Atomic Theory DEMOCRITUS 460 - 370 BC The Greek philosopher Democritus proposed that all matter was made of small, unbreakable particles he called.

Atomic Theory

DEMOCRITUS 460 - 370 BC

• The Greek philosopher Democritus proposed that all matter was made of small, unbreakable particles he called atoms which means unbreakable.

• He believed that atoms were too small to be seen.

• Philosophers are not scientists. They do not test their ideas. Instead they use reasoning to back up their beliefs.

• To them, human reasoning was superior to experimentation.

ARISTOTOLE

• The famous philosopher Aristotle, who also lived at that time, argued that all matter was made of only four elements.

• For the next two thousand years, Aristotle overshadowed Democritus.

• Finally, in the early 1800s, the atomist’s theory was revived by John Dalton.

John Dalton 1766-1844

• In 1809, Dalton by proposing the following:a) All matter was made of atoms.b) Atoms were solid spheres.c) Atoms of different elements differed in

mass.d) Atoms were indivisible and

indestructible.e) Atoms combine to form compounds.

J.J. THOMSON 1856-1940• Before you can understand Thomson’s

experiment, 3 properties about electrical charges:a) There are two types of electrical charge:

positive and negative.b) Opposite charges attract.

c) Like charges repel.• Thomson took Cathod ray tube and added

two plates inside the tube and connected them with a wire.

• When the plates were not charged, the ray shot straight.

Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Passing an electric current makes a beam appear to move from the negative to the positive end


Voltage source

+-

Voltage source


By adding an electric field

+

-

Voltage source

By adding an electric field he found that the moving pieces were negative

+

-


Cathod Ray Tube Conclusion

• Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons.

• Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

• Electrons have so little mass that atoms must contain other particles that account for most of the mass

• Thomson’s model was called the Plum Pudding Model was named after a popular dessert in England at that time.

• It was the first model to propose that smaller charged particles make up the atom.

• Thomson’s model lasted less than two decades but it was first to propose the existence of subatomic particles.• In 1911 another scientist who worked in Thomson’s lab improved on his atomic model.

ERNEST RUTHERFORD 1871-1937• One type of radioactivity

is when an atom throws out a positively charged particle from the nucleus.

• This particle was called an alpha particle (α).

• Rutherford used this alpha particle to investigate the structure

Rutherford and Geiger in the Cavendish Lab

Rutherfold’s Gold Foil Experiement• Uranium is a radioactive element that gives off positive particles

(alpha particles).• Rutherford used these positive particles to invest• Rutherford encased uranium in lead (which absorbs alpha

particles).• This produced a beam of alpha particles traveling in a straight line.

• He fired these positive particles at a thin piece of gold (dense metal).• A screen around the gold to detect where the alpha particles were traveling.

• Rutherford shot alpha particles at a thin sheet of gold to observe what happened when the positive α particles passes through the gold atoms.

• If Thompson’s model was correct the alpha particles should pass through the diffused positive cloud with ease.

Rutherfold’s Gold Foil Experiement

Lead block

Uranium

Gold Foil

Fluorescent Screen

What he expected

Because

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

Rutherfold’s Conclusion

• From his observations Rutherford concluded that the atom had a dense, positive central nucleus composed of + charged protons.

• He stated that the electrons orbited the nucleus - like planets orbiting the Sun.

• In 1909 Rutherford proposed his Planetary Model of the Atom.

• His model created positively charged protons located in the nucleus and placed electrons in orbit around the nucleus – like planets around the sun.

+

Almost no deflection; few greatly deflected

Checking for understandingExplain Thompson’s conclusions in 3 points:1.2.3.

Explain Rutherford’s conclusions in 3 points:1.2.3.

Atomic Structure

Subatomic Particles

• Over the past century scientist have discovered that the atom is composed of 3 subatomic particles:

ProtonsNeutronsElectrons

Checking for understandingDraw this diagram. Label all subatomic particles and include their charges.

The Proton1. Symbol = p+2. Relative Mass =

1 Atomic Mass Unit (AMU).

3. Actual mass = 1.674 x 10 -24 g

4. Location: Inside the nucleus

5. Electrical charge: Positive.6. Importance: The atomic

number which is the identity of the element.

