1. Atomic Number, Mass Number, Atomic Mass and Isotopes
Ms. McCallum
Chemistry

3. Atomic Number (Z): is the number of protons
in the nucleus of the atom.
Z=#p

4. The number of protons (atomic number) determine the identity of an element.

5. Atoms have no overall electrical charge so, an atom must have as many
electrons
as there are
protons
in its nucleus.

6. The atomic number of an element also equals the number of electrons
in a neutral atom of that element.

8. Ex: Sodium What is the atomic number of Sodium?
How many protons does sodium have?
How many electrons does sodium have?

9. Mass Number (A): The sum of the protons and neutrons in the nucleus.
A=#p + #n

10. Notation

11. Other ways to write elements:36
17Cl Cl-36
Mass Number
Mass Number
Atomic Number

12. Nucleons:
protons and neutrons

13. Isotopes of an element have different mass numbers because they have
different numbers of neutrons,
but they have
the same atomic number.

15. Example: Isotopes of Carbon and Hydrogen
protium deuterium tritium
H H H
Isotopes of Hydrogen
Isotopes of Carbon

16. is a unit used to compare
Atomic Mass Unit
is a unit used to compare
the masses of atoms
and has the symbol u or
amu.

17. is approximately equal to
1 amu or u
is approximately equal to
the mass of
a single
proton or neutron.

18. Carbon-12 Chemists have defined the carbon-12 atom as having a mass of
12 atomic mass units.

19. 1 u = 1/12 the mass of a Carbon-12 atom.

20. Atomic Mass is the weighted average mass
of all the naturally occurring
isotopes
of that element.