Download Atomic Mass

How atoms differ
Mass Number
• The sum of the protons and neutrons
• BUT…Why is it not an even number??
Isotopes:
Dalton was wrong in saying that every atom
of an element is the same.
Neutral atoms of an element always have
the same number of protons and
electrons.
But, the number of neutrons of an element
differs in different atoms
Isotopes:
Atoms with the same number of protons in the
nucleus but different number of neutrons
Isotopes
• Isotopes have a different mass
• Isotopes containing more neutrons
have a larger mass
• But, isotopes react the same – WHY?
Examples:
• EX: Potassium has 3 isotopes – one
with 20 neutrons, one with 21 neutrons,
and another with 22 neutrons
• Potassium has 19 protons
• Potassium 39 – 19 protons 20 neutrons
• Potassium 40 – 19 protons 21 neutrons
• Potassium 41 – 19 protons 22 neutrons
39
19
K
40
19
K
41
19
K
Isotopes
• In nature, elements are found as a
mixture of isotopes
• There is a set percent present of each
isotope
• Example – Potassium in a banana
93.25% is 20 neutrons; 6.7302% is 22
neutrons; and 0.017% is 21 neutrons
Mass of an element:
• For elements - Instead of listing
each isotope of every atom –
scientists combine the isotopes
to make the atomic mass of the
element
Atomic Mass:
• Atomic mass of an element is the
weighted average of the mass of the
isotopes of that element
• To find the Atomic Mass
• (Isotope 1 mass X % abundance) +
(Isotope 2 mass X % abundance), etc…
Atomic Mass:
• EXAMPLE
• Chlorine has two isotopes
–Chlorine – 35 with 75.770% abundance
–Chlorine – 37 with 24.230% abundance
Isotope Number of
protons
Number of
neutrons
Mass
Relative
abundance
N14
7
7
14
99.64%
N15
7
8
15
0.36%
• Carbon has two isotopes, C-12 and C-13.
C-12 has a percent abundance of 98.89%.
C-13 has a percent abundance of 1.11%.
Calculate the average atomic mass of
Carbon.