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What are atoms? Describe them. Models of the Atom. - PowerPoint PPT Presentation

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Page 1: What are atoms? Describe them
Page 2: What are atoms? Describe them

What are atoms?Describe them.

Page 3: What are atoms? Describe them

Models of the Atom

Page 4: What are atoms? Describe them

JJ THOMSON MODEL 1907PLUM PUDDING

J.J.Thomson that atoms contained p+ and e-.But neutral atoms must contain equal numbers of p+ & e-.

Thomson proposed that inside an atom was a mixture of equal numbers of p+ and e-, scattered randomly, just as raisins and currants are found inside a plum pudding.

Page 5: What are atoms? Describe them

Plum Pudding Model

Page 6: What are atoms? Describe them

Rutherford ModelLord Ernest Rutherford learned from experiments that atoms contained a very small but heavy central positive nucleus.

In 1911 he proposed the model that inside an atom all the protons were gathered at the center (nucleus) of the atom, with the electrons scattered randomly around.

Page 7: What are atoms? Describe them
Page 8: What are atoms? Describe them

Most a particles went straight through, but the ones that passed closest the Au nucleus were progressively more deflected.

Page 9: What are atoms? Describe them
Page 10: What are atoms? Describe them

The fact that most alpha particles pass straight through the foil suggests:

• The nucleus is positive

• The nucleus is negative

• The atom is mostly empty

• The particles have lots of energy.

Page 11: What are atoms? Describe them

The fact that some alpha particles were deflected by the foils suggests:

• The nucleus is positive

• The nucleus is negative

• The atom is mostly empty

• The particles have lots of energy.

Page 12: What are atoms? Describe them

Gold foil experiment :atom is mostly empty space

with dense positively charged nucleus.

Electrons move in circular orbits about the nucleus.

Page 13: What are atoms? Describe them

Rutherford felt the inertia from circular velocity of e- would balance the electrostatic attraction of the nucleus.

Page 14: What are atoms? Describe them

Flaws: Maxwell had proved that accelerated charges radiate EM energy. Circular motion is acceleration.

The e- should lose energy & spiral into the nucleus.

That does not happen!

Also, how did the positive nucleus stay together?

Page 15: What are atoms? Describe them

Bohr Model

There are several allowed orbits that an e- can occupy.

The orbits are at varying distances from the nucleus.

Closest orbit n=1, the ground state. E=0 for e-

Page 16: What are atoms? Describe them

Orbits further from the nucleus require the e- to absorb E to occupy.

If e- in higher orbit, atom said to be excited.

Page 17: What are atoms? Describe them

Atoms need to absorb E to excite e- to higher orbits.

When e- drops to lower orbit atom gives off E.

Page 18: What are atoms? Describe them

Evidence for Bohr Mode from Spectra

• Absorption- e- absorbs specific photon E.

• Emission - e- emits specific photon E.

Page 19: What are atoms? Describe them

Electric E supplied to gas tubes causes gases to emit light.

Page 20: What are atoms? Describe them

Emission SpectrumWhen viewed through a prism or

spectroscope, we see only certain of light are emitted by each element.

Bright Line Spectra

Page 21: What are atoms? Describe them

AbsorptionContinuous spectrum

From sunlight

Page 22: What are atoms? Describe them

Absorption Spectrum

When light is passed through cool gasses, each gas absorbs only certain

’s of light.

When viewed through a prism, the same ’s that are emitted by each

element, are absorbed by each element.

Page 23: What are atoms? Describe them

Frequencies emitted exactly match the frequencies absorbed.

Page 24: What are atoms? Describe them

Do Now: A photon with an unknown energy hits a metal with Wo 2.11 x 10-19 J. The max KE of the photoelectrons 2.0 x 10-19 J. What is the color on the incident photon?

• Epho = Wo + KE.

• Epho = 2.11 x 10-19 J + 2.0 x 10-19 J.

• Epho = 4.11 x 10-19 J

• E = hf.

• f = 6.2 x 1014 Hz.

Page 25: What are atoms? Describe them

Review

• How does Rutherford view the atom?

• How does Bohr view the atom?

Page 26: What are atoms? Describe them

Rutherford Assumptions

• Atom has dense + nucleus

• Tiny, low mass negative e- orbit in shells.

• Atom mostly empty space.

• Evidence: most +alpha particles passing straight through gold foil.

• Some deflected or repelled

straight back.

Page 27: What are atoms? Describe them

Bohr Assumptions:• Each atomic orbit is associated with a specific E level for e-.

It is quantized.

• Innermost orbit is “ground state”

• When atom absorbs photon energy e- “jump” to higher E outer orbits. Atom is “excited”.

• Atom emits photons when e- “fall” to lower inner E orbits.

• No in between orbits possible, photons absorbed/emitted

comes in discrete E amounts.

