Download Atomic Number

How Atoms Differ
Mr. E Chemistry
Atomic Number = number of protons in the nucleus of an atom
- It really is the identity of the element. What did you
discover yesterday when you changes the amount of
protons in an atom?
- The atomic number identifies the element.
6
C
The # of electrons equals the number of protons in an atom.
If the number of protons does not equal the number of electrons
we call it an ion.
Not all atoms of an element are identical. The different versions of
an element are called isotopes.
Isotope = atoms of the same element with different numbers of
neutrons
Ex: Carbon
~95%
12
6
C
Protons =
Neutrons =
~5%
BUT
14
6
C
Protons =
Neutrons =
Mass Number = # of protons + # of neutrons in a single atom
- Different isotopes have different mass numbers
- NOT on the periodic table!
Ex:
Mass Number
39
19
Atomic Number
Protons =
K
Neutrons =
Mass Number =
Atomic Mass Unit (amu) = defined as 1/12 the mass of a carbon-12
atom
Composed of 2 particles:
1) Proton = positive charge
- mass of 1.673 x 10-24 g = 1 amu (atomic mass unit)
2)Neutron = neutral, no charge
- 1 amu
Average Atomic Mass (or Atomic Mass) = weighted average of all
the isotopes of an element
How to calculate it:
Chlorine-35
Atomic Mass = 34.969 amu
75.770% abundance
Chlorine-37
Atomic Mass = 36.966 amu
24.230% abundance
The average atomic mass of chlorine is closer to the mass of chlorine35 because chlorine-35 is much more abundant in nature.
% Abundance Equation
Electrons = negatively charged particle
- mass =
1
1678
amu … basically no mass!
- Charge = # of protons - # of electrons

Ex:
Mass Number
7
3
Atomic Number
Charge
Li
If there is nothing in the
charge position, the atom
has 0 charge = neutral!
Protons =
Neutrons =
Electrons=
Ex:
24
12
36
17
Mg
+2
Protons =
Neutrons =
Electrons=
Cl
-1
Protons =
Neutrons =
Electrons=
Ex:
24
Na
Protons =
Neutrons =
Electrons= 10
S
Protons =
Neutrons = 16
Electrons= 18