 is used to estimate the age of carbon-containing substances. Carbon atoms circulate between the oceans and living organisms, and the concentration of C-14 in living things keeps 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.