Atoms of the same element are NOT exactly similar

They may have different masses◦ Vast majority of mass is from protons and neutrons

Element is defined as # of protons◦ ∴ protons cannot be different

Electrons must balance protons in neutral atom◦ ∴ electrons cannot be different

# of neutrons may be different between atoms◦ These are isotopes of the same element

Isotopes are atoms of the same element with different masses due to different number of neutrons

View P. 113 in textbook Same Element, different mass due to neutrons

P-7N-7

Mass = 14

P-7N-8

Mass = 15

◦ A = mass number◦ Z = atomic number◦ X = symbol◦ Example:

Full Example: Sodium-24

Short Hand Example: Na-24

Mixed Example: 24Na

The atomic mass unit has 2 common symbols◦ μ◦ amu

1 amu Defined as 1/12th the mass of a carbon-12 atom The size of the atomic mass unit is arbitrary Could have used

◦ 1/24th the mass of a carbon atom or◦ 1/10th the mass of the iron or…….

Three reasons for using 1/12th the mass of a C-12 isotope are:◦ Carbon is a very common element.◦ It results in nearly whole-number atomic masses for most other

elements.◦ Lightest element, hydrogen (H), has a mass of approximately 1 amu.

The mass of C-12 is EXACTLY 12amu (important for sig-figs)

Mass on Periodic table is the average of isotopes naturally found◦ No Silicon isotope with a mass of 28.086amu

This does tell us that likely MOST Silicon isotopes have a mass of close to 28 amu

Some elements have 2-3 common isotopes Others have 5-6 common isotopes

◦ Tend to be heavier elements Some isotopes are highly radioactive

Example:◦ What is the average of the following? (3 Sig-Figs)

Cl-35 Cl-35 Cl-35 Cl-35 Cl-36 Cl-36 Cl-36

◦ 35.4amu Add all masses, divide by the number of atoms

Example:◦ Find the average mass of the following? (3 sig-

figs)

10.8amu

Isotope Number of Atoms

B-10 170

B-11 300

B-12 75

TOTAL: 545

Example◦ One of the dietary sources of potassium is the banana:

93.1% of the potassium atoms are potassium-39 (20 neutrons), 6.88% are potassium-41, and only a trace are potassium-40. Find average mass.

◦ Pretend % is # of atoms and the total # of atoms is 100 39.1amu

Isotope % Abundance

K-39 93.1%

K-41 6.88%

K-40 0.02%

Example◦ Elemental boron is a combination of two naturally

occurring isotopes: boron-10 has a relative abundance of 19.78% and boron-11 has a relative abundance of 80.22%. Find average mass.

10.80amu

Isotope Measured Mass (amu)

% Abundance

B-10 10.001 19.78%

B-11 10.997 80.22%

Read Textbook P.110-117 Textbook Questions

◦ P.112 #’s 17-18◦ P.113 #’s 19-20◦ P.116 #’s 21-22◦ P.117 #’s 23-24

Handout◦ Calculating Average Atomic Mass

Due Next class