topic 7. 1 atomic structure

Topic 7. 1 Atomic Structure

The modern atom has gone through a few stages of development

Dalton’s Atomic Therory – idea of an atomJJ Thompson – 1890 – negative charge

(electrons)Earnest Rutherford – 1911 - positive nucleus

(protons)Niels Bohr – 1913 – orbital shellsChadwick – 1932 – neutrons

7.1.1 Describe a model of the atom that features a small nucleus surrounded by electrons.

This is a VERY simplified idea of the atomNucleus

Protons – positive charge – 1.6 x 10-19CNeutrons – no chargeDiameter order of 10-15m

Electron “cloud”Electrons – negative charge – 1.6 x 10-19CDiameter order of 10-10m


The nucleus is about 100,000 times smaller than the electron orbits.

Imagine a pea in the center of a football field with the track being the orbits.

Protons and Neutrons have very similar mass. Protons and Neutrons are about 1800 times

bigger than electrons.


7.1.2 Outline the evidence that supports a nuclear model of the atomDalton’s Atomic Theory1. All matter is composed of extremely small particles

called atoms.2. All atoms of a given element are identical.3. Atoms cannot be created, divided into smaller

particles, or destroyed.4. Different atoms combine in simple whole number

ratios to form compounds.5. In a chemical reaction, atoms are separated,

combined or rearranged.

Deomcritus

Atoms

Differences in atoms

Dalton’s Atomic Theory1. All matter is composed of extremely small particles

called atoms.2. All atoms of a given element are identical.3. Atoms cannot be created, divided into smaller

particles, or destroyed. (This part proven wrong)4. Different atoms combine in simple whole number

ratios to form compounds.5. In a chemical reaction, atoms are separated,

combined or rearranged.

Deomcritus

Atoms


Dalton• Atoms

• Sameness• Created/destroyed

• Combination• Rearragement

Subatomic Particles and the AtomJ. J. Thomson – 1890-1900

Used cathode ray tube to prove existence of electron.Proposed “Plum Pudding Model”

Cathode ray tubeStream of charged particles (electrons).

http://www.youtube.com/watch?v=YG-Wz-arcaY

http://www.youtube.com/watch?v=O9Goyscbazk

http://www.youtube.com/watch?v=YG-Wz-arcaY



Subatomic Particles and the AtomPlum Pudding

J. J. ThompsonPlum Pudding Model

Deomcritus

Atoms


Dalton• Atoms



Thompson• Atoms composed

of electrons

Ernest RutherfordGold Foil experiment

Used to prove the existence of a positively charged core (Nucleus)

Fired alpha particles(2protons and 2 neutrons) into very thin gold foil.

The results were “like firing a large artillery shell at a sheet of paper and having the shell come back and hit you!”

Ernest RutherfordWhat should have

happened • What DID happened

Ernest RutherfordAfter performing hundreds of tests and calculations,

Rutherford was able to show that the diameter of the nucleus is about 105 times smaller than the diameter of the atom

Deomcritus

Atoms


Dalton• Atoms




of electrons

Rutherford• Positively Charged

Nucleus

Subatomic Particles and the AtomChadwick

Worked with Rutherford.Noted there was energy in the nucleus, but wasn’t the

protons.Concluded that neutral particles must also exist in nucleus.

Subatomic Particles and the AtomJames Chadwick – 1932Bombarded a beryllium target with alpha

particlesAlpha particles are helium nucleus

Discovered that , carbon was produced with another particle.

Concluded this particle had almost identical mass to proton but no charge.

Called it a neutron

Deomcritus

Atoms


Dalton• Atoms




of electrons

Rutherford• Positively Charged

Nucleus

Chadwick• Neutrons

exist in Nucleus

Subatomic Particles and the AtomThree main particles:

ProtonPositiveIn nucleus

NeutronsNeutralIn nucleus

ElectronsNegativeOrbiting the nucleus (not inside)

If Rutherford’s was correct, electrons orbiting would undergo centripetal acceleration.

