11

Mr. Shields Regents Chemistry U01 L05

22

Atomic MassAtomic Mass

Up to now we discussed Atomic mass number in terms ofThe Number of Neutrons and Protons

Ex. 8p + 9n = atomic mass number 17

But mass should tell us how much matter is present.

What does “17” really tell us about how much matter(mass) is present?

33

Atomic MassAtomic Mass

Remember - Neutrons and Protons DON’T reallyHave the same mass.

Mass of a Proton = 1.6726 x 10-24 gramMass of a Neutron = 1.6749 x 10-24 gram

So a neutron is really a little heavier than a Proton.So what’s the implication?

If I have 19 neutrons and 19 protonsin a nucleus it DOES NOT have exactly the same mass asA nucleus with 21 neutrons and 17 protons even thoughThe stated mass number is the same (38)

44

Atomic MassAtomic Mass

Let’s look at the mass of one of the isotopes of Carbon.

Carbon-12: 6 protons + 6 neutrons

6p x (1.6726 x 10-24) + 6 x (1.6749 x 10-24) grams1 atom of carbon-12 = 2.00850 x 10-23 grams

But this is an “awkward” number to work with

55

Atomic Mass UnitsAtomic Mass Units

Instead of actual weights in grams Scientist use a unit called an ATOMIC MASS UNIT

- Abbreviated “AMU”

Scientist decide NOT to base theAMU on either the proton or neutron

Instead an arbitrary “Standard” was chosen

66

Atomic Mass UnitAtomic Mass UnitThe “Standard” they chose was an isotope of Carbon

- Carbon-12 (REMEMBER THIS!!)

The mass of CARBON-12 wasdefined to be EXACTLY equal to12 AMU’s

In other words 1 AMU EQUALS 1/12 the mass of aCarbon-12 Atom

1 AMU is slightly less than the mass of either aneutron or a proton

- 1 AMU = 1.661 x 10-24 g

77

AMU’sAMU’s

For example here’s some examples of AMU’s vs Mass No.

Atomic Mass in AMU’s Mass #Proton 1.007825 1Carbon-12 12.00000 12Oxygen-16 15.994915 16Magnesium-25 24.985837 25Nickel-60 59.930791 60Uranium-235 235.043925 235

EXCEPT for Carbon-12 the MASS of an atom in AMU’s is NEVER EXACTLY the same as it’s MASS NUMBER

88

99

Isotopic Abundance/CompositionIsotopic Abundance/CompositionIn nature an element may have several

isotopes

Isotopes have a specific percentcomposition no matter where thesample is collected on earth.

For example, oxygen in the air we breath has this composition:

% AMUOxygen-16 99.76% 15.994915Oxygen-17 0.038% 16.999132Oxygen-18 0.200% 17.999160

Total: 100.00%

liquid

1010

Relative AbundanceRelative AbundanceThese %’s are known as The “Relative Abundances” of the isotope.

In our example of Oxygen the Average AMU’s of a sample ofOxygen must be between 15.994915 and 17.999160. Why?

The AVERAGE MASS of all the elements isotopes is calledthe ATOMIC MASS or the ATOMIC WEIGHT

THIS IS DIFFERENT THAN ATOMIC Mass Number

1111

Isotopic CompositionIsotopic CompositionSo how do we calculate Atomic Mass?

Well it’s simply a weighted average.

Since we’re considering Oxygen ….

Rel Abundance AMUOxygen-16 99.762% 15.994915Oxygen-17 0.038% 16.999132Oxygen-18 0.200% 17.999160

total: 100%

So, What is the Atomic Mass for Oxygen?

1212

Isotopic CompositionIsotopic Composition

i.e. (Mass x abundance) + (mass x Abundance) etc.

(.9976 x 15.994915) + (.00037 x 16.999132) + (.00204 x 17.999160) =

(15.956527) + (0.006290) + (0.036718) = 15.9995

LOOK AT OXYGEN’S MASS ON THE PERIODIC TABLE.

Is it pretty close to our answer ?

Remember: the Atomic mass is the Avg. Atomic mass of all the elements isotopes & we need to use a weighted avg. to Calculate it.

1313

Estimating Atomic MassEstimating Atomic MassLook at our Calculated average Atomic Mass of Oxygen(15.9995) and the relative abundance of the isotopesOf Oxygen.

Abundance AMUOxygen-16 99.762% 15.994915Oxygen-17 0.038% 16.999132Oxygen-18 0.200% 17.999160

Could you guess what the Calculated Atomic mass would beClose to?

Which isotope is present in the greatest amount?

Isn’t it’s mass pretty close to the calculated value?

1414

Estimating Atomic MassEstimating Atomic Mass1. In the following problem which isotope will have a massClosest to the actual atomic mass?

2. Calculate the atomic mass of Chlorine (Assume these values are correct though they are not)

Abundance AMUChlorine-35 70% 35.0Chlorine-37 30% 37.0

1. Chlorine 352. (0.70 x 35) + (0.30 x 37) = 24.5 + 11.1 = 35.6 AMU