Diagram of a hydrogen atom
Hydrogen is the simplest kind of atom, and in the very earliest days after the Big Bang hydrogen was the only kind of atom in the new Universe. A hydrogen atom, like other atoms, has a nucleus. The nucleus of a hydrogen atom is made of just one proton. Around the nucleus, there is just one electron, which goes around and around the nucleus.
The first hydrogen atoms in space gradually grouped together into clumps called nebulae. When a nebula got dense enough, it formed a star. Inside these stars, there was a lot of heat and gravity. The strong gravity pressed the hydrogen atoms together, and four hydrogen atoms squashed together to form a new atom, with two protons, two neutrons, and two electrons. This is a helium atom. The neutrons had just a little less energy than the protons, and the extra energy shot off into space and became heat and light for us. Inside most of the stars in the Universe, billions of hydrogen atoms are changing into helium atoms every day. When a star runs out of hydrogen, it becomes a supergiant or a red giant star.
Not all the hydrogen stayed inside stars, though. Some of it began spinning around the outside of the star, orbiting the new star. This hydrogen mixed with other kinds of atoms, and gradually gravity pulled them together into planets. There are many hydrogen atoms on Earth (though most of them are combined with other atoms to make molecules).
The first men riding in a hydrogen balloon
(in France, 1783 AD)
Because hydrogen atoms are so simple, they are very light. They are lighter than air, so if you fill a balloon with hydrogen, it will float up to the sky (like a helium balloon that you can get at the store). But the most important things about hydrogen for us are that two hydrogen atoms combine with one oxygen atom to make a molecule of water, and that hydrogen atoms combine with carbon atoms to make hydrocarbons, which all living things are made of.
To find out more about hydrogen and other atoms, check out these books from Amazon.com or from your library: