1. Oxygen is extremely active (chemical, chemically, chemistry, chemist). 2. Oxygen (unites, unit, uniting, united) directly with a number of other elements (form, to form, formed, formation) oxides.
3. In (combines, combining, combined, combination) with hydrogen oxygen forms water.
4. Oxygen is an important (constitutes, constituting, constituent) of the atmosphere.
5. Silicon is the second most abundant element in the Earth’s crust very rarely (to occur, occurrence, occurring) as the pure free element.
6. Silicon (constitutes, constituting, constituent) almost 28% of the earth’s crust mass.
7. Calcium is a reactive element that is not found free in nature because it readily (formation, forming, formed, forms) compounds with oxygen and water.​

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