what are atoms? describe them

What are atoms?Describe them.

Models of the Atom

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.

Plum Pudding Model

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.

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

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.

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.

Gold foil experiment :atom is mostly empty space

with dense positively charged nucleus.

Electrons move in circular orbits about the nucleus.

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

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?

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-

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

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

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

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

Evidence for Bohr Mode from Spectra

• Absorption- e- absorbs specific photon E.

• Emission - e- emits specific photon E.

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

Emission SpectrumWhen viewed through a prism or

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

Bright Line Spectra

AbsorptionContinuous spectrum

From sunlight

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.

Frequencies emitted exactly match the frequencies absorbed.

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.

Review

• How does Rutherford view the atom?

• How does Bohr view the atom?

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.

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.

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

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

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

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.

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.

Bohr Energy Diagrams

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.

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

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

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?

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

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

Use diagrams to find E.

Ephoton = Ei - Ef

Use Epho = hf of the radiation

to find frequency associated with photon of known energy.

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?

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.

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

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.

Film: Quantum Mechanics & Atomic Structure 6:20

http://www.youtube.com/watch?v=-YYBCNQnYNM&annotation_id=annotation_990931&feature=iv

Equivalence of Mass & Energy

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.

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?

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.

Compare that with eV generated by glowing gasses.

What is the graph of E vs. m?

E

m

What is the slope?

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

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.

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

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

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

(A single hydrogen nucleus)

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”

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.

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.

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.

Hwk: Pkt Mod Phys Prac 3

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

Standard Model

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

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.

Quarks compose neutrons & protons.

Diagrams Examples

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.

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

’s that don’t fit circumference cannot exist.

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.

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.

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.

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.

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

Hwk rd 450 –462 Core only

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