Atomic Number, Mass Number and Atomic Mass

Contributed by:

Jonathan James Fri, Jan 14, 2022 06:36 PM UTC

The highlights are:
1. Atomic number
2. Mass number
3. Atomic mass and Average Atomic mass
4. Isotopes

1. NOTES – 4.3:
Atomic Number,
Mass Number,
Isotopes and Atomic Mass

2. Summary of atomic structure:

3. ATOMIC NUMBER:
● Designates the number of protons
in a nucleus of an atom
● Each element has a characteristic
atomic number (the same atomic #
for ALL atoms of the same element)
● The number of electrons equals the
atomic number in a neutral atom

4. MASS NUMBER:
● Designates the total number of
protons + neutrons in an atom
● Number of neutrons =
mass number - atomic number
● Atoms of the same element can
have different mass numbers

5. Periodic Table Symbol Key:
6 Atomic number (Z)
C Element’s symbol
Carbon Element’s name
12.011 Atomic mass (A)
# of protons = Z
# of electrons = # of protons (in a neutral atom)
# of neutrons = A - Z

6. Determining # p+, n, and e- from
chemical symbols:
● Example 1: Mass #
# protons = 6 14
# electrons= 6 C
# neutrons= 14-6 = 8 6
Atomic #
● Example 2: 15
# protons = 7 N
# electrons= 7 7
# neutrons= 15-7 = 8

7. ISOTOPES:
● Atoms with the same number of
protons, but different numbers of
neutrons
● Isotopes of an element have the
same atomic number, but different
mass numbers
● Nuclear Symbol or isotopic symbol
– shows number of protons, neutrons
and electrons in an atom

9. ISOTOPE CHART:
Element Isotope # pro. # elec. # neu.
symbol
Boron- 10 B 5 5 5
Boron- 11 B 5 5 6
Chlorine- 35 Cl 17 17 18
Chlorine- 37 Cl 17 17 20

10. AVERAGE ATOMIC MASS
(= Atomic Weight)
● the weighted average of the
masses of the atoms (isotopes) in a
naturally occurring sample of an
element
● masses are based off of the atomic
mass unit (amu) defined as one
twelfth the mass of a carbon-12
atom
● these values can be fractions

11. Average Atomic Mass Example
● In nature carbon is composed of
98.890% 12C atoms and 1.1100% 13C
atoms. 12C has a mass of 12.000 amu
and 13C has a mass of 13.0034 amu.
What is the average atomic mass of
carbon?

12. Average Atomic Mass Example
● In nature carbon is composed of
98.890% 12C atoms and 1.1100% 13C
atoms. 12C has a mass of 12.000 amu
and 13C has a mass of 13.0034 amu.
What is the average atomic mass of
carbon?
Ave. mass = (.98890)(12.000)
+ (.011100)(13.0034)
= 12.011 amu

13. Example: There are 3 isotopes of magnesium that
occur in nature. Their abundances and masses are
listed below:
Isotope % Abundance Mass (amu)
24 Mg 78.99% 23.98504
25 Mg 10.00% 24.98584
26 Mg 11.01% 25.98259

14. What is the atomic weight of
magnesium?
Atomic weight =
(.7899)(23.98504)
+ (.1000)(24.98584)
+ (.1101)(25.98259)
= 24.31 a.m.u.

15. Carbon-12 and carbon-14 are
a) isomers
b) isotopes
c) radioactive elements
d) different elements
Isotopes are atoms of the same element with
different numbers of neutrons, and therefore
different atomic masses.
While carbon-14 is used in radioactive dating,
carbon-12 has a more stable nucleus and therefore
is not used in this capacity.

16. How many protons, electrons and neutrons are in one
atom of oxygen-17?
a) 17 p, 17 e, 17 n
b) 17 p, 17 e, 1 n
c) 8 p, 8 e, 8 n
d) 8 p, 8 e, 9 n
While most oxygen atoms have a mass of 16 g/mol,
oxygen-17 is an isotope with a mass of 17 g/mol.
The number of protons in an element is the same
for every atom of that element.
# electrons = # protons if element has no charge.

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