Chapter 4

Atomic StructureAlpharetta High School

Dr. Sonha Payne

Why do scientists use models?

Models may be used to represent things that are difficult to visualize.

Scaled-down models allow you to see something too large to see all at once, (solar system) or something that hasn’t been built yet.

Scaled-up models are used to visualize things that are too small to see. (atoms)

1st Model of the

Atom

Democritus (400 BC)

atomos,

meaning uncuttable,

or cannot be divided

Just as a brick is basic

to the structure of a wall,

an atom is basic to the

structure of matter.

Foundations of Atomic Theory

Law of Conservation of Mass (Lavoisier)

Mass is neither created nor destroyedduring ordinary chemical reactions.

Mass products = mass reactants

Foundations of Atomic Theory

Law of Definite Proportions (Proust)

A chemical compound contains the same elements in the same percent by massregardless of the size of the sample or

the source of the sample.

Ex: H2O will always have the same percent by mass, 11.2% H and 88.8% O.

Foundations of Atomic Theory

Law of Multiple Proportions

(Dalton)

When elements combine, they do so in the ratio of small whole numbers.

CO2 and CO H2O and H2O2

Principles of Dalton’s Atomic Theory of Matter (1808)

1. All matter is composed of extremely small, indivisible particles called atoms.

2. All atoms of a given element have identical properties that differ from those of other elements.

3. Atoms cannot be created, destroyed, or transformed into atoms of another element.

4. Compounds are formed when atoms of different elements combine with one another in small whole-number ratios.

5. The relative numbers and kinds of elements are constant in a given compound.

Dalton’s Billiard Ball Model of the Atom

Billiard Ball Modelbecause the atom

is likened to a billiard ball - it is a single, complete

unit of matter.

A solid, indivisible,

indestructible sphere

Voltage source

No Charge Flows When the Glass Tube is Empty (Vacuum, No Gas)

Cathode Ray Tube: 2 metal plates sealed inside a glass tube connected to a source of electricity

Since glass is an insulator, no charge was observed to flow when the tube was empty.

Some means of conduction was needed if charge was to flow.

Charge Flows (a Ray is observed) in the Presence of ANY GAS

When a small amount of any gas was placed in the glass tube and the power source was turned on, a ray was observed striking the

phosphor-coated end of the tube and emitting a flash of light.

Voltage source

+-

The current carriers in the gas are “invisible”and are visualized by the phosphor-coated

screen which fluoresces as the current

carriers strike it.

For current to flow, mobile,

charged particles are required.

Voltage source+-

Observation: A fluorescent screen glows (emits a flash of light) when struck by the “ray”.

Conclusion: Invisible particles traveling from the cathode to the anode are carrying electric current and are visualized by the fluorescent screen.

Observation: When a tiny object is placed in the middle of the tube, a shadow is cast on the screen at the anode

Conclusion: the particles travel in a straight line.

Cathode Rays Originate at the Cathode(Negative Electrode)

The rays were called cathode rays because they originated at the negative electrode(aka the cathode) and moved to the positive electrode (aka the anode).

The entire apparatus is now known as a cathode ray tube (CRT).

Observation: In the presence of a magnetic field, the cathode rays bend.

Conclusion: The cathode ray consists of charged particles.

Observation: In the presence of an external electric field, the cathode rays bent towards the positive plate.

Conclusion: The particles are negatively charged.

ALL gases used in the tube were found to produce identical rays, so the negatively charged particles

of the cathode rays weredetermined to be part of all

matter.

Observation: A paddle wheel in the middle of the cathode ray tube turns.

Conclusion: The particles have mass.

JJ Thomson (1897)

Credited with discovery of the electron

Found the ratio of the mass of the particles to the chargeof the particles in cathode rays

m/e = -5.6856 x 10-9 g/C

Conclusion: all cathode rays are composed of identical, negatively-charged particles.

Conclusion: these negative particles are fundamental particles of matter. (electrons)

In 1897, JJ Thomson discovered that negatively charged electrons were part of all matter.

