1. Protons, Neutrons, and Electrons
Structure of the Atom
Protons, Neutrons, and Electrons

2. Subatomic Particles Name Symbol Charge Relative Mass (amu)
Actual Mass (kg)
proton p+ +1 1
1.67 x 10-27
neutron n0
electron e- -1
1/1840
9.11 x 10-31

3. Basic Structure of the Atom
There are two regions
The nucleus
With protons and neutrons
Positive charge
Almost all the mass
Electron cloud
Most of the volume of an atom
The region where the electron can be found

4. Describing Atoms Atomic Number Mass Number number of protons
number of electrons (if atom is neutral)
is on the Periodic Table
must be the same for atoms of the same element
Mass Number
number of protons + neutrons
is NOT on the Periodic Table
can be different for atoms of the same element

5. Isotopes Atoms of the same element with different mass numbers.
For isotopes of the same element:
The number of protons stays the same.
The number of neutrons varies.

6. Complete Chemical Symbols
Contain the symbol of the element, the mass number and the atomic number. X
Mass
number
Atomic
# protons
+ # neutrons
mass number
# protons

7. Br Symbol Practice 80 35 Find the number of protons number of neutrons
number of electrons
Atomic number
Mass number Br 80 35

8. Isotopes + + Carbon-12 Neutrons 6 Protons 6 Electrons 6 Carbon-14
Nucleus
Neutron
Proton +
Electrons
Nucleus
Nucleus
Neutron
Proton +
Carbon-12
Neutrons 6
Protons 6
Electrons 6
Electrons
Carbon-14
Neutrons 8
Protons 6
Electrons 6
Nucleus

9. Atomic Mass
Atomic mass is a weighted average of the mass of all isotopes of an element.
The measurement unit is the atomic mass unit (amu).
1 amu = 1/12 of the mass of a carbon-12 atom.
1 p+ = amu 1 n0 = amu 1 e- = amu

10. Calculating Atomic Mass
To calculate the atomic mass, you need the mass of each isotope of the element and the relative abundance of each isotope.
The relative abundance is usually expressed as a percentage. When using the percentages in atomic mass calculations, you will need to express the percentage in its decimal form.
25% = .25
Then, multiply each isotope’s mass by its relative abundance (in decimal form). Record each result, and then add the results for all of the isotopes. The total is the atomic mass.

11. Example: Atomic Mass
Calculate the atomic mass of magnesium. The three magnesium isotopes have atomic masses and relative abundances of amu (78.99%), amu (10.00%), and amu (11.01%).