Review Problem Set 1

Plan for this week’s lab

• Lab syllabus, then sign agreement

• Check in with your lab partner

• Safety video

Tuesday, IC 420Section E: 9:00 am, Section F: 1:00 pm

Ch 2. Atoms and Elements

Substances are composed of extremely small particles called atoms.

If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.

Richard Feynman

Atom

Nucleus (Rutherford’s experiment)

Electrons (Thomson’s experiment)

Atomic Nucleus

Nucleus carries positive charges

Each electron carries one negative charge

Nucleus carries almost all the mass of an atom

Atom

Nucleus

Electrons (each carries a negative charge)

Protons (each carries a positive charge)

Neutrons (neutral)

number of electrons = number of protons

= atomic number

Atoms are neutral

Atoms that have the same number of protons belong to

one kind of element.

isotopes: same number of protons, different number of neutrons

mass of a proton ≈ mass of a neutron >> mass of an electron

number of protons + number of neutrons = mass number

OR

Practice – Complete the table

Al2713

Practice – Complete the table

Al2713

C136

Mo9642

Cs13355

No change occurs inside a nucleus in chemistry

Atoms can lose or gain electrons

Na − e− → Na+ positive ion = cation

Cl + e− → Cl− negative ion = anion

Mg − 2e− → Mg2+

O + 2e− → O2−

Practice – Complete the table

3Al

Practice – Complete the table

2Mg

2S

Br

3Al

isotopes: same number of protons, different number of neutrons

mass of a proton ≈ mass of a neutron >> mass of an electron

number of protons + number of neutrons = mass number

1H 1.6735 x 10−24 g

16O 2.6560 x 10−23 g

One atomic mass unit (amu) is defined as 1/12 ofthe mass of a 12C atom.

12C atom: 6 protons, 6 neutrons, 6 electrons.

1 amu = 1.6605 x 10−24 g

1H 1.6735 x 10−24 g

16O 2.6560 x 10−23 g

1 amu = 1.6605 x 10−24 g

2424

1 amu1.6735 10 g 1.0078 amu

1.6605 10 g

2324

1 amu2.6560 10 g 15.995 amu

1.6605 10 g

23

Mass Spectrometer

Tro: Chemistry: A Molecular Approach, 2/e

Mass Spectrum of Natural Copper

69.09 % 63Cu

30.91 % 65Cu

natural abundance:percent of an isotopein nature

Mass Spectrum of Natural Copper

69.09 % 63Cu: 62.93 amu

30.91 % 65Cu: 64.93 amu

Average atomic mass of Cu = ?

How to find the average?

1, 1, 1, 1, 2

Average = (1 + 1 + 1 + 1 + 2) / 5 = 1.2

2.1

4.08.0

2%201%80

25

11

5

45

2114

5

21111

Mass Spectrum of Natural Copper

69.09 % 63Cu: 62.93 amu

30.91 % 65Cu: 64.93 amu

Average atomic mass of Cu = ?

63.55 amu

listed in the periodic table

35Cl 34.967 amu 75.78 %

37Cl 36.966 amu 24.22 %

Average atomic mass of Cl

= 34.967 amu x 75.78 % + 36.966 amu x 24.22 %

= 35.45 amu

listed in the periodic table

mass of a proton ≈ mass of a neutron ≈ 1 amu

mass number of an isotope ≈ atomic mass of the isotope in amu

35Cl 34.967 amu

37Cl 36.966 amu

One atomic mass unit (amu) is defined as 1/12 ofthe mass of a 12C atom.

The number of carbon atoms in exactly 12 g of 12Cis called Avogadro’s number: 6.022 x 1023

One Avogadro’s number of particles is called a mole.

1 mol = 6.022 x 1023 particles

1 pair = 2 particles 1 dozen = 12 particles

(exact number)

Example 2.6

Calculate the number of atoms in 2.45 mol of Cu.

1 mol = 6.022 x 1023 particles

For Practice 2.6

A pure Ag ring contains 2.80 x 1022 Ag atoms. How many moles of Ag atoms does it contain?

1 mol = 6.022 x 1023 particles

Experiment 2 next Tuesday

Read the lab manual before coming.

Bring lab manual, data form, and goggles.

Dress properly according to the syllabus.

Aprons not required for this experiment.

For an element A, its atomic mass is x amu.

What is the mass of 1 mol of A in grams?

The mass of 1 mol of A is x g.

The molar mass of an element is the mass in grams per mole of the element.

Unit: g/mol

moles

grams in massmassmolar

From the definition of molar mass:

(recall d = m/V)

1 mol = 6.022 x 1023 particles

moles

grams in massmassmolar

Unit: g/mol

molar mass and Avogadro’s number are exact numbers

By using these two equations, we can

(1) Find the number of atoms of an element from its mass.

A piece of Cu has a mass of 200. g. How many Cu

atoms are present?

1 mol = 6.022 x 1023 particles

mass in gramsmolar mass

moles

A silicon chip has a mass of 5.68 mg. How many silicon

atoms are present in the chip?

1 mol = 6.022 x 1023 particles

mass in gramsmolar mass

moles

By using these two equations, we can

(1) Find the number of atoms of an element from its mass.

(2) Find the mass of a number of atoms in grams.

Compute the mass in grams of a sample of six

mercury atoms.

1 mol = 6.022 x 1023 particles

mass in gramsmolar mass

moles

Calculate the mass of a sample of cobalt (Co) containing

5.00 x 1020 atoms.

1 mol = 6.022 x 1023 particles

mass in gramsmolar mass

moles

By using these two equations, we can

(1) Find the number of atoms of an element from its mass.

(2) Find the mass of a number of atoms in grams.

(3) Identify an element.

The mass of 500 atoms of an element is 2.24 x 10−20 g.

Identify this element.

1 mol = 6.022 x 1023 particles

mass in gramsmolar mass

moles

Problem Set 2