7. Discovered by: Ernest Rutherford in 1909

The Electron1. Symbol = e-2. Relative Mass = 1 /1836

Atomic Mass Unit.3. Actual mass =

9.11 x 10 -28 g4. Location: Energy level

outside the nucleus

5. Electrical charge: Negative.6. Importance: The number of

electrons located in the last energy level determine the chemical activity of the element.

7. Discovered by: J.J.Thomson in 1897

The Neutron1. Symbol = n2. Relative Mass = 1 Atomic

Mass Unit (AMU).3. Actual mass =

1.675 x 10 -24 g4. Location: Inside the

nucleus

5. Electrical charge: Neutral.6. Importance: Is responsible for isotopes (atoms of the same element with different numbersof neutrons.7. Discovered by: James Chadwick in 1932

Atomic NumberAtomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.

Element # of protons Atomic # (Z)

Carbon 6 6

Phosphorus 15 15

Gold 79 79

Mass NumberMass number is the number of protons and neutrons in the nucleus of an isotope.

Mass # = p+ + n0

Nuclide p+ n0 e- Mass #

Oxygen - 10 - 33 42 - 31 15

8 8 1818Arsenic 75 33 75

Phosphorus 15 3116

IsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.

Isotope Protons Electrons Neutrons NucleusHydrogen–1

(protium)1 1 0

Hydrogen-2(deuterium)

1 1 1

Hydrogen-3(tritium)

1 1 2

Atomic Masses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 protons6 neutrons

98.89%

Carbon-13


1.11%

Carbon-14


<0.01%

Atomic mass is the average of all the naturally occurring isotopes of that element.

Carbon = 12.011

Weight Average Atomic Mass

• The atomic masses given on the periodic table are WEIGHT-AVERAGED masses.

• This is calculated using both the masses of each isotope and their percent abundances in nature.

• For the purposes of simplicity, we will round weight-average mass to the THOUSANDTHS place.

• The weight-average mass is based on the abundance of the naturally occurring isotopes of that element

• To find the weight-average mass of an element given the mass of each isotope and each isotopes percent abundance:

WAM =

(massisotope 1 X % ) + (massisotope 2 X % ) + (massisotope 3 X % ) + etc…

Weight Average Atomic Mass

Atomic Mass Unit (AMU)• amu = atomic mass unit– the ratio of the average mass per atom of

the element to 1/12 of the mass of 12C in its nuclear and electronic ground state.

• An atomic mass unit is actually an average mass, found by taking the mass of a C-12 nucleus and dividing it by 12–Hydrogen = 1amu, 1/12 of C

39

Carbon has two stable isotopesCarbon-12 has natural abundance of 98.89% and 12.000 amuCarbon-13 has natural abundance of 1.11% and 13.003 amu

Calculate the atomic mass

1. GivensCarbon-12 m=12.000 amu Abundance= 98.89%=0.9889Carbon-13 m = 13.003 amu Abundance = 1.11%=0.0111

2. Formula atomic mass of carbon-avg = (mass C-12 x nat.abund) + (mass C-13 x nat.abund.)

3. Plug in the #s(12.000amu x 0.9889) + (13.003 amu x 0.0111)= 12.011 amu= 12.0 amu

4 Types of Electron

Configuration of Elements

1. Shell Configuration

• Shows how many electrons are found in each shell (principal energy level).

• This is the configuration Niels Bohr would have come up with as the discoverer of the energy level!

Shell Number (Principle Electron Level)

Number of Electrons to hold

1 2

2 8

3 8

4 18

5 18

6 32

7 32

Shell Configuration (Bohr Diagrams)

C

1) Draw a nucleus with the element symbol inside.

2) Carbon is in the 2nd period, so it has two energy levels, or shells.

3) Draw the shells around the nucleus.

1) Add the electrons.

2) Carbon has 6 electrons.

3) The first shell can only hold 2 electrons.

C



1) Since you have 2 electrons already drawn, you need to add 4 more.

2) These go in the 2nd shell.