Dalton Postulate 1: All matter is composed of extremely small, indivisible particles called atoms.

This is no longer valid.

Atoms are NOT indivisible particles, but CONTAIN ELECTRONS.

Millikan Determined the Charge on an Electron by Examining the Motion of Tiny Oil Drops (1909)

Small drops of oil with a negative charge are examined.

The diameter of the drop is measured. From this, the volume is determined (4/3pr3).

The mass is calculated using the known density of the oil.

By adjusting the electric field to balance the force of gravity, the charge on the drop is calculated.

ObservationThe charge of each drop is a whole number multiple of some common number.ConclusionEach number is divisible by 1.92 x 10-19. This is the charge of an electron. (modern value = 1.602 x 10-19 C)

Charge in Coulombs

13.458 x 10-19 = 7 x (1.92 x 10-19)

15.373 x 10-19 = 8 x (1.92 x 10-19)

17.303 x 10-19 = 9 x (1.92 x 10-19)

15.378 x 10-19 = 8 x (1.92 x 10-19)

17.308 x 10-19 = 9 x (1.92 x 10-19)

28.844 x 10-19 = 15 x (1.92 x 10-19)

11.545 x 10-19 = 6 x (1.92 x 10-19)

19.214 x 10-19 = 10 x (1.92 x 10-19)

Millikan Determined the Mass of an Electron

Knowing the charge, Millikan was able to use Thomson’s charge-

to-mass ratio to determine the mass of an electron.

1928

8

charge 1.6022 10 Cmass of an electron = = = 9.10 10 g

charge / mass 1.76 10 C/g

Inferences from theProperties of Electrons

Atoms are neutral, so there must be positively charged particles to balance the negatively charged electrons.

Electrons have a tiny mass, so atoms must contain other particles that account for most of their mass.

Even the lightest atom, H, has a mass of 1.7 x 10-24 g, compared to a mass of only 9.1 x 10-28 g for an electron.

That is, the lightest atom is almost 10,000 x heavier than an electron.

Thus, most of the mass of an atom had to come from somewhere else.

Thomson’s Plum Pudding Model of the Atom

•Since electrons are negatively charged, and atoms are neutral, atoms must also contain a positively charged substance.

•Thomson’s model of the atom: the positively charged substance fills the atom and the electrons are embedded throughout the substance like “raisons in plum pudding”.

Rutherford's gold foil experiment (1911)

Discovery of the Nucleus

(helium nuclei) a-particles were known to be heavy positive particles.

When the particles hit the screen, brief flashes of light were seen.

Rutherford’s Expected and Actual Results

If plum-pudding

model was correct.Actual results.

Rutherford’s Expected and Actual Results

• Observation: Most particles went straight through the foil undeflected.

• Conclusion: The atom is mostly empty space.

• Observation: Some particles bounced back!

• Conclusion

• To deflect the energetic a-particles, most of the mass and all of the positive charge must be in a dense central regionwhich he called the nucleus.

• For the fraction deflected to be small, the nucleus must be small, relative to the overall size of the atom.

• Since atoms are neutral particles, the charge of the nucleus must be equal to the sum of the negative charges of the electrons.

1911Rutherford’s

Gold Foil Experiment

Conclusion: An atom is mostly empty space occupied by electrons, and centrally located within that space lies a tinyregion, which he called the nucleus, that contains all the positive charge and essentially all the mass of the atom.

Rutherford proposed that positive particles lay within the nucleus and called them protons.

Summary: The Rutherford Experiment (1911)

Alpha particles (helium nuclei) fired at a thin sheet of gold

Assumed that the positively charged alpha-particles were bounced back if they approached a positively charged atomic nucleus head on

(Like charges repel one another)

Very few particles were greatly deflected back from the gold sheet

Atoms contain very small, very dense, positively-charged nuclei.

The electrons are in clouds surrounding the nucleus at relatively large distances.

Most of the atom is empty space, and if the nucleus were the size of a ladybug, it would be in an atom the size of the Georgia Dome.

