Atomic structure
An atom has a nucleus (protons and neutrons) surrounded by electrons. The atomic number equals the number of protons (and, in a neutral atom, the number of electrons too); the mass number equals protons plus neutrons.
Example 1: An atom has atomic number 17 and mass number 35. Find its number of protons, electrons, and neutrons.
Protons = electrons = atomic number = 17. Neutrons = mass number - atomic number = 35 - 17 = 18.
Electron configuration
Electrons fill shells around the nucleus in order, with each shell holding a maximum number of electrons (2, 8, 8 for the first three shells in simple cases). Writing an atom's electron configuration means listing how many electrons sit in each shell, starting from the one closest to the nucleus.
Example 2: Write the electron configuration of sodium (atomic number 11).
11 electrons fill as 2, 8, 1 -- the first shell takes 2, the second takes 8, and the single electron left goes into the third shell. This lone outer electron is why sodium is so reactive.
Ionic bonding
Ionic bonds form when a metal atom transfers one or more electrons to a non-metal atom, producing a positive ion (cation) and a negative ion (anion) that are held together by electrostatic attraction.
Example 3: Explain how sodium chloride (NaCl) forms.
Sodium (2,8,1) loses its single outer electron to become (2,8), and chlorine (2,8,7) gains that electron to become (2,8,8). Both ions now have full outer shells, and the oppositely charged ions attract to form the ionic compound NaCl.
Covalent bonding
Covalent bonds form when two non-metal atoms each contribute an electron to a shared pair, letting both atoms count that pair towards a full outer shell.
Example 4: Explain the bonding in a water molecule, .
Oxygen needs 2 more electrons for a full outer shell; each hydrogen atom needs 1 more. Oxygen shares one electron pair with each of the two hydrogen atoms, giving two covalent O-H bonds and completing all three atoms' outer shells.