COUNTING ATOMSChapter 3.3

Objectives1. Explain what isotopes are2. Define atomic number and mass number,

and describe how they apply to isotopes.3. Given the identity of a nuclide, determine

its number of protons, neutrons, and electrons.

4. Define mole in terms of Avogadro’s number, and define molar mass.

5. Solve problems involving mass in grams, amount in moles, and number of atoms of an element

Atomic Number Symbol – Z Number of protons in nucleus of atom Determines the identity of the

atom!!!!! C - 6 He - ___ F - ___ Pb - ___

Use periodic table to find these! Since atoms are neutral atomic number is

also number of electrons So C also has 6 electrons He - ____ electrons

2

82

9

2

Isotopes All Hydrogen atoms contain 1 proton Many naturally occurring elements

can contain different numbers of neutrons.

Isotope – at0ms of the same element that have different masses.

n

Protium 99.985 %

Deuterium 0.015 %

Tritium Trace

Three isotopes of Hydrogen

nn

Mass Number Mass number – total number

of protons and neutrons in the nucleus of an isotope

Symbol – A Protium has 1 proton and 0

neutrons Mass number = #p + #n = 1 + 0 = 1

What is the mass number deuterium and tritium?

Find on periodic table

Designating Isotopes Hyphen Notation

Name of atom – mass number Uranium - 235

Nuclear symbol A

X A : mass number, Z : atomic number, X : symbol of element

235U92

Z

Nuclide – general term for any isotope of any element

Number of protons Neutrons Electrons

92 9214

3

Sample Problem A How many protons, electrons, and

neutrons are there in an atom of chlorine-37? atomic number = number of protons =

number of electrons mass number = number of neutrons +

number of protons # protons # electrons #neutrons

17Atomic number from periodic table

17 # protons = # electrons20Mass # = # protons + # electrons

Practice Pg. 801. How many protons, electrons, and

neutrons make up an atom of bromine-80?

2. Write the nuclear symbol for carbon-13.

3. Write the hyphen notation for the isotope with 15 electrons, and 15 neutrons.

Answer : 35 protons, 35 electrons, 45 neutrons

Answer : 13 C6

Answer : phosphorus – 30

Relative Atomic Mass Oxygen – 16 , mass of 2.657 x 10-23 g Easier to use relative scale

Need arbitrarily chosen standard All others are compared to carbon - 12

One atomic mass unit (1 amu) – exactly 1/12 the mass of a carbon – 12 atom.

All others are compared to carbon -12 Hydrogen – 1 , about 1/12 the mass of carbon -12

Precise value is 1.007825 amu

Magnesium – 24 , Slightly less than twice that of carbon -12• Precise value is 23.985042 amu

Average Atomic Mass of Elements Most elements occur naturally as

mixtures of isotopes Average atomic mass : weighted

average of the atomic masses of the naturally occurring isotopes of an element

Ex. of weighted average Box contains two types of marbles, 25%

have mass of 2.00g, and 75% have a mass of 3.00 g. 25 marbles x 2.00 g = 50 g 75 marbles x 3.oo g = 225 g so 50 g + 225 g = 275 g (TOTAL MASS)

275 g ÷ 100 = 2.75 g average marble mass

Calculating average atomic mass Or

(2.00 g x 0.25) + (3.00 g x .75) = 2.75 g Calculate the average atomic mass of copper

Use table 3-4 pg. 80 in text Copper - 63 69.17% with mass of 62.929599 amu Copper – 65 30.83% with mass of 64.927793 amu

(0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu)

=63.55Match with atomic mass on periodic table

Round to two decimal places in calculations Visual Concept – Click

Here

Relating Mass to Numbers of Atoms The MOLE

SI base unit for amount of a substance Abbreviated as mol Defined as : amount of substance that

contains as many particles as there are atoms in exactly 12 g of carbon – 12

Counting unit just like a dozen We don’t order 12 or 24 eggs, we order 1

or 2 dozen In same way, a chemist may want 1 mol of

carbon or 2 mol of iron.63.55 g of copper or 1 mol of copper

Visual Concept – Click Here

Avogadro’s Number The number of particles in a mole –

AVOGADRO’S NUMBER 6.022 x 1023 particles in exactly one

mole of a pure substance

How big is this number? If every person on earth (5 billion people)

counted 1 atom per second, it would take 4 million years to count all the atoms.

Visual Concept – Click Here

Molar Mass Molar mass – mass of one mole of a

pure substance Units : g/mol Numerically equal to atomic mass of

element Mass of 6.02 x 1023 atoms of element

One mole He 6.02 x 1023 atoms 4.oo g

One mole Li 6.02 x 1023 atoms 6.94 g

One mole Cu 6.02 x 1023 atoms 63.5o g

One mole Fe 6.02 x 1023 atoms 55.78 g

Visual Concept – Click Here

Gram/Mole Conversions

Mass In grams

Moles In mol

Atoms In atoms

Molar mass

Avogadro’s

Number

What is the mass in grams of 3.50 mol of the element copper, Cu?

Sample Problem B

3.30 mol Cu

x63.55 g Cu1 mol Cu

222 g Cu

=

Step 1:

Write the given value

Step 2:

Use molar mass from periodic table

Step 3:

Cancel out units

Step 4:

Calculate

Always round to 2 places after the decimal

Sample Problem CA chemist produced 11.9 g of Aluminum, Al. How many moles of aluminum were produced?

11.9 g Al

x26.98 g Al

1 mol Al

0.441 mol Al

=

How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver?

Sample Problem D

3.01 x 1023 atoms Ag

x 0.500 mol Ag

1 mol Ag

=6.02 x 1023

atoms Ag

Avogadro’s number

Sample Problem EWhat is the mass in grams of 1.20 x 108 atoms of copper, Cu?

1.20 x 108atoms Cu

x

1.27 x 10-14 g Cu

1 mol Cu

=

6.02 x 1023 atoms Cu

x1 mol Cu

63.55 g Cu

Avogadro’s #

****Requires 2 conversions

Molar mass

The Ten Commolements

(found at http://members.tripod.com/~smalls42/commolements.html)

1. Thou shalt not use 6.02 x 1023 in vain. 2. Thou shalt not use the term mole if thou has no true

knowledge of the term mole. 3. Thou shalt not kill a mole. 4. Thou shalt not covet your neighbor's mole. 5. Thou shall always remember to celebrate Mole Day. 6. Thou shalt not disparage Mole Day. 7. Thou shalt not use a mole out of season. 8. Thou shalt always honor the one who introduced thou to Mole

Day. 9. You shalt always keep sacred 10/23. 10. Thou shalt always remember these commolments or thou will

never properly celebrate Mole Day.