UnitUnit 4 – Atomic 4 – Atomic StructureStructure

Bravo – 15,000 kilotons

Atoms, Ions, and the Periodic Table

What is an atom?

It is smallest particle of an element that retains the elements properties.

But how did we come to know all the information we have about these tiny particle?

Democritus (460-370 BC)

Democritus (460-370 BC) • Matter is made of tiny, solid, indivisible particles which he

called atoms (from atomos, the Greek word for indivisible).• Different kinds of atoms have different sizes and shapes.• Different properties of matter are due to the differences in

size, shape, and movement of atoms. • Democritus’ ideas, though correct, were widely rejected by

his peers, most notably Aristotle (384-322 BC). Aristotle was a very influential Greek philosopher who had a different view of matter. He believed that everything was composed of the four elements earth, air, fire, and water. Because at that time in history, Democritus’ ideas about the atom could not be tested experimentally, the opinions of well-known Aristotle won out. Democritus’ ideas were not revived until John Dalton developed his atomic theory in the 19th century!

John Dalton (1766-1844)

John Dalton (1766-1844)

– All matter is composed of extremely small particles called atoms.

– All atoms of one element are identical.– Atoms of a given element are different

from those of any other element. – Atoms of one element combine with

atoms of another element to form compounds.

– Atoms are indivisible. In addition, they cannot be created or destroyed, just rearranged.

• Dalton’s theory was of critical importance. He was able to support his ideas through experimentation, and his work revolutionized scientists’ concept of matter and its smallest building block, the atom.

• Dalton’s theory has two flaws:– In point #2, this is not completely true.

Isotopes of a given element are not totally identical; they differ in the number of neutrons. Scientists did not at this time know about isotopes.

– In point #5, atoms are not indivisible. Atoms are made of even smaller particles (protons, neutrons, electrons). Atoms can be broken down, but only in a nuclear reaction, which Dalton was unfamiliar with.

Discovery of the Electron JJ Thomson (1856-1940)

Discovery of the Electron JJ Thomson (1856-1940)

• Discovered the electron, and determined that it had a negative charge, by experimentation with cathode ray tubes. A cathode ray tube is a glass tube in which electrons flow due to opposing charges at each end. Televisions and computer monitors contain cathode ray tubes.

• Thomson developed a model of the atom called the plum pudding model. It showed evenly distributed negative electrons in a uniform positive cage.

• Diagram of plum pudding model:

Discovery of the NucleusErnest Rutherford (1871-1937)

http://www.mhhe.com/physsci/chemistry/animations/chang_2e/rutherfords_experiment.swf

Discovery of the NucleusErnest Rutherford (1871-1937)

• Discovered the nucleus of the atom in his famous Gold Foil Experiment.

• Alpha particles (helium nuclei) produced from the radioactive decay of polonium streamed toward a sheet of gold foil. To Rutherford’s great surprise, some of the alpha particles bounced off of the gold foil. This meant that they were hitting a dense, relatively large object, which Rutherford called the nucleus.

Rutherford then discovered the Rutherford then discovered the protonproton, and next, working with a , and next, working with a colleague, James Chadwick (1891-1974), he discovered the colleague, James Chadwick (1891-1974), he discovered the neutronneutron as well.as well.

Models of the Atom - Niehls Bohr

Models of the Atom - Niehls Bohr

• Developed the Bohr model of the atom (1913) in which electrons are restricted to specific energies and follow paths called orbits a fixed distance from the nucleus. This is similar to the way the planets orbit the sun. However, electrons do not have neat orbits like the planets.

• Diagram of Bohr model:

Quantum Mechanical Model

Quantum Mechanical Model

• This is the current model of the atom. We now know that electrons exist in regions of space around the nucleus, but their paths cannot be predicted. The electron’s motion is random and we can only talk about the probability of an electron being in a certain region.

Modern Atomic TheoryModern Atomic Theory

Atoms of an element have a characteristic average mass which is unique to that element.

Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

All matter is composed of atoms

Atoms of any one element differ in properties from atoms of another element

Discovery of the ElectronDiscovery of the ElectronIn 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

Cathode ray tubes pass electricity through a gas that is contained at a very low pressure.

Conclusions from the Study Conclusions from the Study of the Electronof the Electron

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 Atomic Thomson’s Atomic ModelModel

Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

Rutherford’s FindingsRutherford’s Findings

The nucleus is small The nucleus is dense The nucleus is positively charged

Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected

“Like howitzer shells bouncing off of tissue paper!”

Conclusions:

Atomic ParticlesAtomic ParticlesParticle Charge Mass # Location

Electron

-1 0 Electron cloud

Proton +1 1 Nucleus

Neutron

0 1 Nucleus

The Atomic The Atomic ScaleScale

Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space

“q” is a particle called a “quark”

About Quarks…About Quarks…Protons and neutrons are NOT fundamental particles.Protons are made of two “up” quarks and one “down” quark.Neutrons are made of one “up” quark and two “down” quarks.

Quarks are held togetherby “gluons”

Atomic NumberAtomic 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 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 1818

Arsenic 75 33 75

Phosphorus 15 3116

IsotopesIsotopesIsotopes are atoms of the same element having different masses due to varying numbers of neutrons.Isotope Proto

nsElectron

sNeutron

sNucleus

Hydrogen–1

(protium)

1 1 0

Hydrogen-2

(deuterium)

1 1 1

Hydrogen-3

(tritium)

1 1 2

Atomic Atomic MassesMasses

Isotope Symbol Composition of the nucleus

% in nature

Carbon-12

12C 6 protons6 neutrons

98.89%

Carbon-13

13C 6 protons7 neutrons

1.11%

Carbon-14

14C 6 protons8 neutrons

<0.01%

Atomic mass is the average of all the naturally isotopes of that element.Carbon = 12.011

Isotopes • Isotopes are atoms of an element with the same number of

protons but different numbers of neutrons.• Change in # of n0 = Change in the Mass #

• Most elements on the periodic table have more than one naturally occurring isotope.

