ATOMIC MASS

October 8, 2014

Atomic Mass (Mass #)

• Definition: the ~ mass of one atom of an element (equals #protons + # neutrons)

• Tells us how much one atom weighs in atomic mass units.

Mass

Organization of The Periodic TableOrdered in columns by increasing atomic mass.Columns arranged to form groups with similar properties

Group/Family

Period

Assessment

• Which element has the greatest atomic mass?

– Lithium

– Sodium

– Potassium

– Rubidium

Reactivity

• The ease and speed with which an element combines, or reacts, with other elements or compounds

Pure sodium reacts explosively with air

Groups/families of elements have similar reactivity

Group 1: metals that react violently with water

Group 18: Gases that barely react at all

Atoms have neutral charge

• # Protons (+) = # Electrons (-) = 0 charge

• Example: Hydrogen– 1 proton + 1 electron

– +1 + (-1) = 0

– Thus # protons = # electrons

• How many electrons does C have?– 6 electrons

RECAP!

• Atoms are composed of protons (p+), neutrons (n0), and electrons (e-)

• Protons and neutrons are in the nucleus

• Electrons orbit

• Atoms have an overall neutral charge equal to zero

The Nucleus of the Atom

The nucleus is…

1. Positively charged

2. Extremely small(diameter is 10,000 x smaller than the atom)

3. Extremely dense

4. Contains virtually all of the atom’s mass

Particle Comparison

Particle Symbol Charge Relative mass

Mass (g)

Electron e- -1 1/1840 9.11 x 10-28

Proton p+ +1 1 1.673 x 10-24

Neutron n0 0 1 1.675 x 10-24

Atomic Mass Units

• Due to extremely small mass we use amu’s (atomic mass units)

• Atomic standard is the carbon atom1 carbon atom = 12 amu

1 amu = 1/12 carbon atom mass

*the values for 1 amu and the masses for 1 proton and neutron are slightly different

e- .000549 amu

p+ 1.007276 amu

n0 1.008665 amu

Atomic Mass

• The weighted average mass of the isotopes of that element

*different than mass number (# of protons + # of neutrons)

Atomic Mass: How Do We Get It?

• All isotopes of an element do not exist in equal abundance in the world

• The atomic mass is the weighted average mass of the isotopes of that element

Example #1

Using the data table, calculate the average atomic mass

6Li = .075 x 6.015 = .4517Li = .925 x 7.016 = 6.490

6.941 amu

Isotope Mass (amu)

Percent abundance

6Li 6.015 7.5%

7Li 7.016 92.5%

Convert the percent to a decimal by dividing by 100!

So, 7.5%/100 = .075

Example #2

• A sample of cesium is 75% 133Cs, 20% 132Cs, and 5% 134Cs. What is its atomic mass?

.75 x 133 = 99.75

.20 x 132 = 26.4

.05 x 134 = 6.7

132.85 amu

+

Example #3

• Assume the following three isotopes of element X exist: X-122, X-124, X-128. If the atomic mass of X is 127.55 amu, which of these isotopes is most abundant?

ANSWER: The most abundant is the one closest to the atomic mass…so it would be X-128!