Atoms and Ions

atomic number (Z)

The number of protons — it defines which element it is

mass number

protons + neutrons (never on the periodic table — it's per isotope)

How do you find the number of neutrons?

neutrons = mass number − atomic number

In a NEUTRAL atom, electrons equal what?

The number of protons

isotope

Same element (same protons), different number of neutrons — so different mass

Do isotopes have different charges?

No — isotopes differ only in neutrons. Charge comes from electrons.

ion

An atom that gained or lost electrons, giving it a net charge

cation

Positive ion — LOST electrons. Metals form these.

anion

Negative ion — GAINED electrons. Nonmetals form these.

Formula relating charge, protons, electrons

charge = protons − electrons

When an atom becomes an ion, what never changes?

The number of protons — change those and it's a different element

Why isn't atomic weight a whole number?

It's the weighted average of all naturally occurring isotopes

How do you calculate average atomic mass?

Multiply each isotope's mass by its abundance as a DECIMAL, then add the products

Check on an average atomic mass answer

It must fall between the isotope masses, closer to the more abundant one

Why do atoms form ions at all?

To reach the electron count of the nearest noble gas (full outer shell)

Group 1 ion charge

+1

Group 2 ion charge

+2

Aluminum ion charge

+3

Group 15 ion charge

−3

Group 16 ion charge

−2

Group 17 ion charge

−1

Group 18 (noble gases) ion charge

They don't form ions — outer shell is already full

Silver ion charge

Always +1 (so it skips the Roman numeral)

Zinc ion charge

Always +2 (so it skips the Roman numeral)

Why do most transition metals need Roman numerals?

They form more than one possible charge — the numeral says which

How do you find a transition metal's charge in a formula?

Work backwards: the compound is neutral, so balance against the anion's known charge

alkali metals

Group 1

alkaline earth metals

Group 2

halogens

Group 17

noble gases

Group 18

The seven diatomic elements

H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

The metalloids (the staircase)

B, Si, Ge, As, Sb, Te

Where are metals vs nonmetals?

Metals left of the staircase, nonmetals right of it