1. The atom and Isotopes

2. # protons in an atom = # electrons
Atomic Number
Atoms are composed of identical protons, neutrons, and electrons
How then are atoms of one element different from another element?
Elements are different because they contain different numbers of PROTONS
The “atomic number” of an element is the number of protons in the nucleus
# protons in an atom = # electrons

3. Atomic Number
Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element.
Element
# of protons
Atomic # (Z)
Carbon 6
Phosphorus 15
Gold 79

4. Mass Number always a whole number NOT on the Periodic Table!
mass # = protons + neutrons
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always a whole number
NOT on the Periodic Table!

5. Mass Number
Mass number (a) is the number of protons and neutrons in the nucleus of an isotope:
Mass # = p+ + n0
Nuclide p+ n0 e-
Mass #
Oxygen - 10 - 33 42
- 31 15 18 8 8 18
Arsenic 75 33 75
Phosphorus 16 15 31

6. Symbols
Nuclear symbol:
Mass #
Atomic #
Hyphen notation: carbon-12

7. Symbols 17 37 20 Chlorine-37 atomic #: mass #: # of protons:
# of electrons:
# of neutrons: 17 37 20

8. Br Symbols 80 35 Find each of these: number of protons
number of neutrons
number of electrons
Atomic number
Mass Number 80 Br 35

9. Symbols
If an element has an atomic number of 34 and a mass number of 78, what is the:
number of protons
number of neutrons
number of electrons
complete symbol

10. Symbols
If an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
complete symbol

11. Symbols
If an element has 78 electrons and 117 neutrons what is the
Atomic number
Mass number
number of protons
complete symbol

12. Isotopes
Dalton was wrong about all elements of the same type being identical
Atoms of the same element can have different numbers of neutrons.
Thus, different mass numbers.
These are called isotopes.

13. Isotopes
Frederick Soddy ( ) proposed the idea of isotopes in 1912
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials.

14. Isotopes
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15. Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Protons
Electrons
Neutrons
Nucleus
Hydrogen–1
(protium) 1
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) 2

16. Isotopes Elements occur in nature as mixtures of isotopes.
Isotopes are atoms of the same element that differ in the number of neutrons.

17. Composition of the nucleus
Atomic Masses
Atomic mass is the average of all the naturally occurring isotopes of that element.
Isotope
Symbol
Composition of the nucleus
% in nature
Carbon-12
12C
6 protons
6 neutrons
98.89%
Carbon-13
13C
7 neutrons
1.109%
Carbon-14
14C
8 neutrons
0.001%
Carbon =

18. Average Atomic Mass Avg. Atomic Mass weighted average of all isotopes
on the Periodic Table
Avg.
Atomic
Mass

19. Average Atomic Mass
EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.
Avg.
Atomic
Mass
16.00
amu

20. Average Atomic Mass
EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.
Avg.
Atomic
Mass
35.40 amu