Isotopes and Allotropes

Elements are pure chemical substances consisting of one type of atom. Elements can be metals, metalloids or non-metals. Examples: carbon, oxygen (non-metals), silicon, arsenic (metalloids), aluminium, iron, copper, gold, mercury, and lead (metals).

As of now, 118 elements have been defined of which 98 are known to occur naturally and 80 are stable. Others are either radioactive, decaying into lighter elements over various timescales from fractions of a second to billions of years or synthesized.

Most abundant elements

Hydrogen and helium are by far the most abundant elements in the universe. However, iron is the most abundant element (by mass) making up the Earth. Oxygen is the most common element in the Earth’s crust.

Isotopes

Isotopes have different number of neutrons but same number of protons. The number of protons is called atomic number so all isotopes have same atomic number but different number of nucleons (neutron + proton) due to different number of neutrons. For example C₁₂, C₁₃ & C₁₄ are isotopes. Each of them has 6 protons. But they have 6, 7, 8 neutrons. So their atomic weights vary.  The 80 elements with stable isotopes have atomic number of 1 to 82.

• The two elements between this atomic number range elements 43 and 61 (technetium and promethium) have no stable isotopes.
• 83rd element is Bismuth and from Bismuth onwards all undergo radioactive decay. They don’t have stable nuclei but are found in nature.
• Remaining 24 are artificial or synthetic elements.

The first synthetic element was technetium. Technetium is found in trace amounts in nature and was discovered in 1925 but it was synthesized in 1937.

Allotropes

Some chemical elements are known to exist in two or more different forms because the atoms are bounded together in different manners. Most common example is Carbon which exists in Diamond, graphite, fullerenes etc. Allotropy is for elements and NOT for compounds. For example water and ice are not allotropes. The changes in state between solid, gas and liquid is NOT allotropy.

Oxygen has two allotropes viz. Oxygen and Ozone. These allotropes can stay in all the liquid, gaseous and solid phases. Phosphorus has many allotropes but when melted, all return to the P4 form. The different structural forms of the same element lead to the allotropes to show different physical properties and chemical behaviours.

Some allotropes

Allotropes of Carbon:

• Carbon has allotropes such as Diamond which is extremely hard, transparent crystal in which atoms are arranged in a tetrahedral lattice.
• Another allotrope is Lonsdaleite which is also known as hexagonal diamond. It has a hexagonal lattice in comparison to the diamond which has tetrahedral crystal structure. It is less hard than diamond (Diamond has Mohs hardness 10 while the Lonsdaleite has 7-8). It’s a brown color substance.
• Another allotrope is Graphite which is soft, black and flaky and a moderate electrical conductor. In graphite the C atoms are bonded in flat hexagonal lattices layered in sheets. Each lattice is called a Graphene.
• Amorphous carbon is another allotrope. It has no crystalline structure.
• Fullerenes are other allotropes of Carbon. The fullerenes exist in various shapes such as hollow sphere, ellipsoid, or tubes. Accordingly they have been named buckyballs (for spherical) and buckeytubes for tube fullerenes. Tube fullerenes are the carbon nanotubes.
• The first fullerene discovered was C60 in 1985 and it was called buckministerfullerene.
• Carbon nanotubes display extraordinary properties that make them potentially useful in many applications including in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. They are either single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).

Allotropes of Phosphorus

• The crystalline solid phosphorous is P₄ which is called white phosphorus.
• The Red phosphorus is polymeric.
• There are scarlet phosphorus, Violet Phosphorus, Black Phosphorus and Diphosphorus.
• Black Phosphorous is a semiconductor which is very much equivalent to Graphite.

Allotropes of Oxygen

• Oxygen has two allotropes Dioxygen O², which is colourless and Ozone O³ which is blue.
• Apart from this there is tetraoxygen O₄ which is also called oxozone and it is metastable.
• The solid oxygen is Octaoxygen which exists in 6 phases under various pressures and temperatures; it is also called Red Oxygen.

Allotropes of Nitrogen

• N₂ is stable isotope. Another is unstable Polynitrogen molecule N₄.

Allotropes of Sulfur

• Sulfur is amorphous polymeric solid which is called plastic Sulfur, then there are S₈ molecules which are large crystalline structures called Rhombic Sulfur.
• Another form is fine needle like structures called monoclinic Sulfur.
• Apart from that there are other ring molecules such as S₇ and S₁₂.