Name___________________________
Date____________
Period______
Unit 3 Atomic Structure
ATOM
NUCLEUS
ELECTRONS
PROTONS
NEUTRONS
POSITIVE
CHARGE
NEUTRAL
CHARGE

NEGATIVE CHARGE
Atomic Number
o Atomic number (Z) of an element is the number of protons in the nucleus of each
atom of that element
Element
# of protons
Carbon
6
Phosphorus
Atomic # (Z)
15
79
79

Mass Number
o Mass number is the number of protons and neutrons in the nucleus of an isotope:
Mass # = p+ + n0
p+
Nuclide
N0
Oxygen-
e-
Mass #
10
33
-31

42
15
Complete Symbols
o Contain the symbol of the element, the mass number and the atomic number
X

Practice Problems!
o #1 Find each of these
a. Number of protons:
80
35Br
b. Number of neutrons:
c. Number of electrons:
d. Atomic number:
e. Mass Number:
o #2 If an element has an atomic number of 34 and a mass number of 78 what is the:
a. Number of protons
b. Number of neutrons
c. Number of electrons
d. Complete symbol
o #3 If an element has 91 protons and 140 neutrons what is the
a. Atomic Number:
b. Mass Number:
c. Number of Electrons:
d. Complete symbol:
o #4 If an element has 78 electrons and 117 neutrons what is the
a. Atomic Number:
b. Mass Number:
c. Number of Protons:
d. Complete Symbol:

Isotopes
o Dalton was wrong about all ____________ of the same type being
_______________
o __________________ of the same element ________ have different numbers of
_______________
o Thus, different ______________________
o These are called _____________________
o _________________________________ (1877-1956) proposed the idea of
_____________ in 1912
o ___________________ are atoms of the same _______________ having different
______________, due to varying number of ____________________
o Soddy won the Nobel Prize in __________________ in 1921 for his work with
___________________ and __________________________ materials
o ______________ occur in nature as ______________ of _____________
o Isotopes are ______________ of the same _________________ that differ in the
_____________ of neutrons.

Naming Isotopes-Hyphen Notation
o We can also put the __________ number after the name of the element
o ____________________________
o ____________________________
o ____________________________
__________________ are atoms of the same _________________ having ________________________ due to
varying numbers of neutrons
Isotope
Protons
Electrons
Neutrons
Nucleus
Hydrogen-1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)

Atomic Mass
o How heavy is an atom of oxygen?

o
It depends, because there are different kinds of oxygen atom
We are concerned with the ___________________________________
o This is based on the __________________________ (percentage) of each variety of that
________________ in nature.

We don’t use ________________ for this _____________ because the numbers
would be too small.

Measuring Atomic Mass
o Instead of ____________, the unit we use is the ________________________________ (amu)
o It is defined as _______________ the mass of a _________________________ atom.

_________________ chosen because of its _______________ purity
o Each isotope has its own _________________________, thus we determine the
________________ from _______________ abundance.

Relative Atomic Mass
o
12C
atom=1.992 x 10-23
o ________________________ (amu)
o 1 amu =1/12 the mass of a _____________ atom
o 1 p=____________________________ amu
o _____=1.008665 amu
o 1e-=_____________________________ amu

To calculate the average:
o ____________________ the ________________________ of each isotope by it’s abundance
(expressed as __________________), then add the _______________
o If not told otherwise, the ______________ of the isotope is __________________ in atomic
mass units (amu)

Average Atomic Mass
o ___________________ average of all ___________________
o On the __________________ ________________
o Round to 2 decimal places
o Average Atomic Mass =
o Practice Example: Find Chlorine’s average atomic mass if approximately 8 of every 10 atoms
are chlorine -35 and 2 are chorine-37
o Practice Example: Calculate the average atomic mass of oxygen if its abundance in nature is
99.76% 16O, 0.04% 17O, and 0.20% 18O.