CHAPTER 4 – ATOMIC STRUCTURE:
4.1 DEFINING THE ATOM
Atom – the smallest particle of an element that retains the
identity of the element in a chemical reaction.
Early Models:
a.) Democritus – atoms are invisible and indestructible his
theory was conceptual not experimental.
b. Dalton – studied the ratios in which elements combine
in chemical reactions. His theories reflected the
thoughts of Democritus but were based on the
results of his experiments
Dalton’s Atomic Theory
a. All elements are composed of
tiny particles called atoms
b. Atoms of the same atom are
identical and are different from
all other elements.
c. Atoms of different elements can
physically or chemically combine
in simple whole number ratios to
form compounds
d. Chemical reactions may separate,
join, or rearrange atoms but
never change the properties of
the individual atoms
Sizing Up Atoms:
Copper is an example of an element – if you were to grind a
pure copper penny down to it’s individual atoms you would
have approximately 2.4 x 1022 atom. The earth has only
about 6.9 x 109 people.
There are about 100,000,000 million copper atoms in 1cm
The radii of atoms varies but most are around 5 x 10-11 to 2
x 10-10m
Individual atoms can be observed using a scanning tunneling
microscope.
The ability to manipulate atoms in a nanoscale world opens
science to many opportunities.
STRUCTURE OF THE NUCLEAR ATOM
Subatomic particles:
•Atoms are divisible!
• The three subatomic particles are protons, neutrons, and electrons
Atomic
Part
Location
Charge
Mass
Symbol
Change
Proton
Nucleus
Positive
1 a.m.u.
p+
New
atom
Neutron
Nucleus
None
1 a.m.u.
n0
Isotope
Electron
Orbit
Negative
1/1840
a.m.u.
e-
Charge
J.J. Thomson (1856-1940) – discovered the electron by passing
electric current through gases at low pressure. The cathode
ray, a glowing beam, travels from the negatively charged cathode
to the positively charged anode. He then deflected the beam
using a magnet.
How did he know the charge of the beam?
What did Thompson
discover about the ratio
of the charge of an
electron to its mass?
He found the ratio to be
constant and that the ratio did
not depend on the type of gas or
metal in the electrodes. He
summarized then that electrons
must be part of all atoms
Robert A Millikan – (1868-1953) used the charge
to mass ratio of an electron to calculate the mass
of an electron. His value for charge and mass are
similar to today’s accepted value.
Consider these four presuppositions;
1. Atoms are neutral, no net electric
charge
2. Electric charges are carried by
particles of matter
3. Charges exist in whole-number
multiples, there are no fractions of a
charge
4. A given number of negatively charged
particles must combine with an equal
number of positively charged particle
to keep the particle neutral
Eugen Goldstein – (1850-1930)
observed rays in a cathode ray
tube traveling in a direction
opposite to that of the cathode
ray. He called them canal rays
James Chadwick – (1891 – 1974) – confirmed the
existence of the neutron
Ernest Rutherford – (1871-1937) a former student of Thompson
gave shape to today’s image of an atom.
In 1911 his gold foil experiment yielded evidence of an atomic
nucleus with a large area of space around surrounding that nucleus.
How did he come to this conclusion?
4.3 DISTINGUISHING AMONG ATOMS
Atomic Number – is the number of protons in the nucleus of an atom.
Mass Number – equal the sum of the protons and neutrons in an atom
Give the number of protons, neutrons, and electrons for the
following atoms
ISOTOPE: Atoms that have the same number of protons but
different number of neutrons
Isotopes of Hydrogen
ATOMIC MASS: Although the actual masses of individual atoms are
useful, it is more useful to compare the relative masses of atoms
using a reference isotope as a standard. The isotope that was chosen
is Carbon – 12 and was assigned a mass of exactly 12 atomic mass
units.
Atomic Mass Unit: (amu) Is defined as one
twelfth of the mass of a carbon – 12 atom.
Magnesium has three isotopes. 78.99% magnesium 24 with a
mass of 23.9850 amu, 10.00% magnesium 25 with a mass of
24.9858 amu, and the rest magnesium 26 with a mass of
The atomic mass of
25.9826 amu. What is the weighted average atomic mass of
an element is a
magnesium?
weighted average
mass of the atoms
in a naturally
Weighted average mass = .7899 x 23.9850amu
+ .1000 x
occurring sample of
24.9858amu + .1101 x 25.9826amu
the element
and
= 18.95amu + 2.499amu
+ 2.861amu
reflects both the
= 24.31amu
mass and relative
abundance of t
isotopes of that
atom as they occur
in nature.
PERIODIC TABLE: Is an arrangement of elements
organized in groups or rows with respect to similar traits
shared by different elements.
Groups – Or families, are vertical columns of elements
that have similar chemical and physical properties
Periods – There are 7 horizontal rows known as periods in
which the properties of the atoms vary as you move
across the table. Periods are associated with the energy
levels of the element.