Atomic Mass and Intro to the Mole

How do isotopes of the same element differ from each other?

Class Opener

To define atomic mass and to demonstrate how the average atomic mass of an element is determined

To introduce the concept of the mole

Objectives

Decimal number listed on the periodic table.

Weighted average of all the naturally occurring isotopes of that element.

No atom has this exact mass.

Atomic Mass

You will notice that the average atomic mass of an element is often closest to the atomic mass of the most abundant element.

Chlorine has two naturally occurring isotopes, chlorine-37 and chlorine-35. Which isotope is more abundant? How do you know?

Study Check

The average atomic mass of an element depends on both the mass and the relative abundance of each of the element’s isotopes. Naturally occurring copper consists of 69.15% copper-63, and 30.85% copper-65.

Calculating Average Atomic Mass

To calculate the atomic mass, take the mass times the decimal form of the percent abundance of each. Then you add the numbers together to get the number that appears on the periodic table.

Naturally occurring copper consists of 69.15% copper-63, and 30.85% copper-65

0.6915 x 62.9296= 43.520.3085 x 64.9278= 20.03 63.55 (This is what appears on the periodic table)

Atomic Mass for Copper

Naturally occurring carbon consists of 98.93% carbon-12 with an atomic mass of 12.00, and 1.07% carbon -13 with an atomic mass of 13.00. Calculate the atomic mass.

Study Check

An Introduction to Chemistry’s Favorite Number

The Mole

Quantities in ChemistryImagine trying to

count all the grains of sand in the castle

Easier way:Count all the grains

of sand in 1 gram of sand

Weigh all the sand and convert using your previous measurement

Quantities in ChemistryAnother easy way:

Count all the grains of sand in 1 liter of sand

Measure the volume of the sand and convert using your previous measurement

How Does This Relate to Chemistry?Chemicals react in

fixed ratios at the atomic level

In order to predict how reactions will occur, chemists need to know how many atoms or molecules they have

How Does This Relate to Chemistry?Imagine you want to

burn the spoonful of sugar on the rightEvery sugar molecule

reacts with a fixed number of oxygen molecules

In order to know how much carbon dioxide and water will be produced, you need to know how many molecules of sugar you start with

SI unit for amount of substance is called mole.

A mole measures the number of particles within a substance.

A mole refers to a specific number of particles.

(Counting Unit)Particles can be atoms, ionic

compounds, or molecules

1 mole = 6.02 x 1023 particles

6.02 x 1023 is also known as Avogadro’s Number

1 mole aluminum = 6.02 x 1023 Al atoms

1 mole copper = 6.02 x 1023 Cu atoms

1 mole lead = 6.02 x 1023 Pb atoms

Although 1 mole always contains the same number of particles, the mass of one mole varies depending on the substance.

Molar Mass – mass of one mole of a substance.

Mass of one mole of an element is equal to its ATOMIC MASS expressed in grams.

1 mole of aluminum = 26.98 grams1 mole of copper = 63.55 grams1 mole of lead = 207.2 grams

Molecular mass is the sum of the atomic masses of the component elementsH2O = 2 hydrogen, 1 oxygen = 2(1) + 1(16) = 18CH4 = 1 carbon, 4 hydrogen = 1(12) + 4(1) = 16NaCl = 1 sodium, 1 chlorine = 1(23) + 1(35) = 58

1 mole of molecules will have a weight that equals the molecular mass1 mol H2O = 18 g1 mol CH4 = 16 g1 mol NaCl = 58 g

Relating Mass to Moles

Representative Particles & Moles

Representative Particles and Moles

Substance Representative particle

Chemical formula

Representative particles in 1.00 mol

Copper Atom Cu 6.02 × 1023

Atomic nitrogen Atom N 6.02 × 1023

Nitrogen gas Molecule N2 6.02 × 1023

Water Molecule H2O 6.02 × 1023

Calcium ion Ion Ca2+ 6.02 × 1023

Calcium fluoride Formula unit CaF2 6.02 × 1023