The Lion and the Gazelle

Every morning in Africa, a gazelle wakes up. It knows it must run faster than the fastest lion or it will be killed.

Every morning a lion wakes up. It knows it must outrun the slowest gazelle or it will starve to death.

It doesn't matter whether you are a lion or a gazelle. When the sun comes up, you better start running.

-African Proverb

Section 3-3Counting Atoms

Coach Kelsoe

Chemistry

Pages 77–87

Atomic Number

Atoms of different elements have different numbers of protons.

The atomic number of an element is the number of protons in the nucleus of each atom of that element.

Atomic Number

Atoms of hydrogen have 1 proton; atoms of helium have 2 protons, and so on.

The atomic number identifies an element.

Since atoms are neutral, an atom will have the same number of protons as electrons.

Isotopes

Isotopes are atoms of the same element that have different masses.

Isotopes will have the same number of protons and electrons, but will differ in the number of neutrons.

Tin has the most isotopes (10) For example, Hydrogen has 3 isotopes:

Protium- one proton, no neutrons Deuterium- one proton, one neutron Tritium- one proton, two neutrons.

Mass Number

The mass number of an atom is the total number of protons and neutrons in the nucleus of an isotope.

Designating Isotopes

Only hydrogen has different names for its isotopes.

There are two ways we designate isotopes: Hyphen notation: hydrogen-1, uranium-235 Nuclear symbol: mass # top, atomic # 1H, 235U 1 92

We find the number of neutrons by subtracting the atomic number from the mass number- Ex: 235U 235-92=143 neutrons

92

Sample Problem

How many protons, neutrons, and electrons are found in an atom of Helium-4? Protons-2, Neutrons-2,

Electrons-2 How many protons, neutrons,

and electrons are found in an atom of 120Sn?

50

Protons-50, Neutrons-70, Electrons-50

Nuclides

Nuclide is a general term for any isotope of any element. Nuclides for

hydrogen are protium, deuterium, and tritium.

Relative Atomic Masses

Masses of atoms in grams are very small.

When comparing masses of atoms, we use atomic mass units.

An atomic mass unit is equal to 1/12 the mass of a carbon-12 atom.

Average Atomic Masses

Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element.

To calculate average atomic mass, we multiply the mass of each isotope by the decimal fraction representing its percentage in the mixture.

Sample Calculation

If we had 3 red styrofoam balls with a mass of 4.0 grams and 5 black styrofoam balls with a mass of 5.0 grams, what would it’s average mass be? (3/8)(4.0 g) (.375)(4) = 1.5 g (5/8)(5.0 g) (.625)(5) = 3.125 g 1.5 g + 3.125 g = 4.625 g average

Bellringer

List how many protons, neutrons, and electrons are in each of the following atoms.

Nickel-60

Barium-138

Krypton-86

Nitrogen-14

31P

201Hg

42Ca

184W

15

80

20

74

Relating Mass to Numbers of Atoms

There are three important concepts that provide the basis for relating masses in grams to numbers of atoms: The mole Avogadro’s number Molar mass

The Mole

The mole is the SI unit for amount of substance.

A mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12.

The mole is a counting unit, like dozens.

Not a real mole!

Avogadro’s Number

The number of atoms in a mole has been determined to be 6.0221367 x 1023.

Avogadro’s number – 6.022 x 1023 – is the number of atoms in exactly one mole of a pure substance.

Molar Mass

The mass of one mole of a pure substance is called the molar mass of that substance.

Molar mass is written in g/mol and is numerically equal to the atomic mass of the element.

Even though the numbers are the same, molar mass and atomic mass aren’t exactly the same. Molar mass is measured in g/mol, atomic mass is measured in amu or g.

Gram/Mole Conversions

We can make a conversion factor to convert units from grams to moles and vice versa. For example:

Sample Problems

What is the mass in grams of 2.85 mol of the element potassium, K?

2.85 mol K x 39.10 g K = 111 g K

What is the mass in grams of 5.10 mol of the element oxygen, O?

5.10 mol O x 16.00 g O = 81.6 g O

1 mol K

1 mol O

Sample Problems

How many moles of iron, Fe, are in 7.00 g of iron?

7.00 g Fe x 1 mol Fe = 0.125 mol Fe

How many moles of gold, Au, are in 234.0 g of gold?

234.0 g Au x 1 mol Au = 1.188 mol Au

55.85 g Fe

196.97 g Au

Conversions with Avogadro’s Number

We can make a conversion factor to convert units from moles to atoms and vice versa.

Here’s a GREAT conversion factor:

- 12 National Championships + 5 NCAA probations x smelly cow college =

Sample Problems

How many moles of sulfur, S, are in 89700 atoms of sulfur?

89700 atoms x 1 mol S =

How many moles of sodium, Na, are in 6.2 x 104 atoms of sodium?

6.2 x 104 atoms x 1 mol Na =

6.022 x 1023 atoms

6.022 x 1023 atoms

Sample Problems

How many atoms of lead, Pb, are in 0.325 mol of lead?

0.325 mol x 6.022 x 1023 atoms =

How many atoms of fluorine, F, are in 0.018 mol of fluorine?

0.018 mol x 6.022 x 1023 atoms =

1 mol

1 mol