Atomic structure and the periodic table

Subatomic particles

• Proton: charge +1, mass 1, in the nucleus.
• Neutron: charge 0, mass 1, in the nucleus.
• Electron: charge -1, negligible mass, in energy levels.

Mass number = protons + neutrons. Atomic number = protons.

²³₁₁Na has 11 protons, 12 neutrons, 11 electrons.

Isotopes

Atoms of the same element with the same number of protons but different neutrons: ¹²C and ¹⁴C; ³⁵Cl and ³⁷Cl.

Electron arrangement (first 20 elements)

Shells hold 2, 8, 8 electrons.

• Na (11): 2.8.1
• Cl (17): 2.8.7
• Ca (20): 2.8.8.2

The periodic table

• Groups (vertical columns): same number of outer electrons and similar properties. Group I: alkali metals (Li, Na, K); Group VII: halogens (F, Cl, Br); Group VIII (0): noble gases (He, Ne, Ar).
• Periods (horizontal rows): same number of energy levels.
• Reactivity of Group I metals increases down the group; reactivity of halogens decreases down the group.