3) Add one at a time -starting on the right side and going counter clock-wise.

C


1) Check your work.2) You should have 6 total

electrons for Carbon.3) Only two electrons can

fit in the 1st shell.4) The 2nd shell can hold

up to 8 electrons.5) The 3rd shell can hold

18, but the elements in the first few periods only use 8 electrons.

C

2. Sublevel Electron Configuration• Principal energy levels are made up of

sublevels, much as a town is made up of streets.

• The expanded configuration tells you how many electrons are found in each sublevel of each PEL.

• Most of the time (and for all of the configurations you will be responsible for), one sublevel must fill up completely before the next one can get any electrons.

Electrons in atoms are arranged as

SHELLS (n)

SUBSHELLS (l)

ORBITALS (ml)

row #shell #

possibilities are 1-77 rows

Arrangement of Electrons in an Atom

subshellpossibilities are

s, p, d, or f4 subshells

group ## e-

s subshell : 1 orbital , total 2 e-p subshell : 3 orbital, total of 6 e-d subshell :5 orbital, total of 10 e-

f subshell: 7 orbital, total of 14 e-

Each orbital can be

assigned no more than 2 electrons!

s , orbital shapes

p orbitals are peanut or dumbbell shaped.

d orbitals

f orbitals

1

2

3

4

5

6

7

6

7

1A

2A

3B 4B 5B 6B 7B 8B 8B 8B 1B 2B

3A 4A 5A 6A 7A

8Agroup # = # valence (outside) e-

d p

f

sRow

=# shells

1

2

3

4

5

6

7

6

7

perio

d #

= #

e- s

hells

1A

2A

3B 4B 5B 6B 7B 8B 8B 8B 1B 2B

3A 4A 5A 6A 7A

8Agroup # = # valence e-

d

f

3d4d5d6d

4f5f

Subshells d and f are “special”

Electron Configuration – Spdf notation

Is2row #shell #




group ## valence e-

possibilities are:s: 1 or 2p: 1-6

d: 1-10f: 1-14

Total e- should equalAtomic #

HELIUM – 2 electrons

Electron Configuration – Spdf notation

Is2row #shell #




group ## valence e-

possibilities are:s: 1 or 2p: 1-6

d: 1-10f: 1-14

Total e- should equalAtomic #

HELIUM – 2 electrons

3. Orbital Box Diagram • Shows how many electrons are in each ORBITAL of

each sublevel, and what each electron’s SPIN is. • Orbitals are all the same size, they can all fit up to

two electrons in them. • The spin of electrons is indicated by arrows up and

down.• If the orbital has two electrons in it, the first will

have an up spin, and the second will have a down spin.

• The number of arrows will equal the number of electrons in the sublevel.

Guide to Drawing Orbital Diagrams

Drawing Orbital DiagramDraw the orbital diagram for nitrogen.Step 1 Draw boxes to represent the occupied

orbitals. Nitrogen has an atomic number of seven, which means it has seven electrons. Draw boxes to represent the 1s, 2s, and 2p orbitals.

1s 2s 2p

Drawing Orbital Diagram

Step 2 Place a pair of electrons in the last occupied sublevel in separate orbitals. We place the remaining three electrons in the 2s orbitals.

1s 2s 2p

Drawing Orbital Diagram

Step 3 Place remaining electrons with opposite spins in each filled orbital. First we place a pair of electrons with opposite spins in the 2p orbitals, with arrows in the same direction.

1s 2s 2p

HONORS CHEMISTRY ONLY3a. Quantum Numbers

• Electron energies are addressed in a similar way to a ZIP code. Many addresses in Ulster and northern Orange

• county have 125 as the prefix, with the last two digits signifying the actual postal box.\

• For example, New Paltz is 12561, Wallkill is 12589, Newburgh is 12550, Pine Bush is 12566.

3a. Quantum Numbers

• There are four identifying characteristics of the energy of a specific electron in an atomic, each more specific than the last.

