That is an interesting question. The atomic
number of an atom is the number of protons that
atom has. Almost every atom is made of three
different things: protons, neutrons, and
electrons. Protons and neutrons are in the
middle of an atom and form what is called a
nucleus. Electrons surround the nucleus. A
nucleus and its electrons make up an atom. The
nucleus is very small compared to the space of
the whole atom. If the nucleus were an apple,
the whole atom (the nucleus and electrons) would
be the size of a football stadium!
A hydrogen atom is made of one proton and one
electron. Because hydrogen has one proton, it
has an atomic number of one. It is the only
element whose atoms don't have any neutrons.
Helium has an atomic number of two; helium has
two protons, two neutrons, and two electrons.
Why does hydrogen have only one proton?
It may be because hydrogen was one of the
first elements to exist in the universe. About
700 million years after the Big Bang, things
like protons, neutrons, and electrons began to
come together to form atoms. It was around this
time that hydrogen happened. Because hydrogen
has been around for so long, there is more of it
in the universe than any other element.
Scientists think that about 75% of the
universe's mass is just hydrogen!