Unit 1: Back to the Basics

Section 2: Important Discoveries is Chemistry

* Introduction
* Five Important Foundational Theories in Chemistry
* Discovery of the Electron
* Radioactivity
* Discovery of the Nucleus
* Modern View of the Atom
* Isotopes

Introduction

Learning Chemistry is like climbing a ladder because it is necessary to
learn certain things before they can be applied to learn other things.
Another example of this might be that an architect can not build a building
without the foundation, and this is exact what this unit is for. This unit
is a foundation. In this unit, the objective is to understand how and why
certain principles came into existence. There are five important theories to
be discussed.

Five Important Foundational Theories In Chemistry

The first theory of importance to Chemistry is the Law of Conservation of
Mass. This theory was created by Antoine Lavoisier and its definition is
that mass can not be created nor destroyed. For instance, if 25 grams react
in a chemical reaction, there should be 25 grams of elements left after the
reaction has taken place. For example if 25 grams of water are boiled in a
beaker and all of it evaporates, nothing is left in the beaker, so where did
all the water go? The answer to this is that water evaporated into steam and
floats up into the air, even though there is nothing left in the beaker, in
reality, 25 grams were produced.

The next law of importance was created by Joseph Proust and it is called Law
of Definite Proportions. The Law of Definite Proportions states that a given
compound will always contain exactly the same proportion of elements by
weight. For example, water weighs 18 grams and 2 of the grams are due to the
weight of hydrogen and the other 16 grams are due to the weight of oxygen.
So every time there is a water molecule, there will be 16 grams of oxygen
and 2 grams of hydrogen.

The third theory was created by John Dalton. Dalton postulated (stated) that
when two elements form a series of compounds, the ratios of the masses of
the first element can always be reduced to small whole numbers. In common
terms, in compounds there can not be half of an atom. This theory is called
the Law of Multiple Proportions. Here is an example that will hopefully
clarify things.

Dalton wasn't done theorizing with the Law of Multiple Proportions. He
continued to explain why substances act the way that they do with his atomic
theory. Dalton's Atomic Theory is printed below.

 

Dalton's Atomic Theory

1. Each element is made up of tiny particles called atoms.
2. The atoms of a given element are identical; the atoms of different
elements are different in some fundamental way or ways.
3. Chemical compounds are formed when atoms combine with each other. A
given compound always has the same relative numbers and types of atoms.
4. Chemical reactions involve the reorganization of the atoms.
Reorganization of atoms means that the way in which atoms are held
together are changed.) The atoms themselves are not changed in a
chemical reaction.

The final fundamental theory that is important to understand now is
Avogadro's Hypothesis. Avogadro said that if two gases occupy the same
volumes at the same temperature and pressure, then the number of particles
in each gas will be the same. So basically, if I have 10 liters of oxygen
gas and 10 liters of helium gas in the same room with me, each gas will have
the exact same number of particles in it.

Discovery of the Electron

J. J. Thomson, while experimenting with a cathode and electricity, he
discovered that atoms contain negatively charged particles called electrons.
A cathode is a partially evacuated tube with electrodes located on it at
opposite ends. Electricity was then applied to the electrodes and a stream
of electrons, which he called a cathode ray, ran straight across the tube
from electrode to electrode. He then applied an electric field around the
center of the cathode and noticed that the electrons bent towards the
positive side of the electric field which meant that the ray must be
negatively charged. He then adjusted the amount of pull that the electric
field had on the cathode ray and he was able to determine the mass to charge
ratio of the electron. The mass to charge ratio of an electron is -1.76 x 108 C/g.

There was also another man doing similar experiments named Robert Millikan.
Robert Millikan, using charged oil drops and the mass to charge ratio that
Thomson determined, determined the mass of an electron. The mass of an electron is 9.11x 10-28 grams.

At the time of Thomson's discovery of an electron, scientists had already
determined that an atom was electrically neutral. Because an atom was
electrically neutral, and he had just discovered a negatively charged
particle, Thomson guessed that there must also be a positively charged
particle in the atom. He went even further and made a model of what an atom
looked like. It was called the plum pudding model because he said that there
was a dense cloud which had a positive charge and randomly scattered inside
this cloud were these negatively charged electrons. (His model looked much
like a chocolate chip cookie would look because the dough part would be the
positively charged part and the chocolate chips would be the negatively
charged part.)

Radioactivity

Late in the nineteenth century, Henri Becquerel, discovered that certain
elements spontaneously emit high energy radiation which was called
radioactivity. Three types of particles were given off when radiation
occurred. The first particle was called the alpha particle and it was a +2
charged particle. The second particle was called a beta particle and it was
a high speed electron. The third particle was called a gamma particle and it
was high energy light.

Discovery of the Nucleus

Rutherford discovered the nucleus of an atom and proved Thomson's plum
pudding model wrong. Rutherford shot alpha particles at a thin sheet of gold
foil and observed some very interesting results. The first observation that
he made was that most particles went straight through the gold foil. This
led him to believe that atoms are very spacious and have lots of open space
in them. The second observation that he made was that some of the particles
got bent on their way through. This led him to the conclusion that some of
the positively charged alpha particles came close to the positively charged
particle in the atom. The third observation that he made was that some of
the particles got reflected straight back and this led him to the conclusion
that the alpha particles came in contact with the positively charged
particle in the atom.

Through this whole experiment, Thomson's plum pudding model was proved
incorrect, and Rutherford proposed a new model of the atom which is much
like today's view of an atom. Rutherford concluded that there was a dense
positively charged center of the atom, called the nucleus, and there were
electrons floating around the center at large radii (relative to the size of
the nucleus).

Modern View of the Atom

The atom has gone through many, many experiments throughout the years.
Today, scientists know that the nucleus of the atom is composed of two
different types of particles. The first is called the proton. The proton is
a positively charged particle and has a charge equal to that of the
electron, except it is positively charged, not negatively charged. (Simply
if the proton has a positive charge of 1, then the electron has a negative
charge of 1. This is why the atom has a neutral charge.) The second particle
of the nucleus is called the neutron. The neutron has no charge, but has a
mass equal to that of a proton. The electron is a particle that "orbits" the
nucleus (much like the moon orbits the earth), but has relatively no
mass,when compared to protons and neutrons.

Isotopes

Isotopes are atoms with the same number of protons but a different number of
neutrons. For example, Carbon 12 (Carbon is the name of the elements and 12
refers to the mass of the atom) has 6 protons, 6 neutrons, and 6 electrons,
whereas Carbon 13 is an isotope of Carbon 12 and has 6 protons, 7 neutrons,
and 6 protons.