• They are:– n (principal quantum number) = Principal Energy

Level (1, 2, 3, 4, etc.)– l (levarotary) = Sublevel (s, p, d, f)– m (magnetic) = Orbital– s (spin) = Spin (+ 1/2, - 1/2)

3a. Quantum Numbers

• n , principal quantum number–based on Bohr’s observations of line

spectra for different elements–‘n’ relates to the main energy of an

electron–allowable values: n = 1, 2, 3, 4, …–electrons with higher ‘n’ values have

more energy

3a. Quantum Numbers

• l , The Secondary Quantum Number – based on the observation that lines on line spectra

are actually groups of multiple, thin lines– ‘l ’ relates to the shape of the electrons’ orbits– allowable values: l = 0 to l = n - 1• i.e. for n = 4: l = 0, 1, 2, or 3

– the ‘l ’ values 0, 1, 2, and 3 correspond to the shapes we will call s, p, d and f, respectively

3a. Quantum Numbers

• ml , the Magnetic Quantum Number– based on the observation that single lines on line

spectra split into new lines near a strong magnet– ‘ml ’ relates to the direction/orientation of the

electrons’ orbits– allowable values: ml = - l to + l • i.e. for l = 2: ml = -2, -1, 0, 1, or 2

– electrons with the same l value but different ml values have the same energy but different orientations

3a. Quantum Numbers

• ms , The Spin Quantum Number– based on the observation that magnets could

further split lines in line spectra, and that some elements exhibit paramagnetism

– ‘ms ’ relates to the ‘spin’ of an electron– allowable values: ms = - ½ or + ½ • i.e. for any possible set of n, l, and ml

values, there are two possible ms values– when two electrons of opposite spin are paired,

there is no magnetism observed; an unparied electron is weakly magnetic

ms , The Spin Quantum Number

4. Electron (Lewis) Dot Diagram• VALENCE ELECTRONS– the electrons in the outermost shell (furthest

energy level from the nucleus), which is also called the valence shell.

– The number of valence electrons that an atom has can be determined by the last number in the basic electron configuration.

The number of valence electrons that an atom has determines its physical and chemical

properties

Group 1 (alkali metals) have 1 valence electron

Group 2 (alkaline earth metals) have 2 valence electrons

Group 13 elements have 3 valence electrons




Group 17 (halogens) have 7 valence electrons

Group 18 (Noble gases) have 8 valence electrons, except helium, which has only 2

Transition metals (“d” block) have 1 or 2 valence electrons

Lanthanides and actinides (“f” block) have 1 or 2 valence electrons

Lewis Dot Diagram

• using dots in groups of 2 around the symbol of the atom to represent the valence electrons.

• For every atom, the valence electrons will occupy only s and p orbitals.

• The s electrons fill up first, then the p electrons fill, up electrons first, followed by the downs, just like in the box diagram.

The Electron Dot diagram for Nitrogen

Nitrogen has 5 valence electrons.

First we write the symbol.

NThen add 1 electron at a time to each side.

Until they are forced to pair up.

Checking for understandingDraw orbital diagrams

Draw Lewis dot diagrams

Carbon

Helium

Fluorine

Nuclear Chemistry

Nuclear Chemistry• Nucleus of an atom contains

protons and neutrons• Strong forces (nuclear force) hold

nucleus together– Protons in nucleus have electrostatic

repulsion– however, strong force overcomes this

repulsion– Strong force: the interaction that

binds nucleus together– Nuclear force (strong force) is MUCH

stronger than electrostatic force– Strong force increases over short

distances

Radioisotopes

• Radioisotopes- unstable isotopes that gain stability by releasing particles.

Unstable isotope stable

alpha

betagamma

Characteristics of Some RadiationProperty Alpha

radiationBeta radiation

Gamma radiation

Composition Alpha particle (He nucleus)

Beta particle (electron)

High energy EM radiation

Symbol , 42 He , 0

-1 e

Penetrating power

low moderate Very high

Shielding Paper, clothing

Metal foil Lead, concrete

Atomic number (Z) = # protons in nucleus

Mass number (A) = # protons + # neutrons

= atomic # (Z) + # neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

U23592 U238

92

Mass Number Atomic Number

Element Symbol

Review

Alpha emission (decay) 4 2 He

238 92 U

Identify the product formed when uranium-238 alpha decays

4 2 He + 234

90 Th

Determines which atom from the periodic table

Beta emission (decay) 0-1 e

14 6 C

Identify the product formed when carbon-14 emits beta particle

0 -1 e + 14

7 N

Gamma emission (decay) 00 γ

14 6 C

Identify the product formed when carbon-14 releases gamma rays

0 0 γ + 14

6 C

Nuclear Reaction

Nuclear Fission• Reaction splits a large

nucleus apart to form two smaller ones.

