Why is the earth atmosphere mostly hydrogen?

The Earth's atmosphere is not mostly hydrogen; it is primarily composed of nitrogen (about 78%) and oxygen (about 21%). Hydrogen is the most abundant element in the universe, but it has largely escaped Earth's gravitational pull due to its lightness and the planet's relatively low gravity. Additionally, the processes that formed the Earth and its atmosphere led to the retention of heavier gases over lighter ones, resulting in a composition dominated by nitrogen and oxygen.