http://www.ck12.org Chapter 4. Atomic Structure
The incredibly small mass of the electron was found to be approximately 1/1840 the mass of a hydrogen atom, so
scientists realized that atoms must also contain other, far more massive particles. Additionally, at least one of these
particles must carry a positive charge because complete atoms are electrically neutral.
Participate in a simulation of Millikan’s oil drop experiment at this site: http://www.dlt.ncssm.edu/core/Chapter3-A
tomic_Str_Part1/Chapter3-Animations/OilDrop.htm.
Protons and Neutrons
If cathode rays are electrons that are given off by the metal atoms of the cathode, then what remains of the atoms
that have lost those electrons? We know several basic things about electrical charges. They are carried by particles
of matter. Millikan’s experiment showed that they exist as whole-number multiples of a single basic unit. Atoms
have no overall electrical charge, meaning that each and every atom contains an exactly equal number of positively
and negatively charged particles. A hydrogen atom, the simplest kind of atom, contains only one electron. When
that electron is removed, a positively charged particle should remain.
In 1886, Eugene Goldstein (1850-1930) discovered evidence for the existence of this positively charged particle.
Using a cathode ray tube with holes in the cathode, he noticed that there were rays traveling in the opposite direction
from the cathode rays. He called these canal rays and showed that they were composed of positively charged
particles. Theprotonis a positively charged subatomic particle that is present in all atoms. The mass of the proton
is about 1840 times the mass of the electron.
In 1932, English physicist, James Chadwick (1891-1974), discovered a third subatomic particle. Theneutronis a
subatomic particle with no electrical charge and a mass that is approximately the same as the mass of a proton.
Below is a summary of the properties of the three fundamental subatomic particles (Table4.1).
TABLE4.1: Properties of Subatomic Particles
Particle Symbol Relative Electrical
ChargeRelative Mass
(amu)∗Actual Mass (g)Electron e− 1- 1/1840 9.11× 10 −^28
Proton p+ 1+ 1 1.67× 10 −^24
Neutron n^001 1.67× 10 −^24∗1 amu (atomic mass unit) = 1.66× 10 − (^24) g
Discovery of the Atomic Nucleus
The next step after the discovery of subatomic particles was to figure out how these particles were arranged in the
atom. This is a difficult task because of the incredibly small size of the atom. Therefore, scientists set out to design
a model of what they believed the atom could look like. The goal of eachatomic modelwas to accurately represent
all of the experimental evidence about atoms in the simplest way possible. Following the discovery of the electron, J.
J. Thomson developed what became known as the “plum pudding” model (Figure4.9). In this model, the electrons
were suspended in a uniform lump of positive charge like blueberries in a muffin. This model of the atom soon gave
way, however, to a new model developed by New Zealander, Ernest Rutherford (1871-1937).
In 1911, Rutherford and coworkers Hans Geiger and Ernest Marsden initiated a series of groundbreaking experi-
ments that would completely change the accepted model of the atom. The experimental setup is shown below (
Figure4.10). When they bombarded very thin sheets of gold foil with fast moving alpha particles, they got some
unexpected results. An alpha particle is a type of positively charged particle whose mass is about four times that of
a hydrogen atom. It occurs naturally as a product of radioactive decay.