Allotropes vs Isotopes
We are familiar with the word “element,” because we learn about them in the periodic table. There are about 118 elements given in the periodic table according to their atomic number. An element is a chemical substance, which consists of only a single type of atoms; hence, they are pure.
Allotropes are different forms of the same element in the molecular level. They show different physical properties. Carbon, oxygen, sulfur and phosphorous are the commonly known elements, which have allotropes. Carbon has a large number of allotropes. The eight allotropes of carbon differ largely from each other. For example, diamond is known as the strongest allotrope of carbon, whereas graphite is less strong. Carbon nanotubes, fullerene, and amorphous carbon are some other allotropes of carbon. For the element oxygen, there are two common allotropes as O2 and O3. From this, O2 is abundant than O3. Normally, in nature, some allotropes are more abundant than the others because of the stability. Phosphorus has three allotropes as red, white and black phosphorus. From these, red and white phosphorus are the most common. Allotropes differ from each other due to atomic arrangement, number of atoms, etc.
Atoms of the same element can be different. These different atoms of the same element are called isotopes. They are different from each other by having a different number of neutrons. Since the neutron number is different, their mass number also differs. However, the isotopes of the same element have the same number of protons and neutrons. Different isotopes present in varying quantities, and this is given as a percentage value called relative abundance. For example, hydrogen has three isotopes as protium, deuterium and tritium. Their number of neutrons and relative abundances are as follows.
1H – no neutrons, relative abundance is 99.985%
2H- one neutron, relative abundance is 0.015%
3H- two neutrons, relative abundance is 0%
The number of neutrons a nucleus can hold differs from element to element. Among these isotopes, only some are stable. For instance, oxygen has three stable isotopes, and tin has ten stable isotopes. Most of the time simple elements have the same neutron number as the proton number. But in the heavy elements, more neutrons are there than the protons. The number of neutrons is important to balance the stability of the nuclei. When the nuclei are too heavy, they become unstable and, therefore, those isotopes are becoming radioactive. For example, 238U emits radiation and decays to much smaller nuclei. Isotopes may have different properties because of their different masses. For example, they may have different spins, thus their NMR spectra differs. However, their electron number is similar giving rise to a similar chemical behavior.
A mass spectrometer can be used to get information about isotopes. It gives the number of isotopes which an element has, their relative abundances and masses.
What is the difference between Allotropes and Isotopes?
• Allotropes are considered in the molecular level, whereas isotopes are considered in the atomic level.
• Allotropes are different forms of the same element in the molecular level. Allotropes of an element differ on the way atoms are arranged. Isotopes are atoms of the same element with differences in the number of neutrons.
• Allotropes cannot be seen in all the elements. But comparatively larger number of elements has isotopes.