The table is the theory

Word	What it means
Proton	Positive, in the nucleus. The number of protons is the element.
Neutron	Neutral, in the nucleus. Changing the count gives an isotope — the same element, behaving the same way chemically.
Electron	Negative, in the space around the nucleus. Chemistry is almost entirely about these.
Atomic number	The number of protons. It is what the table is ordered by.
Mass number	Protons plus neutrons.
Isotope	Atoms of one element with different neutron counts.
Valence electrons	The electrons in the outermost shell. They decide how an element reacts, which is why a group behaves alike.
Group / family	A column. Same number of valence electrons, so similar behaviour.
Period	A row. Properties change steadily across it rather than repeating.
Ion	An atom with a charge, because it has lost or gained electrons. Never because it lost protons.

Electrons occupy shells that hold 2, then 8, then 8 for the first twenty elements. An element's group number tells you its valence electrons, and valence electrons decide reactivity.

• Group 1 has one valence electron to lose, so these metals are very reactive.
• Group 2 has two to lose, forming 2+ ions.
• Group 17 needs one more, so these non-metals are very reactive and form 1− ions.
• Group 18 is already full, which is why the noble gases react with almost nothing.

Trends worth knowing. Reactivity increases down group 1 and up group 17. Both have the same cause: the further the outer electron is from the nucleus, the more easily it is lost — and the harder one is to attract.

A historical note that is really a method note. Mendeleev ordered by atomic mass and had to swap a few pairs to make the families work, and left gaps for elements nobody had found. Moseley later showed the right ordering was by proton count, which removed the exceptions. A theory that predicts missing things is doing more than a chart.

Protons = atomic number  ·  Neutrons = mass number − atomic number  ·  Electrons = protons, if neutral

Worked. Carbon-14 has 6 protons (carbon's atomic number) and a mass number of 14, so 8 neutrons. It is carbon because of the 6, and it is an isotope because of the 8 — carbon-12 has the same 6 and only 6 neutrons.

Ions. Charge = protons − electrons. Magnesium is in group 2, so it loses two electrons and becomes Mg²⁺. Oxygen is in group 16 and needs two more, so it becomes O²⁻.

The trap. An ion never forms by gaining or losing protons — that would change the element. Only electrons move.

	Metals	Non-metals
Where	Left and middle	Upper right
Conduct?	Yes, heat and electricity	Mostly not
Solid form	Malleable, ductile, shiny	Brittle, dull
Electrons	Lose them, forming positive ions	Gain them, forming negative ions

Elements along the staircase — silicon, germanium — are metalloids, with properties of both. Silicon's halfway conductivity is exactly why semiconductors work.

• Saying an ion lost protons. Only electrons move.
• Confusing mass number with atomic mass. Mass number is a count for one atom; atomic mass on the table is an average over isotopes.
• Reading a group number as an ion charge for the transition metals. The pattern works for the main groups.
• Saying electrons are in the nucleus.