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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
15
21
Medium

This is a halogen element, lustrous solid, which volatilizing at ordinary temperatures into a blue-violet gas with an irritating odor. Ample amounts of this element are required by the human body for the proper function of the thyroid gland.

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53

I

Iodine
35

Br

Bromine
85

At

Astatine
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Iodine
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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