Atomic Mass

Atomic NumberAtoms are arranged on the periodic table in order of atomic number, or the number of protons.

P

N

N

P

e

e

2 protons

2 protons

+ 2 neutrons

Ignore

electrons

Atomic Number

Isotopes are atoms of the same element that have the same number of protons but a DIFFERENT number of neutrons.

Each isotope is a specific nuclide

Isotopes

Note that the # of protons stays the same, so the nucleus’ charge stays the same

Note that isotopes do not vary chemically in any significant way

Mass number = # protons + # neutrons

Nuclear Symbol is the proton # and the mass number next to the Element symbol. We’re going to write them with the mass # on the top left and the atomic # on the bottom left

Ex: Helium-3 has 2 protons and 1 neutronsMass# =

Helium-4 has 2 protons and 2 neutronsMass# =

Mass Number

Element and Isotope Practice

Isotope # of Protons

# of Neutron

s

# of Electro

nsMass # Nuclear

Symbol

Hydrogen-1Hydrogen-3Copper-63

Copper-65

6 6

6 8

These are relative mass units: we assign the mass of 1 proton to be 1/12th of a Carbon-12 atom

Why do you think we do this?

Scientists decided they liked carbon a lot 1 carbon atom has roughly 12 times more mass than hydrogen (1

proton) Because the mass of a proton and neutron are very miniscule and

hard to think of in terms of grams

Atomic Mass Units

If we know there are different Isotopes of 1 element, how can we arrange the elements on the periodic table by mass correctly?

Remember when we didn’t mass our filter paper in the Separation Lab? How did we compensate for the fact that our filter paper would be similar to another sheet of filter paper, but not identical? How did we get a good value?

A weighted average!

Average Atomic Mass

Let’s say we have an element named Derpium from the alternate universe of the Land of Ooo.

Derpium exists in 2 isotopes, one with an atomic mass of 16 amu and the other with an atomic mass of 19 amu.

Derpium-19 is less stable and thus less common in the Land of Ooo, so it’s only found 25% of the time.

Derpium-16 is more stable and thus found 75% of the time.

How to do a Average Atomic Mass

If the head wizard of the Land of Ooo had a chunk of Derpium Ore and didn’t feel like explaining to the King that 25% of The Derpium had less mass than the rest of the 75% of the Derpium, he could create a weighted average

25% of the ore is 19 amu: 19 * 0.25 =

75% of the ore is 16 amu: 16 * 0.75 =

Average Atomic Mass of Derpium =

How to do a Average Atomic Mass

A. Copper-63 occurs 69.15% of the time B. Copper-65 occurs 30.85% of the time

Step 1: Turn the percentage of A into a decimal:

Step 2: Multiply the amu of A by this decimal:

Step 3: Turn the percentage of B into a decimal:

Step 4: Multiply the amu of B by this decimal:

Step 5: Add the result of Step 2 and Step 4 together

Practice Average Atomic Mass