Physics, 8th grade. Section 29. Structure of atoms.
Everything around us consists of atoms. Atoms are composed of protons, neutrons, and electrons.
At the center of an atom is a large nucleus consisting of protons and neutrons, with electrons orbiting around it.
The masses of protons and neutrons are approximately equal, but the mass of an electron is almost 1840 times smaller than the mass of a proton.
Protons have a positive charge, electrons have a negative charge, and neutrons have no charge.
The number of protons in an atom is equal to the number of electrons and is equal to the atomic number of the chemical element in the periodic table.
Note that the periodic table also lists the atomic weight—the sum of the number of protons and neutrons. Atomic weight is not a whole number. The fractional part is due to isotopes, where the number of protons and neutrons may differ from the normal number.
The number of protons in an atom is equal to the number of electrons. The charge of a proton is equal in magnitude to the charge of an electron, but opposite in sign.
The total charge of an atom is zero—the total charge of the electrons balances the total charge of the protons.
The number of neutrons is greater than or equal to the number of protons, but this does not affect the charge.
This is the typical structure of a normal atom. But if nothing changed in it, we would have no electricity. In fact, electrons orbiting in the outer orbit can leave their atom and become free—that is, free to move between other atoms. Accordingly, other—"foreign"—electrons can arrive in the outer orbit.
From an electrical perspective, we are only interested in whether an atom can have extra electrons or have "holes"—vacant spaces for electrons. If an atom has extra electrons, it has a negative charge; if an atom is missing electrons, its charge is positive. Such atoms are called ions—accordingly, an ion can be negative or positive. Unlike a normal, uncharged atom, an ion is a charged particle, and the behavior of charged particles is influenced by an electric field.
Since electrons can move freely, from an electrical perspective, the only constant in an atom is the charge of the nucleus.
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