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20
20
Hard

This element is classified as a lanthanide and it is the most abundant of the rare-earth elements of the earth’s crust. It is the most reactive of the rare-earth metals and is used as a catalyst to refine petroleum.

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58

Ce

Cerium
68

Eu

Erbium
71

Lu

Lutetium
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Cerium
12
21
Medium

This is a non-metal and it is a structural element of all biomolecules and living beings. Three naturally occurring allotropes of this element are known to exist: amorphous, graphite and diamond.

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6

C

Carbon
1

H

Hydrogen
15

P

Phosphorus
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Carbon
18
20
Hard

This element is a noble gas which is gas at room temperature. It is characterized by its brilliant green and orange spectral lines. This noble gas is used in fluorescent light bulbs, luminous signs, high-speed photography.

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36

Kr

Krypton
54

Og

Xenon
86

Rn

Radon
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Krypton

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