Page 28: What are atoms? Describe them
Page 29: What are atoms? Describe them

Bohr ModelEmission & absorption spectra form from e- either emitting or absorbing EM energy photons as they change orbits.

Page 30: What are atoms? Describe them

How stuff works. How is light produced? 2:15

• http://www.youtube.com/watch?v=GCvjo3em7EQ

Page 31: What are atoms? Describe them

The frequencies/colors of the observed spectral lines correspond to the exact energies (E=hf) that e- are absorbing or emitting as they move between allowed orbits in the atom.

Page 32: What are atoms? Describe them

Film: Quantum Mechanics & Atomic Structure 6:20 minutes

http://www.youtube.com/watch?v=-YYBCNQnYNM

Hwk Concepts: Rd Tx 840 – 847Do pg 847 #2 – 6 Full Sentences &

Mult Choice sheet.

Page 33: What are atoms? Describe them

Bohr Energy Diagrams

Page 34: What are atoms? Describe them

Orbital Energy Levels/ Ionization Energy

Each orbit is associated with a specific energy which corresponds to the minimum energy needed to totally strip an e- from that orbit.

This ionization energy > E needed to jump between orbits.

If an atom absorbs E = to the orbit energy it becomes ionized (charged).

Orbits are named by quantum number.

Page 35: What are atoms? Describe them

Ionization Energy: e- stripped from atom if photon with sufficient energy absorbed.

Page 36: What are atoms? Describe them

When e- is in lower/closer orbits to nucleus. It takes more E to ionize/strip it out of atom.

Page 37: What are atoms? Describe them
Page 38: What are atoms? Describe them

Ex 1: How much energy would be needed to ionize (completely strip) an electron:

In the n=1 level of of Hydrogen?

in the n = b or level of Mercury?

In the n = 2 level of Hydrogen?

Page 39: What are atoms? Describe them

For e- to jump to higher orbits it must absorb exact E between orbits.

Page 40: What are atoms? Describe them

The photon energy absorbed & emitted during transitions between e- orbits:

Page 41: What are atoms? Describe them

Use diagrams to find E.

Ephoton = Ei - Ef

Use Epho = hf of the radiation

to find frequency associated with photon of known energy.

Page 42: What are atoms? Describe them

Ex 2:

a) How much E is absorbed when a Hydrogen e- jumps directly from the n=1 to n=3 orbit? b) How much E is released when the Hydrogen e- drops from n=3 to n=1?

c) When the e- drops back down to the n=1 from n=3 orbit, what f photon is emitted?

d) To which type of radiation does that photon correspond? e) How many different photons are possible to be emitted by electron dropping from the n=3 to n=1 level?

Page 43: What are atoms? Describe them

n =3 to n = 1 Ephoton = Einitial - Efinal.

-13.6 eV - (-1.51 eV)= -12.1 eV

(12.1 eV)(1.6 x 10-19 J/eV) = 1.936 x 10-18J.

E = hf. f = E/h

f = 1.936 x 10-18J/(6.63 x 10-34 Js)

f = 2.92 x 1015 Hz. Look up.

Page 44: What are atoms? Describe them

Ex 3: A Mercury Atom has an e- excited from the n=a to the n=e energy level.

• What is the frequency it will absorb?

• To which radiation does the frequency correspond?

• If the e- drops down from the e to the b level, what type of radiation will it emit.

1.61 x 10 15 Hz

UV

Orange

Page 45: What are atoms? Describe them

Read Rev Book pg 333 – 334do pg 334 “try it 1-3” & 338 #17 -19,

22-23, 26-27, & 30, 32 – 34

Write out equations, calculations with units on separate sheet for credit.

Hwk check.

Page 47: What are atoms? Describe them

Equivalence of Mass & Energy

Page 48: What are atoms? Describe them

Einstein: “EM E, acts like tiny bit of matter, at the smallest scale matter/energy same thing”.

E stored in the nucleus of mass obeys Einstein’s equation:

E = in J.

E = mc2. m = mass kgc = 3 x 108 m/s

E can be released when nucleus is transformed.

Page 49: What are atoms? Describe them

Ex 1: How much energy is produced when 2.5 kg of matter are completely converted to energy?

How much energy is that in eV?

Page 50: What are atoms? Describe them

E = mc2.

=(2.5 kg )(3x108 m/s)2. = 2.25 x 1017 J

in eV

(2.25 x 1017 J)(1 eV / 1.6 x 10 –19 J) = 1.4 x 1036 eV.

Page 51: What are atoms? Describe them

Compare that with eV generated by glowing gasses.

Page 52: What are atoms? Describe them

What is the graph of E vs. m?

E

m

What is the slope?

Page 53: What are atoms? Describe them

Atomic Mass Units: amu or u

• Mass of atoms very small so they are measured in amu or u rather than kg.

• Analogous to J and eV.

