Chapter 3

Atoms: The Building Blocks of Matter

Chapter 3

The Atom: From Philosophical Idea To Scientific Theory

Chapter 3

The Atom: From Philosophical Idea To Scientific Theory

Remember the idea that matter is made of atoms dates back to 400 BC.

Not proven experimentally until 1700’s

The GreeksHistory of the Atom

In 400 B.C the Greeks tried to understand matter (chemicals) and broke them down into earth, wind, fire, and air.

Greek Model

Greek philosopherIdea of ‘democracy’Idea of ‘atomos’

Atomos = ‘indivisible’‘Atom’ is derived

No experiments to support idea

Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

Democritus

Alchemy After that chemistry was ruled by alchemy.

They believed that that could take any cheap metals and turn them into gold.

Alchemists were almost like magicians.elixirs, physical

immortality

Alchemy

. . . . . . . . . . . .. . .

GOLD SILVER COPPER IRON SAND

Alchemical symbols for substances…

transmutation: changing one substance into another

In ordinary chemistry, we cannot transmute elements.

Contributionsof alchemists:

Information about elementsInformation about elements - the elements mercury, sulfur, and antimony were discovered- properties of some elements

Develop lab apparatus / procedures / experimental techniquesDevelop lab apparatus / procedures / experimental techniques - alchemists learned how to prepare acids. - developed several alloys - new glassware

Timeline

2000 1000 300 AD

American Independence

(1776)

Issac Newton(1642 - 1727)

400 BC

Greeks (Democratus ~450 BC)

Discontinuous theory of matter

ALCHEMY

Foundations of Atomic TheoryObservations and chemical reactions led to the following scientific laws that describe how compounds are formed.

Foundations of Atomic Theory

Law of Definite Proportions (Proust)

The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.

Law of Multiple Proportions (Dalton)

If two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a fixed mass of the first element is always a ratio of small whole numbers.

Law of Conservation of Mass (Lavoisier)

Mass is neither destroyed nor created during ordinary chemical reactions or physical changes.

Conservation of Mass

2 H2 + O2 2 H2O

4 atoms hydrogen2 atoms oxygen

4 atoms hydrogen2 atoms oxygen

H

H

O

O

O

O

H

H

H

H

H

H

H2

H2

O2

H2O

H2O

+

Legos are Similar to Atoms

Legos can be taken apart and built into many different things.

H

H

O

O

O

O

H

H

H

H

H

HH2

H2

O2

H2O

H2O

+

Atoms can be rearranged into different substances.

45 g H2O? g H2O

Conservation of Mass

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

Highvoltage

Before reaction

electrodes

glasschamber

5.0 g H2

80 g O2

300 g (mass of chamber)+385 g total

H2O2

Highvoltage

After reaction

0 g H2

40 g O2

300 g (mass of chamber)+385 g total

O2

H2O

Law of Definite ProportionsJoseph Louis Proust (1754 – 1826)

Each compound has a specific ratio of elements

It is a ratio by mass Water is always 16 grams of

oxygen for every 2 grams of hydrogen

Law of Definite Proportions

103 g ofcopper carbonate

53 g ofcopper

40 g of oxygen 10 g of carbon

+ +

Whether synthesized in the laboratory or obtained from

various natural sources, copper carbonate always has

the same composition.

Analysis of this compound led Proust to formulate

the law of definite proportions.

Law of Multiple ProportionsJohn Dalton (1766 – 1844)

If two elements form more than one compound, the ratio of the second element that combines with the first element in each is a simple whole number.

H2O H2O2

water hydrogen peroxide Ratio of oxygen is 1:2 (an exact ratio)

Dalton’s Atomic TheoryEnglish chemist in the

early 1800’sDalton stated that

elements consisted of tiny particles called atoms

He also called the elements pure substances because all atoms of an element were identical and that in particular they had the same mass.

Dalton’s Atomic Theory

1. All matter consists of extremely small particles that are indivisible and indestructible called atoms.

2. Atoms of a given element are identical in their physical and chemical properties. Atoms of different elements have different physical and chemical properties.

3. Atoms of different elements combine in simple, whole number ratios to form chemical compounds.

4. Atoms cannot be subdivided, created or destroyed.

5. In chemical reactions, atoms are combined, separated, or arranged.

Although some exceptions have been discovered, the theory still stands today, and has been expanded and modified.

Daltons’ Models of Atoms

Carbon dioxide, CO2

Water, H2O

Methane, CH4

Structure of AtomsScientist began to wonder what an atom

was like.Was it solid throughout with no internal

structure or was it made up of smaller, subatomic particles?

It was not until the late 1800’s that evidence became available that atoms were composed of smaller parts.

