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Structure of the Atom
Basic Parts of the Atom
nucleus –the positively charged center of the
protons – positive charged particle with a relative
mass of 1
neutrons – neutrally charged particle in the nucleus
with a relative mass of 1
electron cloud – negatively charged, mostly
empty space
electrons – negatively charged particles with a
relative mass of 1 / 1840
Atomic Structure
Forces in Atoms
Electromagnetic Force:
Strong Force:
Opposes the electromagnetic force between
protons in the nucleus.
The strongest of the four forces, but only acts over
short distances.
Weak Force:
Like charges repel, opposite charges attract.
The force responsible for radioactive decay.
The force of attraction between all objects in the
The weakest of the four forces.
Atomic Number
The atomic number is equal to the number
of protons in the nucleus of an atom
This number is expressed as a whole
number on the periodic table.
 Since + and - charges are equal in atoms,
it is also the number of electrons
Mass Number
The mass number is equal to the sum of the
protons and neutrons in the nucleus of an
This number is expressed as a
Number of neutrons in a atom = (mass number,
as a whole number) - (atomic number)
How many neutrons are in an atom of zinc? . . . .
65 - 30 = 35 neutrons in Zn
Atomic Number and Mass Number
Atoms which have the same number of
protons with different amounts of
Carbon-12 has 6 neutrons, while Carbon14 has 8 neutrons.
 The mass number for an element on the
periodic table represents the average for
all the isotopes in a sample of the element.
Hydrogen has 3 common isotopes
Carbon also has 3 common isotopes
1)6p, 6n
2)6p, 7n
3)6p, 8n
mass # = 12
mass # = 13
mass # = 14
nuclide name
carbon - 12
carbon - 13
carbon - 14
Atomic Mass
The average mass of the mixture of an elements
Atomic mass is measured in Atomic Mass Units
where 1amu = (1/12) mass of carbon measured in
In order to find this average you need to know:
number of stable isotopes of the atom
mass of isotopes
percent abundance in nature of isotopes
Atomic Mass
Example: A sample of cesium is 75%133Cs,
20% 132Cs, and 5% 134Cs.
What is its average atomic mass?
Answer: .75 x 133 = 99.75
.20 x 132 = 26.4
.05 x 134 = 6.7
Total = 132.85 amu