Complex sentence: Nitrogen is the most abundant element in Earth's atmosphere, making up about 78% of the atmosphere by volume: it is the most common uncombined element on Earth, since all the more common elements occur primarily as compounds.
Split into simpler sentences: Nitrogen is the most abundant element in Earth's atmosphere, making up about 78% of the atmosphere by volume. It is also the most common uncombined element on Earth, since all the more common elements occur primarily as compounds.