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The two most common elements in the universe are also the two smallest ones: hydrogen and helium. Hydrogen is by far the most common element in the universe because it was the first element in the universe to be created. The Big Bang caused all of the hydrogen in the universe to be formed at the beginning. This hydrogen was then the basis for all heavier elements to be formed through fusion reactions. Since helium is formed by the fusion of two hydrogen atoms, it is the second most common element in the universe. The heavier elements like iron are much less common in the universe since they require more fusion reactions to occur.
Abundance of elements in Earth's upper crust is not necessarily dependent upon atomic number. Rather it depends upon their ability to remain in the Earth’s crust either in free state or as compounds. A few of the criteria that must be met for this to happen are: a) The compound must be hard, high melting solid b) Insoluble in molten iron core, so that they do not sink deeper into the Earth's core, c) should not be so volatile that they are released in the atmosphere as gases. Most reactive elements of the p-block that form solid compounds are perfect candidates. Silicon and oxygen constitute such a pair. So these are the two most abundant elements, followed by aluminium, in free state as well as in the form of oxide.
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