September 20th, 2012

Do Now:

Clear Desk except for calculator/writing utensil for Quiz

Do Now (Part 2)

Complete this problem in your notebook :

a)The radius of an atom of krypton is about 1.9 Angstroms. Express this distance in nanometers and in picometers.

b) If the atom is assumed to be a sphere, what is the volume in cubic cm of a single Kr atom?

Which Is Which!?

Discuss with your neighbor, what question could we try and answer today?

How does an atom of silver differ from an atom of platinum?

Subatomic Composition! Compare Platinum and Silver on the

periodic table. If you were missing the element symbol, how might you determine which is which?

Let's break it down...

Elements are symbolized by one or two letters.

Each element has a specific letter symbol...

Why are some elements symbols not the first letter of their name?

Atomic Number (z)

Why do all atoms of the same element have the same number of protons and electrons?

According to Dalton's law of constant composition, do all atoms of the same element have the same atomic number? Why?

A = Z + n

Atomic mass number (A): the total number of protons and neutrons in the nucleus of an atom. (Total # of nucleons)

Practice!

Fill in the following information for each of the following elements.

1) Ca 2) S 3) Ar 4) X

5) X

__#e

__#n

__#p

__#e

__#n

__#p

__#e

__#n

__#p

__#e

__#n

__#p

__#e

__#n

__#p

Predict why those two x's have different mass numbers?

Atoms with identical atomic numbers but different mass numbers are called isotopes.

How does carbon-12 differ from carbon-14?

Isotopic atoms have the same number of protons, but different number of neutrons.

Let's take a look...

Practice!

Magnesium has three isotopes, with mass numbers 24, 25, and 26.a) write the complete chemical symbol

(subscript and superscript) of each.b) how many neutrons are in an atom of

each isotope?

How many protons, neutrons, and electrons are in an atom of phosphorus-31?

So...

So if isotopes exist, how did we get the atomic mass for the periodic table? Discuss with your neighbor.

A sample of an element in nature consists of a mixture of its naturally occurring isotopes.

Atomic Mass (with isotopes)

Atomic Mass of an element is a weighted average of all isotopes, resulting in fractional values.

Mass spectrometry is used to find the mass and percentage of each isotope.

Mass Specto...what?

Here is another example...

So how does it look?

Calculating Atomic Mass

Weighted Average

Atomic mass = (%A) (mass of isotope A)+ (%B) (mass of isotope B) 100 100

Example!

Naturally occurring chlorine is 75.78% chlorine-35 (which has an atomic mass of 34.969 amu), and 24.22% chlorine-37 (which has an atomic mass of 36.966 amu).

Calculate the average atomic mass (atomic weight) of chlorine.

Practice!

1) Naturally occurring carbon is a mixture of two isotopes, 12C (98.89%) and 13C (1.11 %). Individual carbon atoms therefore have a mass of either 12.000 or 13.003354 amu. What is the atomic mass of a carbon sample.

2) Calculate the atomic mass for Ne with these isotope:

Mass # Mass (amu) %

20 19.992 90.51

21 20.994 0.27

22 21.991 9.22