• Since mass is equivalent to energy,

• 1 u = 931 MeV or 931 x 106 eV

• 931 MeV

Page 54: What are atoms? Describe them

Ex 2: Calcium has an average mass of 40 u. How much energy in Joules is stored in the nucleus of each atom of Calcium?

• 40 u x 931 MeV = 37, 240 MeV.

u

(37, 240 x 106 eV) x 1.6 x 10-19 J = 6 x 10-9 J. eV.

Page 55: What are atoms? Describe them

Ex 3: Calculate the mass in kg of one universal mass unit.

Page 56: What are atoms? Describe them

Convert MeV to Joules.

• (1 u) x (931 x 106 eV/u) x (1.6 x 10 –19 J / eV) =

• 1.49 x 10-10 J

• E = mc2 so m = E/c2.

• (1.49 x 1010 J) / (3x108 m/s)2 =

• 1.66 x 10 –27 kg

Page 57: What are atoms? Describe them

1 mass unit is close to the mass of a proton or 1H.

(A single hydrogen nucleus)

Page 58: What are atoms? Describe them

The Atomic Bomb• Some of the mass of U is converted to E.

• The total mass after the reaction is less than the initial mass.

• Should be “Law of conservation of mass/energy”

Page 59: What are atoms? Describe them

Summary:

• EM energy can be thought of as tiny particles (photons) related to f.

• Matter can be thought of as E stored in nucleus.

• E in matter E = mc2. Joules.

• 1 u = 931 MeV.

• E in EM radiation/photons E = hf.

• E measured in eV and J.

• 1 eV = 1.6 x 10-19 J.

Page 60: What are atoms? Describe them

4. An e- and an anti-electron have the same mass. When they meet they completely annihilate each other. Look up the mass and calculate the energy released from the destruction in joules.

• E = mc2.

• = 2(9.11 x 10-31 kg)(9 x 1016 m2/s2)

• =1.64 x 10 -13 J.

Page 61: What are atoms? Describe them

5: A Helium nucleus consists of 2 protons & 2 neutrons. How much energy in MeV would be released if it were completely converted?

• Each nucleon is 1 unit u.

• 1 u = 931 MeV.

• 4 x 9.31 x 102 MeV/ nucleon.

• 3724 MeV.

Page 62: What are atoms? Describe them

Hwk: Pkt Mod Phys Prac 3

Page 63: What are atoms? Describe them

http://www.youtube.com/watch?v=lPSxIuQLQVI&feature=related• http://www.youtube.com/watch?v=KWGLS

7Ck1qs&feature=related

Einstein’s Big Idea 51 min historical view.

http://www.youtube.com/watch?v=jqiRoKy0Gyo&feature=related

Page 64: What are atoms? Describe them

Standard Model

http://www.particleadventure.org/standard-model.html

Page 65: What are atoms? Describe them

Standard Model:Matter is composed of small subatomic particles called quarks & leptons.

Forces also have particles that transfer information through tiny particles.

See review book xerox.

Page 66: What are atoms? Describe them

Quarks compose neutrons & protons.

Page 67: What are atoms? Describe them

Diagrams Examples

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Page 71: What are atoms? Describe them

Bohr’s model could not explain why e- could occupy only certain orbits.

DeBroglie’s hypothesis for the wave nature of matter helped explain how only certain orbits were allowed.

Each e- has = h/mv.

DeBroglie proposed that each e- is a standing wave.

Page 72: What are atoms? Describe them

Proposed e- standing waves. Only ’s that fit certain orbits are possible.

Page 73: What are atoms? Describe them

’s that don’t fit circumference cannot exist.

Page 74: What are atoms? Describe them

Heisenberg’s uncertainty principle 1927.

It is impossible to be make simultaneous measurements of a particle’s position and momentum with infinite accuracy.

When you try to look to see where an e- actually is, you must give it energy. If you give it energy, it moves.

Page 75: What are atoms? Describe them

Alpha Rays

• A rays are helium nuclei, (2p+ and 2no), that are emitted from nucleus.

• They can be easily stopped by skin or thin sheet of paper.

• More likely to knock e- from orbits because they lose all their KE at once.

• Charge = +2e

• Mass 4 units

• Energy is KE = ½ mv2.

Page 76: What are atoms? Describe them

Beta Rays

• More penetrating than alpha.

• Less capable of ionizing because their energy is lost over greater distance.

• They are fast moving e-.

• Charge = -e.

• mass = e.

• KE = ½ mv2. v can be sig portion of c.

• Need a few mm of Al to stop them.

Page 77: What are atoms? Describe them

GammaPenetrating power greatest. Can pass thru human body, concrete, and lead.

Lowest ionizing power.

They are EM waves.

No charge. No mass.

Energy described by E = hf.

Travel with vel of light in vacuum.

No maximum stopping range.

Page 78: What are atoms? Describe them

How could we distinguish the different types of radiation? What could we observe?

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Hwk rd 450 –462 Core only

Do quest pg 451 1-5p 457 1-4p 458 1-3p 462