Moseley,1913 X-rays, Atomic Number and the Proton

A beam of electrons shot at a sample of an element gives off x-rays.

Observation

The frequency of x-rays given off is unique to that element.

Higher energy rays are given off when the nuclear charge is higher.

Conclusion

The atomic number can be determined from the x-ray data

Each element on the periodic chart differs from the next by having one more positive charge in the nucleus

These fundamental positive charge units are the protons

Bohr’s Theory of the Hydrogen Atom

When white light is passed through a prism, a

continuous spectrum of colors results which

contains all of the wavelengths of visible light

Visible Light

Spectrum

The

Hydrogen

Line

Emission

Spectrum

When a sample of hydrogen gas is excited by electricity and passed through a prism, only a few

lines are seen, each of which corresponds to a discrete wavelength.

Every Element Has Its Own

Unique Emission Line Spectrum

Bohr Attributed the Emission of Radiation to the

Electron Dropping From a Higher Energy Orbit

to a Lower One

1. Electrons orbit the

nucleus in circular orbits.

2. Only orbits of certain radii

are permitted.

3. An electron in a permitted

orbit has a specific

energy.

4. Energy is emitted or

absorbed as a photon

when the electron

changes its orbit.

What color of light is emitted when an excited electron in the

hydrogen atom falls from:

a) n =5 to n = 2 b) n = 4 to n = 2 c)n = 3 to n = 2

The Hydrogen e- Visualized as a

Standing Wave Around the Nucleus

Only certain circular orbits have a

circumference into which a whole number

of wavelengths of the standing electron

wave will “fit”.

Circular orbits with any other

circumference produce destructive

interference of the standing electron wave

and are not allowed.

Electron Diffraction Experiments Demonstrated that

Electrons Exhibit the Wave Property of Interference

Experiments have shown that electrons do indeed possess wavelike

properties:

X-ray diffraction pattern of

aluminum foil

Electron diffraction pattern of

aluminum foil.

The Heisenberg uncertainty principle states that it is impossible to

know simultaneously both the momentum p and the position x of a

particle with certainty.

Δx is the uncertainty in position in meters

Δp is the uncertainty in momentum

Δu is the uncertainty in velocity in m/s

m is the mass in kg

Quantum Mechanics

If the position of a particle is known more

precisely, then it’s velocity measurement

must become less precise

In the picture, we

know the exact

location of the

cars, but we have

no idea how fast

they are moving.

If the velocity of a particle is measured more

precisely then the position must become

correspondingly less precise

In the

picture, we

know the

speed of the

cars, but we

have no idea

exactly

where they

are.

43

Which of the following statements regarding

Dalton’s atomic theory are still believed to

be true?

I. Elements are made of tiny particles called atoms.

II. All atoms of a given element are identical.

III. A given compound always has the same relative

numbers and types of atoms.

IV. Atoms are indestructible.

Modern Atomic Theory1. All matter is made up of very tiny particles called

atoms.

2. Atoms of the same element are chemically alike.

3. Individual atoms of an element may not all have the same mass. However, the atoms of an element have a definite average mass that is characteristic of the element.

4. Atoms of different elements have different average masses.

5. Atoms are not subdivided, created, or destroyed in chemical reactions.

Modern Atomic Theory An atom is an electrically neutral, spherical entity composed of a

positively charged central nucleus surrounded by one or more negatively charged electrons. The “cloud” of rapidly moving, negatively charged electron occupies virtually all of the atom’s volume and surrounds the tiny nucleus.

The nucleus is very dense as it contributes 99.97% of the atom’s mass but occupies only about one ten-trillionth of its volume

An atom’s diameter (~10-10 m) is about 10,000 times the diameter of it’s nucleus (~10-14 m).

The Modern Model of the AtomThe precise paths of electrons cannot be

determined accurately. Instead, the PROBABILITY

of finding electrons in a specific location can be

determined.

The location and energy of electrons can be

specified using three terms:

Shell (aka level)

Subshell (aka sublevel)

Orbital

An additional fourth term, spin number, indicates

whether the electron is spinning clockwise or

counterclockwise.

