Chapter 3 Section 3 Counting Atoms Lesson Starter Imagine that your semester grade depends 60% on exam scores and 40% on laboratory explorations. Your exam scores would count more heavily toward your final grade. In this section, you will learn that the atomic mass of an element is a weighted average of the masses of the naturally occurring isotopes of that element. Chapter 3

Section 3 Counting Atoms Objectives Explain what isotopes are. Define atomic number and mass number, and describe how they apply to isotopes. Given the identity of a nuclide, determine its number of protons, neutrons, and electrons. Define mole, Avogadros number, and molar mass, and state how all three are related. Solve problems involving mass in grams, amount in moles, and number of atoms of an element. Chapter 3

Section 3 Counting Atoms Atomic Number Atoms of different elements have different numbers of protons. Atoms of the same element all have the same number of protons. The atomic number (Z) of an element is the number of protons of each atom of that element. Chapter 3 Section 3 Counting Atoms Isotopes

Isotopes are atoms of the same element that have different masses. The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. Most of the elements consist of mixtures of isotopes. Chapter 3 Section 3 Counting Atoms Mass Number The mass number is the total number of protons and neutrons that make up the nucleus of an isotope.

Chapter 3 Section 3 Counting Atoms Designating Isotopes Hyphen notation: The mass number is written with a hyphen after the name of the element. uranium-235 Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number. 235 92 U

Chapter 3 Section 3 Counting Atoms Designating Isotopes, continued The number of neutrons is found by subtracting the atomic number from the mass number. mass number atomic number = number of neutrons 235 (protons + neutrons) 92 protons = 143 neutrons

Nuclide is a general term for a specific isotope of an element. Chapter 3 Section 3 Counting Atoms Designating Isotopes, continued Sample Problem A How many protons, electrons, and neutrons are there in an atom of chlorine-37? Chapter 3 Section 3 Counting Atoms

Designating Isotopes, continued Sample Problem A Solution Given: name and mass number of chlorine-37 Unknown: numbers of protons, electrons, and neutrons Solution: atomic number = number of protons = number of electrons mass number = number of neutrons + number of protons Chapter 3 Section 3 Counting Atoms

Designating Isotopes, continued Sample Problem A Solution, continued mass number of chlorine-37 atomic number of chlorine = number of neutrons in chlorine-37 mass number atomic number = 37 (protons plus neutrons) 17 protons = 20 neutrons An atom of chlorine-37 is made up of 17 electrons, 17 protons, and 20 neutrons. Chapter 3 Section 3 Counting Atoms Relative Atomic Masses

The standard used by scientists to compare units of atomic mass is the carbon-12 atom, which has been arbitrarily assigned a mass of exactly 12 atomic mass units, or 12 amu. One atomic mass unit, or 1 amu, is exactly 1/12 the mass of a carbon-12 atom. The atomic mass of any atom is determined by comparing it with the mass of the carbon-12 atom. Chapter 3 Section 3 Counting Atoms Average Atomic Masses of Elements Average atomic mass is the weighted average of the

atomic masses of the naturally occurring isotopes of an element. Calculating Average Atomic Mass The average atomic mass of an element depends on both the mass and the relative abundance of each of the elements isotopes. Chapter 3 Section 3 Counting Atoms Average Atomic Masses of Elements, continued Calculating Average Atomic Mass, continued Copper consists of 69.15% copper-63, which

has an atomic mass of 62.929 601 amu, and 30.85% copper-65, which has an atomic mass of 64.927 794 amu. The average atomic mass of copper can be calculated by multiplying the atomic mass of each isotope by its relative abundance (expressed in decimal form) and adding the results. Chapter 3 Section 3 Counting Atoms Average Atomic Masses of Elements, continued Calculating Average Atomic Mass, continued

(0.6915 62.929 601 amu) + (0.3085 64.927 794 amu) = 63.55 amu The calculated average atomic mass of naturally occurring copper is 63.55 amu. Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms The Mole The mole is the SI unit for amount of substance. A mole (abbreviated mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12.

Avogadros Number Avogadros number6.022 1415 1023is the number of particles in exactly one mole of a pure substance. Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Molar Mass The mass of one mole of a pure substance is called the molar mass of that substance. Molar mass is usually written in units of g/mol.

The molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units. Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Gram/Mole Conversions Chemists use molar mass as a conversion factor in chemical calculations. For example, the molar mass of helium is 4.00 g He/ mol He.

To find how many grams of helium there are in two moles of helium, multiply by the molar mass. 4.00 g He 2.00 mol He = 8.00 g He 1 mol He Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Conversions with Avogadros Number Avogadros number can be used to find the number

of atoms of an element from the amount in moles or to find the amount of an element in moles from the number of atoms. In these calculations, Avogadros number is expressed in units of atoms per mole. Chapter 3 Section 3 Counting Atoms Solving Mole Problems Chapter 3 Section 3 Counting Atoms

Determining the Mass from the Amount in Moles Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem B What is the mass in grams of 3.50 mol of the element copper, Cu? Chapter 3

Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem B Solution Given: 3.50 mol Cu Unknown: mass of Cu in grams Solution: the mass of an element in grams can be calculated by multiplying the amount of the element in moles by the elements molar mass. moles Cu grams Cu = grams Cu

moles Cu Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem B Solution, continued The molar mass of copper from the periodic table is rounded to 63.55 g/mol. 63.55 g Cu 3.50 mol Cu = 222 g Cu 1 mol Cu

Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem C A chemist produced 11.9 g of aluminum, Al. How many moles of aluminum were produced? Chapter 3 Section 3 Counting Atoms

Relating Mass to Numbers of Atoms, continued Sample Problem C Solution Given: 11.9 g Al Unknown: amount of Al in moles Solution: moles Al grams Al = moles Al grams Al The molar mass of aluminum from the periodic table is rounded to 26.98 g/mol.

1 mol Al 11.9 g Al = 0.441 mol Al 26.98 g Al Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem D How many moles of silver, Ag, are in 3.01 1023 atoms of silver?

Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem D Solution Given: 3.01 1023 atoms of Ag Unknown: amount of Ag in moles Solution: moles Ag Ag atoms = moles Ag Avogadro's number of Ag atoms

3.01 10 23 1 mol Ag Ag atoms = 23 6.022 10 Ag atoms 0.500 mol Ag Chapter 3 Section 3 Counting Atoms

Relating Mass to Numbers of Atoms, continued Sample Problem E What is the mass in grams of 1.20 108 atoms of copper, Cu? Chapter 3 Section 3 Counting Atoms Relating Mass to Numbers of Atoms, continued Sample Problem E Solution Given: 1.20 108 atoms of Cu Unknown: mass of Cu in grams

Solution: Cu atoms moles Cu grams Cu = grams Cu Avogadro's number of Cu atoms moles Cu The molar mass of copper from the periodic table is rounded to 63.55 g/mol. 1 mol Cu 63.55 g Cu

1.20 10 Cu atoms = 23 6.022 10 Cu atoms 1 mol Cu 8 1.27 10 14 g Cu