When the structure of the atom was first being
studied, people had no idea what they would find,
or even that atoms existed at all. However, after
many experiments, it was found that atoms are
built of smaller components, some of which are
charged (protons and electrons).
It was also discovered that the charge of
the electron was exactly equal and opposite to the
charge of the proton. Since the overall
atoms were uncharged, the number of
protons and electrons had to be the same in each
atom. However, some atoms gain or lose
electrons, and thus become charged. These
charged atoms are called ions, and they
don't have equal numbers of protons and
electrons. For example, common table salt,
sodium chloride, is made of sodium ions and
chloride ions. Sodium ions have 11 protons and 10
electrons; chloride ions have 17 protons and 18
electrons. But, a fundamental law of
physics--the law of charge conservation--states
that the total amount of charge in a system always
stays the same. So, in any uncharged
system, the sum of charges must be zero.
Since the universe is thought to be uncharged,
that means that the total number of protons in the
universe must equal the total number of electrons.
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