What are the particles that make up an

atom and where are they located?

Particle Symbol Charge Relative Mass (amu) Actual Mass (g)

Proton p+ +1 1 1.7 x 10-24

Neutron n 0 1 1.7 x 10-24

Electron e- -1 0 9.1 x 10-28

Atomic Mass Unit (amu): the Unit Used For

Masses of Atoms and Subatomic Particles

Atoms have an very small mass

(e.g. hydrogen 1.67 x 10–24 g)

It is hard to work with these small numbers

so a relative mass scale was introduced:

atomic mass unit (amu or u)

1 u = 1/12 the mass of a carbon-12 atom

Particle Symbol Charge Relative Mass (amu) Actual Mass (g)

Proton p+ +1 1 1.7 x 10-24

Neutron n 0 1 1.7 x 10-24

Electron e- -1 0 9.1 x 10-28

# Protons Defines an

Atom

The number of protons distinguishes atoms of one element from

atoms of all of the other elements.

Atomic number = # protons in the nucleus

What Distinguishes One Element from

Another Element? The # of Protons!

For a Neutral Atom,

# protons = # electrons

For a neutral atom,

the positive charges (+1 per proton)

and negative charges (-1 per electron)

must add up to zero.

Complete the table.

Element Atomic number

Protons Electrons

Pb 82

8

30

Mass Number = protons + neutrons

The mass number is NOT on the periodic table!

Always a whole number

Nuclear Symbol and Hyphen Notation are Interchangeable

Carbon-12

Hydrogen-2

Atoms of the Same Element That Have

Different # of Neutrons are ISOTOPES

Isotopes of elements have the same # of protonsbut different # of

neutrons.

Magnesium-24

Magnesium-25

Magnesium-26

An isotope of an element is identified by the mass number

How many protons, electrons, and neutrons do each of these isotopes have?

Determine # p+, # e-, # n0 for the following. Name each isotope and write its symbol.

Element Atomic number

Mass number

Neon 10 22

Calcium 20 46

Oxygen 8 17

Iron 26 57

Zinc 30 64

Mercury 80 204

Complete each of the following isotope symbols:

206

84?a. 224

?Rab.

197

?Auc. 84

36?d.

Average Atomic Mass

Atomic mass is the mass of an atom in atomic mass units (amu).

1 amu = 1/12 the mass of a carbon-12 atom

The average atomic mass on the periodic table represents the average

mass of the naturally occurring mixture of isotopes.

Average mass (C) = (0.9893)(12.00000 amu) + (0.0107)(13.003355 amu)

Isotope Isotopic mass (amu)Natural

abundance (%)

12C 12.00000 98.93

13C 13.003355 1.07

= 12.01 amu

Isotopes Used in MedicineSome radioactive isotopes are useful in the medical field.

I131

53

Used to

detect

thyroid

problems

Co60

27

Used in

cancer

therapy to

kill

cancerous

tissue

P32

15

Used in

leukemia

therapy

Cs137

55

Used to

irradiate food

to kill bacteria

and other

organisms

Isotopes

Is it possible to have a hydrogen

isotope with 3 neutrons,

hydrogen-4?

No, only certain combinations of

protons and neutrons are

possible; the others are

unstable.

Not all elements have the same

number of isotopes.

Every element up to lead (Z = 82)

has at least 1 stable isotope.

Isotope AbundanceSome elements have only 1 isotope,

while others have many more.

Not all isotopes are present in equal amounts.

Note: the sum of the percentages is 100%

Chlorine-35 75.77%

Chlorine-37 24.23%

Total 100.00%

Oxygen-16 99.762%

Oxygen-17 0.038%

Oxygen-18 0.200%

Total 100.00%

The atomic mass of an element

(the number on the periodic table)

is the weighted average mass of all its isotopes expressed in atomic mass units.

Chlorine (Cl)

How to determine

the atomic mass of an element

What is the atomic weight of chlorine?

