Chemistry - A Molecular Science

Indeed, Rutherford observed that most of the

-particles did pass through the gold α

foil, and some were deflected slightly, yet

one in 20,000 was deflected at a sharp angle.

This one particle in 20,

was totally unexpected, but it provided the information that

Rutherford needed. “It was as if you fired a 15-

inch shell at a piece of tissue paper, and it

came back to hit you!” he would later exclai

m. Rutherford proposed a new model of the

atom that was consistent with his observations.

1 in 20,000 particles

deflected at a sharp angle

some particles are

deflected at smaller angles

gold atom

trajectory ofone

particlea

most particles

are undeflected

nucleus

Figure 1.5 Rutherford’s model of the atom used to explain the gold foil experiment The positive charge and most

of the mass resides in the

nucleus, which is represented as a small black dot in the center of the atom. Particles

that were deflected at acute

angles collided with the nucleus. If drawn to proper scale, the nucleus would be too small to be seen.

Observation

: Most of the

-particles passed through undeflected. α

Conclusion:

Most of the volume of the atom is empty space.

Observation

: Some minor deflections were observed.

Conclusion:

A very large force would be required to repel these high-energy particles.

α-

particles are positively charged, so he assumed that the deflections were due to collisions with another positively-charged and massive particle.

Observation

: Only one

-particle in 20,000 experienced acute deflections. α

Conclusion:

Only one

α-particle in 20,000 collided with

the positively-charged particle in

the atom, so the cross-sectional area of this particle must be less than 1/20,000th that of the atom; yet it must also contain almost all of the mass!

Rutherford’s model of the atom (Figure 1.5) placed the positive charge and essentially

all of the mass of the atom in a very small particle, which he called the

nucleus

. The

nucleus occupies almost none of the volume of the atom. He suggested that the electrons orbit the nucleus much as the planets orbit the sun.

Rutherford’s model was another giant step

in our understanding of matter, but there

were still dramatic changes to come over the

next ten years that would revolutionize the

way we would think about atoms and molecul

es. Nevertheless, the Rutherford model had

identified an essential feature of the atom: electrons with negative charge surround a nucleus with positive charge. In fact, the electrons in an atom, as well as their interactions with the electrons and nuclei of other atoms, are the focus of chemistry. The electrons, their location about the nucleus, and their number relative to the number of positive charges in the nucleus dictate

the chemistry of an element.

To appreciate just how small and dense the nucleus is, consider the following two

analogies: •

If the nucleus of an atom were the size of a

baseball, the atom would be a sphere with a one-

mile diameter.

(^) •
A matchbox filled with nuclei (not atoms) would weigh 2.5 billion
tons
.
Chapter 1 The Early Experiments
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