This would mean they would radiate electromagnetic waves.

Meaning they would loose energyMeaning the atom would collapse on it’s self

7.1.3 Outline one limitation of the simple model of the nuclear atom.7.1.4 Outline evidence for the existence of atomic energy levels.

If low-pressure gases are heated or current is passed through them they glow.

Different colors correspond to their wavelengths.

Visible spectrum 400nm(violet) to 750nm(red)


Gas – slit – slit – prism – viewing screenWhen single element gases such as hydrogen

and helium are excited only specific wave lengths were emitted.

These are called emission line spectra


Light – gas vapor – slit – slit – prism – viewing screen

If white light is pass through the gas the emerging light will show dark bands called absorption lines.

They correspond to the emission lines.


Rutherford’s model didn’t explain why atoms emitted or absorbed only light at certain wavelengths.

1885 JJ Balmer showed that hydrogen’s four emission lines fit a mathematical formula.

This “Balmer series” also show the pattern continued into non-visible ultra-violet and infra-red.

LIMITATION

Bohr called these “energy levels” Reasoned that the electrons do not lose

energy continuously but instead, lose energy in discrete amounts called “quanta”.

He agreed with Rutherford that electrons orbit the nucleus but only certain orbits were allowed.

LIMITATION

The electric force between protons and electrons holds electrons in orbit

Electron never found between these levels. (“jumps” instantly)

Only radiates energy when it “jumps” down.Absorbs energy when it “jumps” up.Total energy stays constant

LIMITATION

Bohr explained the emission and absorption line spectra with the idea that electrons absorbed only certain quantity of energy that allowed it to move to a higher orbit or energy level.

Each element has its own “finger print”.

LIMITATION

Ground state – lowest energy level – smallest possible radius

Excited state – when an electron absorbs energy and jumps to a higher energy level.

Once an electron jumps back to a lower state it gives off energy in the form of a photon.

These photons are the emission spectrum.

Energy Level Diagram

The amount jumped correlates to the energy of the photon.

Greater the jump means the greater the energy is emitted.

Each jump corresponds to a different amount of energy being released. This means we can calculate the frequency and wavelength of light that will be produced.

Energy Level Diagram

E = hf

E = energy of a quantumh = Planck’s constant (6.63 x 10-34Js)f = frequency

Energy of a light quantum

An electron in a hydrogen atom drops from energy level E4 to energy level E2. What frequency of the emitted photon, and which line line in the emission spectrum corresponds to this event?

Sample Problem C

First find the amount of energy lost

Elost = E4 – E2 Elost = (-0.85eV) – (-3.40eV)Elost = 2.55 eV

Sample Problem C

Second, convert eV into J.

1eV = 1.6 x 10-19J

Answer: 4.08 x 10-19J

Sample Problem C

Third use Planck’s equations to find the frequency.

E = hf

f = 6.15 x 1014 Hz

Sample Problem C

Fourth decide which line corresponds to this even.

Answer: Green light

v = f λ

Sample Problem C

Practice C, pg 769 in book, #2 – 5Your practice

DefinitionsNucleon – any of the constituents of a nucleus.

Protons and neutrons.Atomic Number – The number of protons in the

nucleus.Nucleon Number – The number of nucleons in the

nucleus. AKA the mass number. (protons + neutrons)

Isotope – Nuclei which contain the same number of protons but different numbers of neutrons.

Nuclide – the nucleus of an atom. The nuclides of isotopes are different, even though they are the same element.

7.1.5 Explain the terms nuclide, isotope and nucleon7.1.6 Define nucleon number A, proton number Z, and neutron number N.

