Review of Atomic Model
How does the modern atomic theory describe the parts that make up the atom?
Atomic Number: Periodic Table
Atomic number:
number of protons in
the nucleus. This
number is found above
element on a periodic
table.
Atomic Number: Symbol
The atomic number of an element is
usually written on lower left corner
of symbol.
Protons and Neutrons
Two particles in the nucleus: Protons have a positive charge.
Neutrons do not have a charge.
Neutron Number
The following table shows the most common number of neutrons found in an element.
1
Mass Number
Mass number refers to the total number of protons and neutrons.
Example:
fluorine: 9 protons 10 neutrons
Atomic number is 9. Mass number is 9 + 10 = 19
Mass Number: Symbol
The mass number is written on upper left corner of element.
Electrons
Electrons have a negative charge and move outside the nucleus.
The number of electrons is equal to the number of protons in a neutral
atom.
This lithium atom has
3 protons and 3 electrons.
Atomic Mass
Atomic mass is stated in units called atomic mass units (amu).
The symbol for 1 amu can be a “u” or a µ.
The amu is based on 126C.
1 amu is defined as 1/12 the mass of 126C.
Atomic Mass Periodic Table
The atomic mass is found below the symbol for an element on the periodic
table.
Atomic Mass Defined
The mass of 126C is said to
be 12 m.
One atomic mass unit
(amu) is defined as being
1/12 the mass of 126C.
Hydrogen has a mass of 1 µ
(1/12 the mass of 126C).
Helium has a mass of 4 µ
(4/12 the mass of 126C).
2
Masses of Particles
A proton is a “heavy” particle. It mass a mass of 1 µ.
A neutron is also a “heavy” particle. It mass a mass very close to 1 µ.
An electron is very very light. It has a mass of about 1/2000 µ, so sometimes we say that it
has a
mass of 0 µ
(even though it does
have some mass).
Checklist
1. Use the modern atomic theory to describe the meaning of the terms atomic number, mass
number, atomic mass.
2. Using the form ABX, write the proper symbol for an element.
3. Predict the number of protons, electrons, and neutrons knowing the atomic number and
mass number of an atom.
4. State the charge, location, and mass of the subatomic particles.
3
uranium
uranium
4