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2014-09-21T16:57:05+05:30
a very light colorless element that is one of the six inert gasses; the most difficult gas to liquefy; occurs in economically extractable amounts in certain natural gases (as those found in Texas and Kansas)
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2014-09-21T18:31:06+05:30

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Helium the second element of our periodic table after Hydrogen. It is denoted by He. It has 2 protons. 2 electrons and 2 neutrons. Its atomic number is 2 and weight 4.  It is chemically a inert gas or noble gas. There is one atom in one molecule of Helium.

Physically Helium is a gas at room temperature. Its melting point and boiling point at normal pressure is very close to 0 deg Kelvin and lowest of all elements. 

physical structure is the way atoms are arranged in the element. Helium atoms are arranged in a hexagonal close packing structure (HCP) in 3-dimensions. It is symmetric in three dimensions. The structure has a high packing density.

In a regular hexagon, there are six helium atoms tangentially touching each other and the edges of the hexagon. These hexagons form one layer of helium atoms. There are twelve neighbors for each atom. There are gaps between atoms. Four atoms share one type of gaps and six atoms share another type of gaps.

The distance between two atoms (neighboring) is one diameter of helium atom. The size of hexagon is 1.5 times diameter of helium atom. The packing factor or space utilized is 74%. That means gaps take 26% of space.

Every alternate layer is identical. 

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