Download 19.2 – Masses of Atoms - Trimble County Schools

19.2 – Masses of Atoms
Objectives – LT#5-7
Compute the mass number of an atom.
 Relate the atomic number to the
composition of an element.
 Identify the components of isotopes.

Atomic Mass

Nucleus contains majority of mass

Protons & neutrons each have a mass
equal to 1 atomic mass unit (amu)


In reality their mass is 1.67 x 10-24 g
Electrons so small their mass is not
calculated

In reality their mass is 9.11 x 10-28 g
Electron
Proton/Neutron
Atomic Properties

Atomic Number = number of protons in
an atom


Elements on Periodic Table are arranged by
atomic number
Mass # = sum of protons + neutrons

# Neutrons = mass # - # protons
(Atomic #)
Isotopes

Atoms of same element that have
different number of neutrons


Because # neutrons is different, mass # is
different
Most elements have more than 1
isotope, each with a different mass
Average Atomic Mass
Weighted-average mass of all isotopes
 The # you find on the periodic table

Chlorine’s Isotopes
Chlorine-35
Chlorine-37
75.53%
24.47%
Therefore, the average
mass (atomic mass) of
chlorine is 35.45.
In-Class Assignment/Homework
 19.1&19.2
Directed reading
 19.2 Enrichment WKT
#3  # of moles x mass from periodic table
 #4  # of grams divided by mass from
periodic table