The Structure of the Atom

Chapter 3

Structure of AtomsThe cathode ray tube led to the

discovery of electrons, small, negatively charged particles at the turn of the century.

J. J. Thomson - English physicistMade a piece of equipment called a

cathode ray tube.It is a vacuum tube - all the air has been

pumped out.

A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Thomson’s Experiment

+-

vacuum tube

metal disks

voltage source

Thomson’s Experiment

+-voltage sourceOFF

ON

Passing an electric current makes a beam appear

to move from the negative to the positive end

Thomson’s Experiment

+-voltage sourceOFF

ON

Thomson’s Experiment

+-voltage sourceOFF

ON

+

-

By adding a magnetic field…

he found that the moving pieces were negative.

J.J. ThomsonHe proved that ALL

atoms of any element must contain these negative particles.

He knew that atoms did not have a net negative charge and so there must be balancing the negative charge.

J.J. Thomson

Plum Pudding Model

In 1910 proposed the Plum Pudding modelNegative

electrons were embedded into a positively charged spherical cloud.

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

Spherical cloud ofPositive charge

Electrons

Television Picture Tube

Fluorescent screen

Shadow mask

Glass window

Blue beam

Green beam

Red beam

Electron gun

Electronbeam

Deflectingelectromagnets

Fluorescentscreen withphosphor dots

Red beam

Green beam

Blue beam

Shadow mask

Ernest Rutherford (1871-1937)Learned physics in

J.J. Thomson’ lab.Noticed that

‘alpha’ particles were sometime deflected by something in the air.

Gold-foil experiment

Rutherford ‘Scattering’In 1909 Rutherford undertook a series of experimentsHe fired (alpha) particles at a very thin sample of gold foilAccording to the Thomson model the particles would only

be slightly deflectedRutherford discovered that they were deflected through

large angles and could even be reflected straight back to the source

particlesource

Lead collimator Gold foil

Rutherford’s Apparatus

beam of alpha particles

radioactive substance

gold foil

circular ZnS - coated

fluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

What He Expected

The alpha particles to pass through without changing direction (very much).

Because…The positive charges were spread

out evenly. Alone they were not enough to stop the alpha particles

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What he expected…

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What he got…

Density and the Atom

Since most of the particles went through, the atom was mostly empty.

Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

Small volume and big mass = big density

This small dense positive area is the nucleus

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Rutherford’s Experiment

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 56

The Rutherford Atom

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

n +

Size of an atom

Atoms are incredibly tiny.Measured in picometers (10-12 meters)

Hydrogen atom, 32 pm radiusNucleus tiny compared to atom

Radius of the nucleus near 10-15 m.Density near 1014 g/cm

If the atom was the size of a stadium, the nucleus would be the size of a marble.

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AtomsAll atoms have similar structure

protons and neutrons cluster together to form a nucleus, or central core

electrons orbit the space surrounding the nucleus

then things change depending on the element

Counting Atoms

Chapter 3

AtomsEach element has a characteristic number of protonsSi- silicon 14 protonsH- hydrogen 1 ptotonAg- silver 47 protons

↓ Atomic number

Atomic ParticlesProtons are constant for an element, but

electrons and neutrons can varyWhen electrons vary, the charge of

the atom changesWhen neutrons vary, you have a

different isotope of the atomIsotope- one of 2 or more atoms having the

same number of protons but different numbers of neutrons

IsotopesHydrogen3 isotopes

protium deuterium tritium

1 proton 1 proton 1 proton

0 neutrons 1 neutron 2 neutronsisotopes have very similar chemical properties

Deuterium or hydrogen-2

Isotopes

Contain the symbol of the element, the mass number and the atomic number

X Massnumber

Atomicnumber

# protons

# protons + # neutrons mass number

Isotopes

Atomic Number = number of protons# of protons determines kind of atom/element

Atomic Number = number of electrons in a neutral atom

Mass Number = the number of protons + neutrons

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Mass Number

mass # = protons + neutrons

• always a whole number

• NOT on the Periodic Table!

