Atomic Number

• The number of protons in an atom

• Also, the number of electrons in a neutral atom

atomic number atomic number atomic number 1 6 79

Atomic Mass

• The number of protons + neutrons in the nucleus

• Since the mass of an electron is soooo much smaller than the mass of protons or neutrons, we do not include it in the atomic mass

Showing atomic number and mass

7 Li 3

3 protons3 electrons7-3 = 4 neutrons

How many protons, neutrons, & electrons?

Ion

• An atom that has gained an electron is called an anion. It has a net negative charge.

• An atom that has lost an electron is called a cation. It has a net positive charge.

Isotope

Atoms of the same element that have different atomic masses (but same atomic number)

WHY do they have different atomic masses?

Isotope

• If two atoms are of same element, they MUST have the same number of protons (the same atomic number)

• But…. they can have different number of NEUTRONS. The more neutrons, the heavier the atom

Hydrogen

• Hydrogen is the smallest atom.• Most hydrogen atoms have only one

proton and one electron (no neutrons)

1H

1

One isotope of Hydrogen has a neutron

Another isotopes of Hydrogen has 2 neutrons

Isotopes of Hydrogen

Isotopes of Lithium

3 neutrons 4 neutrons 5 neutrons

Isotopes of carbon