Dr. Saidane
Chem 200
Lecture Notes
Chapter 2, Part 1
Atoms

Historical Development of the Model of the Nuclear Atom. A series of discoveries
were made that led to the modern model of the atom.
1. In the late 1700, scientists studied chemical reactions and their discoveries led to
three basic laws:
a) The Law of Conservation of Mass, which states that mass is neither
destroyed nor created during ordinary chemical reactions.
b) The Law of Definite Proportions, which states that a chemical compound
contains the same elements in exactly the same proportions by mass
regardless of the size of the sample or source of the compound.
c) The Law of Multiple Proportions, which states that, if two or more
compounds are composed of the same two elements, then the ratio of the
masses of the second element combined with a certain mass of the first
element, is always a ratio of small whole numbers.
2. In 1808 John Dalton proposed an explanation of the laws stated above. Dalton’s
atomic theory can be summed up by the following statements:
a) All matter is composed of extremely small particles called atoms.
b) Atoms of a given element are identical in size, mass, and other properties.
c) Atoms cannot be subdivided, created, or destroyed. This statement
explains the law of conservation of mass.
d) Atoms of different elements combine in simple whole-number ratios to
form chemical compounds. This explains the law of definite proportion as
well as the law of multiple proportions.
3. The modern atomic theory was developed between late 1800 and early 1900.
During this period several scientists discovered that contrary to the third
statement in Dalton’s theory, atoms could be subdivided in subatomic particles
named electrons, protons and neutrons.
a) Discovery of the electron. Several scientists investigated the relationship
between electricity and matter. They performed several experiments using
cathode rays tubes. They hypothesized that particles were formed inside
the cathode ray tubes, which moved from the cathode to the anode. In
1897, J. J. Thompson was able to measure the ratio of the charge of
cathode-ray particles to their mass. He concluded that all cathode rays are
composed of identical negatively charged particles, which were later
named electrons. In 1909, Robert Millikan measured the charge and the
mass of the electron.
b) Discovery of the atomic nucleus. In 1911, Ernest Rutherford bombarded
a gold foil with fast moving alpha particles. He observed that most of the
particles passed through the foil, and a small number of these particles
were deflected. Rutherford concluded that each atom in the gold contained
a small, dense, positively charged nucleus surrounded by electrons.
c) Discovery of neutrons and protons.
Protons were discovered by
Rutherford in 1919. James Chadwick discovered neutrons in 1932.

The Nuclear Atom. The current model of the atom is called the nuclear atom and
can be summarized as follows:
1.
Atoms are made up of subatomic particles called electrons, protons, and
neutrons.
2.
The protons and neutrons are jointly known as nucleons. They form a compact,
central body called the nucleus of the atom.
3.

The electrons are distributed in space like a cloud around the nucleus.
Properties of subatomic particles. The properties of the three subatomic particles
are summarized as follows:
Symbol
Charge
Mass (g)
Relative mass (a.m.u*)
Electron
e-
-1
9.109 x 10-28
0.0005486
Proton
p+
+1
1.673 x 10-24
1.007276
Neutron
n0
0
1.675 x 10-24
1.008665
Particle
* 1 amu (atomic mass unit) = 1.660540 x 10–24 g
The mass of an electron is negligible compared to the mass of a neutron and a proton.
Therefore the mass of an atom is considered to be the mass of its nucleus.

The atomic number (Z) of an element is the number of protons in the nucleus of
each atom of that element. The atomic number identifies an element.

The mass number (A) is the total number of protons and neutrons in the nucleus of
an atom.

Isotopes are atoms of the same elements have same number of protons and electrons,
but different number of neutrons. Isotopes of the same element have same atomic
number but different mass number. All the isotopes of a given element have the same
number of electrons around the nucleus. Isotopes of the same element are called
nuclides. Ex: Hydrogen has three nuclides:
Hydrogen-1 (protium), Hydrogen-2
(deuterium), and Hydrogen-3 (tritium).

The mass number and the atomic number of an element are used to determine the
number of subatomic particles in each atom. The atomic number Z determines the
number of protons, which is also equal to the number of electrons in a neutral atom.
The difference between A and Z determines the number of neutrons in an atom.

Names of the Elements. Each element has a name and is represented by a unique
chemical symbol made up of one or two letters. The first letter of a symbol is always
uppercase and the second letter, always lowercase. Some elements have symbols
formed from the first letter of the name and a later letter. Ex: Helium He, Aluminum
Al, Magnesium Mg, Chlorine Cl. Others have their symbol derived from their Latin
or Greek name. Ex: Gold (Au) from Aurum, and Silver (Ag) from Argentum.

Periodic table. Elements are arranged in the periodic table according to their atomic
number. The vertical columns of the periodic table are called groups. The horizontal
rows of elements in the periodic table are called periods. Elements in the same group
have similar chemical properties. Physical and chemical properties change somewhat
regularly across a period. The periodic table is an arrangement of the elements that
reflects their family relationships; the members of a group typically show a smooth
trend in properties.

The elements in the periodic table can be classified as metals, nonmetals, and
metalloids or semimetals.
1. A metal is an element that is good conductor of heat and electricity. Metals are
shiny. At room temperature, most metals are solids. They are ductile, which
means that they can be drawn into a wire, and malleable, which means that they
can be hammered or rolled into thin sheets.
2. A nonmetal is an element that is a poor conductor of heat and electricity. Many
nonmetals are gases at room temperature. Solid nonmetals are brittle rather than
malleable and ductile.
3. A metalloid is an element that has some characteristics of metals and some
characteristics of nonmetals. A metalloid has the appearance and some properties
of a metal but behave chemically like a nonmetal. All metalloids are solid at room
temperature. They are less malleable than metals but not as brittle as nonmetals.
Their ability to conduct electricity is intermediate between that of metals and that
of nonmetals.
 Skills you should have mastered
Conceptual
1. Explain the essential points of Dalton’s atomic theory and their relationship to the
law of conservation of mass, the law of definite proportions, and the law of
multiple proportions.
2. Describe the structure of the nuclear atom.
3. Predict the characteristics of elements in different regions of the periodic table.
4. Distinguish by their properties metals, metalloids, and nonmetals and locate them
in the periodic table.
Problem-solving
1. State the numbers of neutrons, protons, and electrons in an isotope, given its
atomic number and mass number.
2. Write the symbols for the elements, given their names, and vice versa.
Descriptive
1. Describe the historical development of the model of the nuclear atom.