There are 88 naturally occurring elements/atoms, but there are 118 atoms found in total.

Element:
A collection of only the same atom, like H2, O2, N2, F2, Cl2, Br2, and I2
Neutral AKA 0 charge

Subatomic particles:
Particles that make up the atom, like the nucleus, electrons, and quarks.

Atomic Number: Amount of protons, and usually electrons.

Isotop: An atom that has different amounts of neutrons, but still keeps the same amount of protons and electrons.

Mass Number:
In units(u) where protons and neutrons are 1u and electrons are 1/1836u, so mass basically just comes from the protons and neutrons.
ALSO, the reason for the weird mass numbers on the periodic table is because the atomic mass on the periodic table is the AVERAGE of all of the isotopes AND their abundance.

Unit:
One atomic mass unit is equal to 1/12th of a single Carbon-12 atom.

The number of electrons:
Affect the chemical properties, so the same group on the periodic table has about the same chemical properties.

Fundamental Charge:
Amount of energy/Coulomb in 1 Electron.
e = 1.602e-19 C
Modern value (charge/mass):
e = 9.109e-31 kg

The more energy the electron has, the farther away from the nucleus it is.

Valence electron: The most outer electrons.

Core electrons: The inner electrons shells.

Formula for amount of electrons in each shell: 2*n2, hvor n = skal nummer

Shell Numbers and Letters:
Shell 1: K
Shell 2: L
Shell 3: M
Ect: N, O, P, Q