+

+

+

+

+

+

Nucleus

Electrons

Nucleus

Neutron

Proton

Carbon-12Neutrons 6Protons 6Electrons 6

Isotopes

Find the number of protonsnumber of neutronsnumber of electronsAtomic numberMass number F19

9

= 9

= 10= 9

= 9

= 19

+

Isotopes

Find the – number of protons– number of neutrons– number of electrons– Atomic number– Mass number

Br8035

= 35

= 45

= 35

= 35

= 80

Isotopes

Find the number of protonsnumber of neutronsnumber of electronsAtomic numberMass number Na23

11

Sodium atom

Isotopes

Find the number of protonsnumber of neutronsnumber of electronsAtomic numberMass number Na23

11

1+

Sodium ion

= 11

= 12

= 10

= 11

= 23

Isotopes

If an element has an atomic number of 23 and a mass number of 51 what is the

– number of protons

– number of neutrons

– number of electrons

– Complete symbol

V5123

= 23

= 28

= 21

+2

IsotopesIsotopes

If an element has 60 protons and 144 neutrons what is the

– Atomic number

– Mass number

– number of electrons

– Complete symbol

Nd20460

= 60

= 204

= 60

56

Can atoms be counted or measured?

atomic mass- the mass of an atom in atomic mass units

atomic mass unit- 1/12 of the mass of the carbon-12 isotope

Atoms too small to measure in grams. Created new unit. Carbon-12 atom was assigned a value of 12 atomic mass units (amu).

57

•22.990 is the average atomic mass of all the isotopes

•a weighted average

IsotopesThe percent natural abundances The percent natural abundances for mercury isotopes are:for mercury isotopes are:

Hg-196 0.146%Hg-196 0.146% Hg-198 10.02%Hg-198 10.02% Hg-199 16.84%Hg-199 16.84% Hg-200 23.13%Hg-200 23.13% Hg-201 13.22%Hg-201 13.22% Hg-202 29.80%Hg-202 29.80% Hg-204 6.85%Hg-204 6.85%

(0.00146)(196) + (0.1002)(198) + (0.1684)(199) + (0.2313)(200) + (0.1322)(201) + (0.2980)(202) + (0.0685)(204) = x

0.28616 + 19.8396 + 33.5116 + 46.2600 + 26.5722 + 60.1960 + 13.974 = x

x = 200.63956 amu

Hg200.59

16

59

Example: Si

92.21% of Si atoms in nature have mass of 27.98 amu

4.70% of Si atoms in nature have mass of 28.98 amu

3.09% of Si atoms in nature have mass of 29.97 amu

What is the average of these masses, taking into consideration how often they are found in nature (abundance)?

(92.21)(27.98) + (4.70)(28.98) + (3.09)(29.97)

100= 28.09 amu

60

Example: Cl2 isotopes occur naturally, chlorine-35, which

has a natural abundance of 75% and a mass of 34.969 amu and chlorine-37 which occurs only 25% of the time and has a mass of 36.966 amu. What is the average atomic mass of chlorine?

chlorine-35 75% 34.969 amuchlorine-37 25% 36.966 amu

(75)(34.969) + (25)(36.966) = 35.453 amu 100

61

Molediscussing tiny particles and tiny amounts

cannot create and work with in lab

chemists derived a new unit as a bridge between the microscopic and macroscopic worlds

fundamental SI unit used to measure the amount of a substance

62

collection of 6.022137 X 1023 particles or usually 6.022 X 1023 (Avogadro’s number)

1 mole of any substance contains 6.022 X 1023 particles

1 mole of oxygen contains 6.022 X 1023 atoms

1 mole of water contains 6.022 X 1023 molecules

Just like a dozen is always 12 pieces!!!

63

molar mass- the mass in grams of 1 mole of a given substance

molar mass = average atomic mass in grams

C → 1 mol = 6.022 X 1023 C atoms = 12.011 g of C

Fe → 1 mol = 6.022 X 1023 Fe atoms = 55.85 g of Fe

Mo → 1 mol = 6.022 X 1023 Mo atoms = 95.94 g of Mo

So…

64

1 mol = 6.022 X 1023 atoms for any substance

can be written as conversion factor

1 mol or 6.022 X 1023

6.022 X 1023 1 mol

65

How many atoms in 2.5 mols of Si?

2.5 mol Si1 mol = 6.022 X 1023 atoms

? = # of atoms

2.5 mol Si • 6.022 X 1023 atoms 1 mol

= 1.5 X 1024 atoms Si

66

How many mols is 9.7 X 1024 atoms of Cu?

9.7 X 1024 atoms1 mol = 6.022 X 1023 atoms

9.7 X 1024 atoms • 1 mol 6.022 X 1023 atoms

= 16 mol Cu

67

Determine the mass in grams of 3.5 mols of Cu.

3.5 mol Cumolar mass of Cu = 63.55g, which means 1 mol = 63.55g of Cu

mass of Cu = ?

3.5 mol Cu • 63.55 g Cu = 222 g Cu

1 mol

68

What is the mass of a single atom of Si?

molar mass of Si = 28.09g1 mol = 6.022 X 1023 atomsmass of 1 atom = ?

28.09g Si • 1 mol

1 mol 6.022 X 1023 atoms= 4.665 X 10-23 g/atom