1. Calculating Atomic Mass

2. Major Subatomic Particles
Name
Symbol
Charge
Relative Mass (amu)
Actual Mass (g)
Electron e- -1
1/1840
9.11x10-28
Proton p+ +1 1
1.67x10-24
Neutron no
Atoms are measured in picometers, meters
Hydrogen atom, 32 pm radius
Nucleus tiny compared to atom
If the atom were a stadium, the nucleus would be a marble
Radius of the nucleus is on the order of m
Density within the atom is near 1014 g/cm3

3. Elemental Classification
Atomic Number (Z) = number of protons (p+) in the nucleus
Determines the type of atom
Li atoms always have 3 protons in the nucleus, Hg always 80
Mass Number (A) = number of protons + neutrons [Sum of p+ and nº]
Electrons have a negligible contribution to overall mass
In a neutral atom there is the same number of electrons (e-) and protons (atomic number)

4. Nuclear Symbols
Every element is given a corresponding symbol which is composed of 1 or 2 letters (first letter upper case, second lower), as well as the mass number and atomic number E A Z
elemental symbol
mass number
atomic number

5. W F Br 184 74 19 9 80 35 Find the number of protons number of neutrons
number of electrons
atomic number
mass number W
184 74 F 19 9 Br 80 35

6. If an element has 91 protons and 140 neutrons what is the:
If an element has an atomic number of 34 and a mass number of 78 what is the:
number of protons in the atom?
number of neutrons in the atom?
number of electrons in the atom?
complete symbol of the atom?
If an element has 91 protons and 140 neutrons what is the:
atomic number?
mass number?
number of electrons?
complete symbol?

7. Isotopes
Atoms of the same element can have different numbers of neutrons and therefore have different mass numbers
The atoms of the same element that differ in the number of neutrons are called isotopes of that element
When naming, write the mass number after the name of the element H 1
Hydrogen-1 2
Hydrogen-2 3
Hydrogen-3

8. How heavy is an atom of oxygen?
There are different kinds of oxygen atoms (different isotopes)
16O, 17O, 18O
We are more concerned with average atomic masses, rather than exact ones
Based on abundance of each isotope found in nature
We can’t use grams as the unit of measure because the numbers would be too small
Instead we use Atomic Mass Units (amu)
Standard amu is 1/12 the mass of a carbon-12 atom
Each isotope has its own atomic mass

9. Calculating Averages
Average = (% as decimal) x (mass1) (% as decimal) x (mass2) (% as decimal) x (mass3) + …

10. Try Again…THINK!!
Your test and quiz mark is 30%. 10% is made up of quiz, 20% is made up of tests If you made 85%, 78% and 79% on your quizzes If you made 87% on your only test… What would be your test quiz mark for your report card?

11. Calculating Averages
Calculate the atomic mass of copper if copper has two isotopes
69.1% has a mass of amu
The rest (30.9%) has a mass of amu
Magnesium has three isotopes
78.99% magnesium 24 with a mass of amu
10.00% magnesium 25 with a mass of amu
The rest magnesium 26 with a mass of amu
What is the atomic mass of magnesium?

12. Average Atomic Masses
If not told otherwise, the mass of the isotope is the mass number in amu
The average atomic masses are not whole numbers because they are an average mass value
Remember, the atomic masses are the decimal numbers on the periodic table

13. Isotope Worksheet Question 2a: 10B = 18.8 %, 11B = 81.2 %
Avg = (10)(0.188) + (11)(0.812) = amu
Skip 3f and 3g, unless you really want a challenge 