List each isotope, it’s mass in atomic

mass units, and it’s abundance in nature.

Mass (amu) Isotopic AbundanceIsotope

Cl-35

Cl-37

34.97

36.97

75.78% = 0.7578

24.22% = 0.2422

How to determine

the atomic mass of an element

Multiply the isotopic abundance by the mass

of each isotope, and add up the products.

34.97 x 0.7578

36.97 x 0.2422

=

=

26.5003 amu

8.9541 amu

35.4544 amu = 35.45 amu

Answer

4 sig. figs.

The sum is the atomic weight of the element.

4 sig. figs.

Calculating Atomic Mass Using Isotopes

Example

average atomic mass of Cl:

 

Average atomic mass = isotope %( ) isotope mass( )å

Chlorine-35 75.77%

Chlorine-37 24.23%

Total 100.00%

Most elements have two or more

isotopes that contribute to the

atomic mass of that element

Average

Atomic

Mass of

Mg

Calculating Atomic Mass Using Isotopes

Example

average atomic mass of O

Oxygen-16 99.762%

Oxygen-17 0.038%

Oxygen-18 0.200%

Total 100.00%

 

= 99.762%( ) 15.99491464( ) + 0.038%( ) 16.9991306( ) + 0.200%( ) 17.99915939( ) = 16.00 u

 

Average atomic mass = isotope %( ) isotope mass( )å

Oxygen is the most abundant element in both Earth’s crust and the human

body. The atomic masses of its three stable isotopes, O (99.757 percent), O

(0.038 percent), O (0.205 percent), are 15.9949, 16.9991, and 17.9992 amu,

respectively. Calculate the average atomic mass of oxygen using the relative

abundances given in parentheses.

Calculating the Average Mass of an Element

Solution

(0.99757)(15.9949 amu) + (0.00038)(16.9991 amu) + (0.00205)(17.992 amu)

= 15.9994 amu

16

8

17

818

8

Think About It The average atomic mass should be closest to the atomic

mass of the most abundant isotope (in this case, oxygen-16) and, to four

significant figures, should be the same number that appears in the periodic

table on the inside front cover of your textbook (in this case, 16.00 amu).

Strategy Each isotope contributes to the average atomic mass based on its

relative abundance. Multiplying the mass of each isotope by its fractional

abundance (percent value divided by 100) will give its contribution to the

average atomic mass.

The Modern Periodic Table

Groups contain elements with similar properties

and are arranged in vertical columns.

Periods are the horizontal rows of elements.

Two Numbering Systems for Groups The old method uses the letter A for the representative

elements (1A to 8A) and the letter B for the transition

elements.

The new method numbers groups 1–18 from left to right.

Special Names of

Groups

Alkali Metals

Halogens

Shape of the Periodic Table with

Lanthanides and Actinides Inserted Into the Main Section

Metals, Nonmetals, and Metalloids

The heavy zigzag line

separates

metals and nonmetals.

Metals are located to the left.

Nonmetals are located to the

right.

Metalloids are located along

the heavy zigzag line between

the metals and nonmetals

(except for Al)

79

Metals solids at room temperature, except Hg

reflective surface (shiny)

conduct heat and electricity

Malleable (can be shaped)

Ductile (can be pulled into wires)

lose electrons and form cations in reactions

About 75% of the elements are metals.

80

Nonmetals Exist as s, l, g

poor conductors of heat and electricity

Solids are brittle.

gain electrons in reactions to become

anions

upper right on the table

except H Chlorine, Cl2(g)Sulfur, S(s)Bromine, Br2(l)

Metalloids

show some properties of

metals and some of

nonmetals

also known as

semiconductors

Properties of Silicon

shiny

conducts electricity

does not conduct heat

well

Properties of Metals, Nonmetals,

and MetalloidsMetals Metalloids Nonmetals

Shiny (s) Dull (s, l, g)

Ductile,

malleable

Brittle

Good

conductors

Better conductors than

nonmetals, but not as

good as metals

Good insulators