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The Atom
The model of the atom has evolved from Dalton's concept of it as a
"solid billiard ball" to a highly complicated model. At first it was felt
that it could not be broken down. It was soon discovered the atom
could be broken down into 3 sub-atomic particles known as electrons,
protons and neutrons.
ELECTRON:

a particle which has a relative charge of -1

much smaller in mass than neutrons and protons
PROTON

a particle which has a relative charge of +1
NEUTRON:

uncharged particle

very similar in size and mass to protons
Protons and neutrons (nucleons) are located in the middle of the atom
in an area known as the nucleus (Rutherford Gold Foil Experiment).
ATOMIC NUMBER

refers to the number of protons in the nucleus

it determines the identity of the element (every atom of each
element has its own unique number of protons)
ATOMIC MASS

the mass of the atom expressed in atomic mass units

It was impossible for scientists to determine the mass of
individual atoms (they are too small!), therefore they attempted
to assign relative atomic masses that agreed with the known
compositions of compounds.

a new unit was developed to mass atoms . C-12 was chosen as the
reference standard. An atom of C-12 was arbitrarily assigned a
mass of 12 atomic mass units. The masses of all other atoms are
compared with the mass of this type of carbon atom. According
to this definition, an atomic mass unit is defined as 1/12 the mass
of a carbon-12 atom.

the most important sub-atomic particles you have studied thus
far have the following atomic mass units:
electron: 0.000 549 u
proton:
1.0073 u
neutron: 1.0087 u
MASS NUMBER

the total number of protons and neutrons in an atom (symbolized
by the letter A)
ISOTOPE

All atoms of the same element must have the same number of
protons in the nucleus, but they do not have to have the same
number of neutrons

atoms whose nuclei contain the same number of protons but a
different number of neutrons
NEUTRAL ATOM

an atom in which the number of protons equals the number of
electrons
All the elements can be represented as symbols that are organized in
the periodic table.
Carbon is represented by
6 protons
6 neutrons
12
refers to the number
of protons
6
C
In most atoms the number of protons does NOT equal the number of
neutrons. There is usually more neutrons than protons.
MASS NUMBER
- number of protons
and neutrons
A
ATOMIC NUMBER
- number or protons
Z
X
CHEMICAL
element
SYMBOL
of
the
Atomic Mass Calculations
Isotopes are atoms of the same element that have different numbers
of neutrons.
For example, carbon has two naturally occurring isotopes: Carbon-12
and Carbon-13. In a sample of carbon 98.89% of the carbon is
1.11% of the carbon is
13
12
C and
C. To calculate the Average Atomic Mass of
carbon both isotopes must be taken into consideration.
Average = (Atomic mass of A)(% of A) + (Atomic mass of B) (% of B)
Atomic Mass = (12 u)(0.9889) + (13 u)(0.0111)
= 11.87 u + 0.144 u
= 12.01 u