• There are a couple of ways to represent the different isotopes. One way is to put the mass after the name or symbol: Carbon-12 or C-12

• Another way is :

C126

##

massatomic

Determining Average Atomic Mass • The atomic mass on the periodic table is

determined using a weighted average of all the isotopes of that atom.

• In order to determine the average atomic mass, – convert the percent abundance to a decimal– multiply it by the mass of that isotope– values for all the isotopes are added to together

to get the average atomic mass

Formula: (Mass1 * Abundance1) + (Mass2 * Abundance2) + (Mass3 * Abundance3)

+ etc….

Example of Average atomic mass calculation

Given:12C = 98.89% at 12 amu13C = 1.11% at 13.0034 amuCalculation:(98.89%*12 amu) + (1.11%*13.0034 amu) =

(0.9889*12 amu) + (0.011*13.0034 amu) (0.9889*12 amu) + (0.011*13.0034 amu) ==

12.01 amu12.01 amu

Now you try one:

• Neon has 3 isotopes:  Neon-20 has a mass of 19.992 amu and an abundance of 90.51%.  Neon-21 has a mass of 20.994 amu and an abundance of 0.27%.  Neon-22 has a mass of 21.991 amu and an abundance of 9.22%.  What is the average atomic mass of neon?

• The answer is: (0.9051 * 19.992 amu) + (0.0027 * 20.994 amu) + (0.0922 * 21.991 amu) =

20.179 amu Now compare this mass for Neon to the mass on

the periodic table!

The MoleThe Mole

1 dozen =1 gross =

1 ream =

1 mole =

12

144

500

6.02 x 1023

There are exactly 12 grams of carbon-12 in one mole of carbon-12.

Avogadro’s NumberAvogadro’s Number6.02 x 1023 is called “Avogadro’s Number” in honor of the Italian chemist Amadeo Avogadro (1776-1855).

Amadeo Avogadro

I didn’t discover it. Its just named after

me!

MOLE

MOLAR MASS 6.02 x 1023 particles

Molecular Mass MoleculesAtomic Mass (really amu’s) AtomsFormula Mass Formula Units

Calculations with Moles:Calculations with Moles:Converting moles to gramsConverting moles to grams

How many grams of lithium are in 3.50 moles of lithium?

3.50 mol Li= g Li

1 mol Li

6.94 g Li24.29

Calculations with Moles:Calculations with Moles:Converting grams to molesConverting grams to moles

How many moles of lithium are in 18.2 grams of lithium?

18.2 g Li= mol Li

6.94 g Li

1 mol Li2.62

Calculations with Moles:Calculations with Moles:Using Avogadro’s NumberUsing Avogadro’s Number

How many atoms of lithium are in 3.50 moles of lithium?

3.50 mol Li = atoms Li

1 mol Li

6.02 x 1023 atoms Li 2.11 x 1024

Calculations with Moles:Calculations with Moles:Using Avogadro’s NumberUsing Avogadro’s Number

How many atoms of lithium are in 18.2 g of lithium?

18.2 g Li

= atoms Li

1 mol Li 6.02 x 1023 atoms Li

1.58 x 1024

6.94 g Li 1 mol Li

(18.2)(6.022 x 1023)/6.94

Nuclear SymbolsNuclear Symbols

23592U

Element symbol

Mass number (p+ + no)

Atomic number(number of p+)

Types of Radioactive DecayTypes of Radioactive Decay

alpha production (): helium nucleus

beta production ():

92238

24

90234U He Th

90234

91234

10Th Pa e

24

He

10 e

2+

Alpha Alpha RadiationRadiation

Limited to VERY large nucleii.

Beta Beta RadiatioRadiatio

nn

Converts a neutron into a proton.

Types of Radioactive DecayTypes of Radioactive Decay

gamma ray production ():

positron production :

electron capture: (inner-orbital electron is captured by the nucleus)

92238

24

90234

002U He Th

1122

10

1022Na e Ne

80201

10

79201

00Hg e Au

10 e

Types of RadiationTypes of Radiation

Deflection of Decay ParticlesDeflection of Decay Particles

Opposite charges_________ each other.

Like charges_________ each other.

attract

repel

NuclearNuclearStabilityStability

Decay will occur in such a way as to return a nucleus to the band (line) of stability.

Half-life ConceptHalf-life Concept

Sample Half-LivesSample Half-Lives

A A Decay Decay SeriesSeries

A radioactive nucleus reaches a stable A radioactive nucleus reaches a stable state by a series of stepsstate by a series of steps

Nuclear Fission and FusionNuclear Fission and Fusion

•Fusion: Combining two light nuclei to form a heavier, more stable nucleus.

•Fission: Splitting a heavy nucleus into two nuclei with smaller mass numbers.0

192

23556

1423691

013n U Ba Kr n

23

11

24

10He H He e

Energy and MassEnergy and MassNuclear changes occur with small but measurable losses of mass. The lost mass is called the mass defect, and is converted to energy according to Einstein’s equation:

E = mc2

m = mass defectE = change in energy

c = speed of light

Because c2 is so large, even small amounts of mass are converted to enormous amount of energy.

FissionFission

Fission ProcessesFission Processes

Event

NeutronsCausingFission Result

subcritical < 1 reaction stopscritical = 1 sustained reactionsupercritical > 1 violent explosion

A self-sustaining fission process is A self-sustaining fission process is called a chain reaction.called a chain reaction.

A Fission ReactorA Fission Reactor

FusionFusion