Structure of the Atom

What you already know about the atom

Nucleus contains

protons (+) and neutrons (neutral)

Electrons (-) orbit the nucleus in “shells”

1st shell:

2 e-

2nd shell:

8e-

Dalton’s Atomic Theory (1808)

Which of Dalton’s four postulates do we believe to be correct today?

1. All matter is made up of small indivisible particles called atoms. Atoms cannot be created or destroyed.

(F: The atom was split in W W II.)

2. Atoms of one element cannot be converted to atoms of another element.

Too bad for alchemists.

(F: Particle accelerators can convert the nucleus of one atom into that of another atom. This is nuclear chemistry and does not involve chemical change.)

3. Atoms of a particular element are identical in terms of mass, size, etc. Atoms of each element have unique properties.

(F: Existence of isotopes. An isotope of an element has the same number of

protons in the nucleus, but different number of

neutrons.

Isotopes of CarbonIsotope # p+ #no mass #12

6C

6 6 1213

6C

6 7 1314

6C

6 8 14

NOTE: The different macroscopic forms of carbon—and some other elements—are called

allotropes.The allotropes of C are:diamond

graphitebuckyballs

nanotubes.

4. Atoms combine in specific proportions—small whole number ratios—to form compounds.

(T: Consider NaCl, H2O, H2O2, CH4, C6H12O6,)

History of Modern Atomic Theory1. Discovery of the electron

2. JJ Thomson determined mass to charge ratio for the electron

3. Millikan’s Oil Drop Exp’t (mass of e-)

4. Discovery of the proton

5. Thomson’s Plum Pudding Model of the atom

6. Rutherford’s Gold Foil Experiment (discovery of nucleus)

7. Mass Spectrometer—Atomic Mass

8. Discovery of the Neutron

9. Electron Configuration (next slide show)

Discovery of the Electron

Sir Wm Crookes (1832 – 1919)

A “man of science”.Discovered thallium in 1861.Worked on pure and applied science, economic

and practical problems, and psychic research.

Cathode Ray Tube

Crookes did a lot of work with a CRT

Schematic Diagram of CRT

The inside of the CRT had a phosphor coating, that gave off light when struck by the beam.

Crookes placed an electric field just outside the CRT. He noticed that the beam was bent towards the external (+) charge.

Crookes placed a Maltese cross in the path of the beam.

He observed that the shadow of the beam fell on the anode.

What Crookes Observed

What did these two observations suggest?

1. That the CRT “beam” had a (-) charge;

2. That the beam emanated from the cathode—the (-) electrode—and traveled to the anode (+).

There’s more . . .

Crookes placed a paddlewheel in the CRT.

The beam caused the paddle wheel to turn.

Crookes coated the tips of the paddle with a phosphor.

They glowed when struck by the CRT beam.

http://www.youtube.com/watch?v=lM8TYe-PrnY

This indicated

that the beam had mass.

ie. The beam was made up of (-) charged particles.

• Same behaviour observed when the CRT filled with different gases (all at low pressure) and when different metals used for the anode and cathode.

• This suggested

that the (-) particle that made up the beam was common to all elements.

• This particle is called

an electron.

• Cathode ray’s path was also bent by external magnetic field.

British Physicist JJ Thomson also experimented with a CRT. He varied external magnetic and electric fields

to determine the charge to mass ratio of an e- to be 1.76 x 108 C/g.

(The coulomb (C) is the SI unit for electric charge.)

Another Diagram of Thomson’s Apparatus

Millikan’s Oil Drop Experiment

In 1909 Robert Millikan (1868-1953) of the University of Chicago measured the charge of an electron.

Apparatus for Oil Drop Experiment

Explanation of Oil Drop Exp’t

• Small drops of oil, when zapped with x-rays, picked up extra electrons.

• Charged oil drops were allowed to fall between two electrically charged plates.

• Millikan monitored the drops, measuring how the voltage on the plates affected their rate of fall.

• From these data, he calculated the charges on the drops.

• His experiment showed that the charges were always whole # multiples of 1.60 x 10-19 C, which he deduced was the charge of a single electron.

• Millikan then calculated the mass of the electron by using his value for the charge, 1.60 x 10 -19 C, and Thomson's charge-to-mass ratio, 1.76 x 108 C/g:

1.76 x 108 C = 1.60 x 10-19 C e- charge

1 g ? g e- mass

? = 9.10 x 10-28 g

is the mass of one e-.

This is ca. 2000 X lighter than a H atom.

Discovery of the ProtonEugene Goldstein made a CRT with a perforated

cathode.

 

Canal Rays

He noticed a beam traveling in the opposite

direction from the cathode rays (violet glow).

He called these canal rays.

While the beam of electrons could be deflected by an external magnetic field, the canal rays are barely affected.

The canal rays are composed of protons (+).

If low pressure H2(g) is in the CRT, here’s what we have:

H2(g) + E(from e- beam) 2H(g)2H + E(from e- beam) 2H+ + 2e-

protons

travel towards cathode (-)

(protons are much heavier than e-s)

Another diagram of CRT with perforated cathode

Fun with a Crooke’s tube

This demonstrates the phenomenon of discharge at different pressures of gas inside the tubes. 

Thomson’s Plum Pudding Model of the Atom

The discovery of the electron lead to a simple model of the atom.

Electrons randomly distributed in a positively charged “pudding”.

Seemed to make sense at the time.

Mass Spectrometer

In 1920s , F.W. Aston developed the mass spectrometer.

This allowed the determination of atomic mass.

And it showed a problem.

Helium, for example, was observed a mass of 4 amu, not 2, as suggested by its 2 protons.

What was missing from the atom?

The neutron. Discovered in 1932.

Why did it take so long?

Neutrons are neutral and don’t respond to electric or magnetic fields.

When James Chadwick

shot alpha particles at beryllium

(atomic number 4) the beryllium

emitted a neutral radiation that

was later determined to be a

stream of neutrons.

So now we have the three subatomic particles:

protons

neutrons

electrons.

Rutherford’s Gold Foil Exp’t

Rutherford and his co-experimenter Ernst Geiger shot a beam of alpha (α) particles through a thin sheet of gold foil—only a few thousand atoms thick.

α particle = He2+ (He nucleus)

mass of α particle =

4 amu

Rutherford’s Apparatus

Based on the Plum Pudding model of the atom, what would you expect if an α particle (4 amu) was shot at a proton (1 amu)?

Rutherford expected the α particle to

pass straight through the foil.

What Rutherford Observed

predictionbased on P.P. Model

what was actually

observed—some α

particles defected or even bounced back!

Interpretation of G.F. Exp’t

Atom has a very small, dense core—contains protons and neutrons—called the nucleus.

We now know that the nucleus fills about a billionth of the atom’s volume.

Of his experimental observations, Rutherford said: “It was as if you fired a 15 inch shell against a piece of tissue paper, and it bounced back at you.”

We’ve accounted for the subatomic particles and the nucleus. But where are the electrons?

In shells, you say?

It’s a little more involved than that . . .

Stay tuned.