A substance consisting of atoms which all have the same number of protons - i.e. the same atomic number.
Elements are chemically the simplest substances and hence cannot be broken down further using chemical methods. Elements can only be changed into other elements using nuclear methods.
At 75 percent, hydrogen is the most abundant element in the universe, followed by helium at 23 percent and oxygen at 1 percent. The other elements make up the remaining 1 percent.
In the earth's crust, oxygen (47 %) is the most abundant element, followed by silicon (28 %) and aluminum (8 %).
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