Chemistry - A Molecular Science

Chapter 1 The Early Experiments

determine the mass of the droplet. He then fired X-rays into the drum, which removed electrons from some of the molecules in the air.

Some of the released electrons attached to

the oil droplet creating a negative charge (

q) on its surface. Next, he applied an electric

field (

E) across the two plates creating a positive charge on the top plate and a negative

charge on the bottom plate. As the electric fi

eld increased, the rate of descent of the

droplet slowed as it was increasingly attracted to the positive upper plate and repelled by the lower plate.

Millikan adjusted the voltage across th

e two plates until the droplet became

suspended, moving neither up nor down. At this point, the electrostatic force (

qE

) that

pulled the droplet up equaled the gravitational force (

mg

*) that pulled it down, so

q =

mg

/E

.

E,

m

, and

g were all known, so he was able to

determine the charge on the droplet.

Various experiments yielded different values of

q for different droplets, but all of the

measured charges were multiples of the same number,

-1.6x

• 
  

C

†.

Millikan reasoned

that the charges on the droplets were differen

t because each droplet

had a different number

of electrons;

i.e.

, q

= n

qe

, where n is the number of electrons and q

is the charge on each e

electron. In this way he was able to dete

rmine that the charge on an electron must be

qe

=

-1.

x^10

• 
  

C.

He then used Thomson’s charge-to-mass ratio

q

/m

= -1.76x

11 C

.kg

-1, and

his charge determine that

the mass of the electron was m

= e

-1.6x

• 
  

C/-1.76x

11 C

.kg

-^

= 9.1x

• kg, which is approximately 1/1800 the mass of the hydrogen atom
  .^

Clearly, it was time to refine Dalton’s atomic model. One proposed model was the

‘raisin pudding’ model of Lord Kelvin and J.

J. Thomson. They reasoned that because

atoms are uncharged themselves, they must c

ontain enough positive charge to balance the

negative charge of the electrons. In the

Kelvin-Thomson model, the atom resembled

raisin

pudding

with the negatively-charged electrons (the raisins) embedded in a mass of diffuse

positive charge (the pudding). E. RUTHERFORD AND THE NUCLEAR ATOM (1911) A new model called for a new test, and the

test of the Kelvin-Thomson model was not

long in coming. In another classic experime

nt, a British physicist, Ernest Rutherford,

tested the ‘raisin pudding’ model by bombarding a very thin gold foil with alpha (

) α

particles (particles with atomic masses of f

our and charges of +2) moving at 10,000 mi/s

(Figure 1.4). If the Kelvin-Thomson model was correct and the positive charge in the atom was diffuse, then these particles should pass straight through the foil with, at most, only minor deflections.

* g is the acceleration due to gravity. g = 9.8 m

-2.s

on earth.

† C is the abbreviation for the

Coulomb

, the basic unit of electrical charge.

Gold Foil

CircularFluorescentScreen

Scatteredparticlesa

Most particlesare undeflected

Source ofparticlesa

Beam ofparticlesa

1 in 20,000 particlesdeflect at acute angles

Figure 1.4 Rutherford’s ‘gold foil’ experiment A thin gold foil was bombarded with positively charged

α-particles. The

path of the particles (red lines) through the foil was determined by noting where the particles

struck a fluorescent screen.