Atomic Number (proton number), ZHow many protons there are.This is what defines the element.Ex. Hydrogen Z =1, Oxygen Z = 8 Carbon Z = 6

Nucleon Number (mass number), AHow many nucleons there are. Protons + neutrons

Number of neutrons, NMass number = atomic number + number of

neutronsA = Z + N


Standard notation is: A over Z in front of element(X)

*****Draw on board*****

IsotopesMore evidence for neutrons is the existence of

isotopes. When nuclei of the same element have different

numbers of neutrons.Carbon has 6 isotopes: Carbon-11, Carbon-12,

Carbon-13, Carbon-14, Carbon-15, Carbon-16.All have 6 protons but each has different number of

neutrons.


The different isotopes don’t exist in nature in equal amounts.

Carbon: C – 12 is most abundant (98.9%)C – 13 is next (1.1%)

This is where atomic mass comes from. It’s the weighted average mass of all the different isotopes.


Nuclei of different atoms are known as nuclides. Ex. C – 12, C – 14Both are carbon but different isotopesTheir nuclei have different numbers of neutrons.These are different nuclides.


How do like charge (protons), stay stuck together?

We already know that like charges repel each other.

We have also seen that they are stronger than gravitational forces.

Strong Force – The force that binds the nucleus together.

It is an attractive force that acts between all nucleons.

Short – range interactions only (up to 10-15m)

7.1.7 Describe the interactions in a nucleus

7.3.3 - Define the term unified atomic mass unit.7.3.4 - Apply the Einstein mass-energy

equivalence relationship.7.3.5 - Define the concepts of mass defect,

binding energy and binding energy per nucleon.7.3.6 - Draw and annotate a graph showing the

variation with nucleon number of the binding energy per nucleon.

7.3.7 - Solve problems involving mass defect and binding energy.

Because the mass of an atom is so small a new unit was created.

Some times called “Atomic mass unit”1 u = 1.66053886 x 10-27 kg12u = one atom of carbon-12

Unified Mass Unit

Mass of a nucleus is sometimes expressed in terms of rest energy.

A particle has a certain amount of energy associated with its mass.

Relationship between rest energy and mass:ER = mc2

Resting Energy

It doesn’t always happen with nuclear processes.

Some times mass is converted or lost in the form of energy.

1u = 931.49 MeV

Conservation of mass

So that means that one proton IS 938.3MeV of energy.

Mass is energy, energy is mass THEY ARE THE SAME THING!!! AHHHHHH!!!!!!

Check out the table

Conservation of mass

What happens when you place two negative charged particles next to each other?

What happens when you place two positively charged particles next to each other?

Nuclear Stability

So why doesn’t a nucleus explode?It shouldn’t stay together.

Strong Force Attractive forceIndependent of electric chargeVery short range

Neutrons!!!Spread the protons apart to help balance

electrical repulsion and strong attraction

Particles in a stable nuclease need an input of energy to break the strong nuclear force.

When to unbound particles come together energy is released. (think nuclear reactions)

Turns out these quantities of energy are the same.

Called the binding energyBinding energy is the energy it takes to hold

the atom together. Equal to the

Binding Energy

Recall that mass is energy. Carbon – 12

Atom of carbon – lighter, less rest energyConstituent parts of – heavier, more rest energy

What happen to that little bit of matter?It is used as the energy to bind together the

atom.

The difference in the two masses is known as mass defect (∆m)

Think of it this way….

Binding energy = mass defect x (speed of light)2

Ebind = ∆m c2

E = mc2

Put it all together

The nucleus of the deuterium atom, called deuteron, consists of a proton and a neutron. Given that the atomic mass of deuterium is 2.014 102u, calculate the deuteron’s binding energy in MeV.

Answer: 2.224MeV

Example

If the phosphorus has a mass of 30.973 762u, then what is the binding energy that holds the nucleus together in MeV?

Answer:

Together

Practice A, pg 795 in book, #1,3-4

Answers:1) 160.65MeV, 342.05MeV2) 0.764MeV3) 7.933MeV4) 7.5701 MeV/nucleon

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