• Reaction is unknown in the natural world, is a form of artificial transmutation

• Reaction can take place at any temperature or pressure

• Reaction is currently being used to produce electricity for our use

• Requires mining to extract uranium ore• Produces THOUSANDS of times more energy than conventional chemical explosives• Produces radioactive wastes

Nuclear Fusion• Reaction combines two small

nuclei together to form one larger one.

• All stars are powered by nuclear fusion

• Reaction requires temperatures of millions of degrees and vast pressures

• Reaction requires temperatures of millions of degrees and vast pressures

• Hydrogen is the most abundant element in the universe• Produces MILLIONS of times more energy than conventional chemical explosives• Produces essentially no radioactive waste

Common to Both Fission and Fusion

• Both generate their energy the same way by converting small amounts of mass (MASS DEFECT) into extraordinary amounts of energy.

Checking for understandingCompare and contrast nuclear fusion and fission

Half - Life

Half- Life• The half-life of a radioactive isotope is defined as the

period of time that must go by for half of the nuclei in the sample to undergo decay.

- Half of the radioactive nuclei/isotope in the sample decay into new, more stable nuclei/isotope

• After one half-life, half (50%) of the original amount of the sample will have undergone radioactive decay.

• After a second half-life, one quarter (25%) of the original sample will remain undecayed.

• After a third half-life, one eighth (12.5%) of the original sample will remain undecayed.

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and turns red. C14 – blue N14 - red

As we begin notice that no time has gone by and that 100% of the material is C14

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

97

The grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red

Halflives


0 100% 0% no ratio

1 50% 50% 1:1

After 1 half-life (5730 years), 50% ofthe C14 has decayed into N14. The ratioof C14 to N14 is 1:1. There are equalamounts of the 2 elements.

98

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red .C14 – blue N14 - red

Halflives


0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

Now 2 half-lives have gone by for a totalof 11,460 years. Half of the C14 that waspresent at the end of half-life #1 has nowdecayed to N14. Notice the C:N ratio. Itwill be useful later.

99

The blue grid below represents a quantity of C14. Each time you click,one half-life goes by and you see red. C14 – blue N14 - red

Halflives


0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

3 12.5% 87.5% 1:7

After 3 half-lives (17,190 years) only12.5% of the original C14 remains. Foreach half-life period half of the materialpresent decays. And again, notice the ratio, 1:7 100

101

Radioactive Dating

Radioactive Dating

• Radioactive Decay is a RANDOM process. It is not possible to predict when a particular nucleus will decay, but we can make fairly accurate predictions regarding how many nuclei in a large sample will decay in a given period of time.

Radioactive Dating

• used to determine the age of a substance that contains a radioactive isotope of known half-life.

• Step 1: Determine how many times you can cut your original amount in half in order to get to your final amount. This is the number of half-lives that have gone by.

• Step 2: Multiply the number of half-lives by the duration of a half-life

Age of Sample = # Half-Lives X Half-Life DurationSee Reference

chart

Reference Chart

• The oldest rocks on Earth have been found to contain 50% U-238 and 50%Pb-206 (what U-238 ultimate decays into). What is the age of these rocks?

First, find out how many half-lives have had to go by so that you have gone from 100% U-238 to 50% U-238:

100 50 ONE half-life has gone by!

Age of Sample = # Half-Lives X Half-Life Duration = 1 half-life X (4.51 X 109 years) = 4.51 X 109 years old!

Atomic Theory DEMOCRITUS 460 - 370 BC The Greek philosopher Democritus proposed that all matter was made of small, unbreakable particles he called.

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