isotope

• Are atoms of the same element that have different masses. Due to a different number of neutrons in the nucleus.

Nuclide

• Is a general term for any isotope of any element.

Atomic number

• Represented in mathematical equations by the letter Z in the book examples.

• Remember. The number of protons is represented by the atomic number, and will never change. It is the “fingerprint” of the element.

Mass number

• Is the total number of protons and neutrons in the nucleus of an isotope.

• The neutron and the proton mass values are each approx 1.

• The periodic table reports the average atomic mass of the many nuclides of a particular atom

Atomic mass unit

• The mass of atoms is extremely small.

• Therefore, we use relative atomic masses because it is easier to do calculations.

• Arbitrarily chose a standard and assigned a relative mass value.

• Selected the carbon –12 nuclide.

Atomic mass unit or amu

• Carbon -12 has been assigned the relative value of 12 amu.

• Or 1 amu is exactly 1/12th the mass of a carbon-12 atom ( nuclide)

• Mass of any nuclide is compared to the carbon –12 nuclide

• An atom that is 4x the mass of the carbon-12 atom will be 4 x 12 amu = 48 amu.

continued

• An atom that is 2/3 the mass of the carbon-12 atom will have a mass of 2/3 x 12 amu = 8 amu.

Finding average atomic mass

• Multiply the abundance factor, (%), by the mass number of the isotope

• Add up all the isotopes

• Divide by 100

example

• Neon has two isotopes Ne-20 has a mass of 19.992 amu and occurs in nature 90% of the time. Ne-22 has a mass number of 21.991 amu and occurs 10% of the time.

• ((90 x 19.992) + (10 X 21.991) ) / 100= 20.192 amu

How many in a dozen?

• How many in 2 dozen

• If you have 36 shoes, how many dozen?

Mole

• The chemist dozen

• Instead of 12 units of something it is

6.02 x10 23 units of something.

• 6.02 x 1023 is called Avogadro’s number– Or the number of particles in exactly one mole

of a substance.

Molar mass

• The mass of one mole of a substance.

• Units are g/mol

• Numerically equal to the atomic mass listed on the periodic table.

• The molar mass of an element contains one mole, or 6.02 x1023 atoms of that element.

conversions

• He has a molar mass of 4.00 g/mol

• How many grams of He are there in 5 moles?

• 4.00 g/mol x 5 mol = 20 grams

• Note: the mole unit crosses out.

problem

• How many grams will there be in 4 moles of oxygen?

again

• How many grams are there in 4.5 moles of copper?

• Page 83 sample problem 3-2 1,2,3,4

Now the other way

• Given the grams calculate the moles.

• You have 72 grams of copper, how many moles do you have?

• 72 grams / 63.55 g/mol = 1.13 moles of Cu

• Page 83 sample problem 3-3 1,2

How many moles is a given # of atoms?

• If you have 2.0 x 10 23 atoms of Ca, how many moles do you have.

• 2.0 x 1023 atom Ca x 1 mol Ca/6.02x 1023 atom of Ca = 0.33 mol Ca

• Page 84 sample problem 3-4 1,2,3

Given # atoms find the mass

• Find the mass of 5 x 10 16 atoms of copper.

• 5 x 10 16 atoms Cu x 1mole/6.02x10 23atoms multiplied by 63.55 g/mol = 1.31 x 10-9

• Page 85 practice problem 3-5 1,2,3