1. Goals To define ions and isotopes
To interpret data given about ions and isotopes

2. Why is there usually a decimal place in the atomic mass number?
It is due to isotopes.
Isotopes – atoms of the same element with the same # of protons but a different # of neutrons

3. Why is there usually a decimal place in the atomic mass number?
Remember: Protons and neutrons have a relative mass of 1, and electrons are 1/1840, so e- don’t really count when determining the mass
Since most of the mass of the element is from protons and neutrons, changing the number of neutrons changes the mass of the atom

4. Why is there usually a decimal place in the atomic mass number?
To determine the atomic mass, scientists take an AVERAGE mass of all isotopes of that atom. This gives us decimal places.
When identifying isotopes, the mass number is given.

5. Mass Number
The mass number is a whole number (no decimals)

6. Example Carbon-12 has a mass number of 12.
How many protons does it have? Why?
6 p+ (atomic number)
How many electrons does it have? Why?
6 e- (same as p+)
How many neutrons does it have? Why?
6 n0 (mass number given in name – atomic number)

7. Example Carbon-14 has a mass number of 14 How many p+ does it have?
How many e- does it have?
How many neutrons does it have? Why?

8. Why is there usually a decimal place in the atomic mass number?
When scientists take the average of these masses to determine the atomic mass, they also consider which is more abundant (common) in nature.
C-12 is more often found in nature so the atomic mass of C is , which is closer to 12 than 14

9. IN SUMMARY….
Mass number is a whole number and only true for one isotope of that atom
Atomic mass has decimal places b/c it’s an average of the masses of all isotopes.
Isotopes are atoms of the same kind (so they have the same # p+ and e-), but have different #s of neutrons.