1. Distinguishing Among Atoms
Chemistry 4.3 Notes
Distinguishing Among Atoms

2. What we know…
Pg. 110 – Atoms are composed of electrons, protons, and neutrons. Protons and neutrons make up the small, dense nucleus. Electrons surround the nucleus and occupy most of the volume of the atom.
**How do atoms differ from each other then?

3. I. Atomic Number A. Def – the # of protons found in an atom.
1. Found in the nucleus
2. Above symbol on P.T.
3. Atoms differ from each other due to the number of protons.
4. Ex: Hydrogen  1 proton
5. Ex: Oxygen  8 protons

5. Atomic Number 2 4 10 42 82 50 56
6. He 
Be 
Ne 
Mo 
Pb 
Sn 
Ba 

6. II. Mass Number
A. Def – the total # of neutrons and protons in the nucleus of an atom.
1. Found above chemical symbol on P.T.
2. Formula: Mass # = protons + neutrons
3. Ex: Na  23
4. Ex: Al  27

7. Mass Number 11 7
222 52 48 65
5. B 
Li 
Rn 
Cr 
Ti 
Zn 

8. III. What about Electrons?
A. **Assume electrons = the # of protons in an atom**
1. All atoms have a charge of zero
2. Complete the following table in your notes.

9. Name Symbol Atomic Number Protons Neutrons Mass Number Electrons
Scandium
Copper
Krypton
Neon
Phosphorus
Gallium
Manganese

10. Name Symbol Atomic Number Protons Neutrons Mass Number Electrons Sc 21
Scandium Sc 21 24 45
Copper Cu 29 35 64
Krypton Kr 36 48 84
Neon Ne 10 20
Phosphorus P 15 16 31
Gallium Ga 39 70
Manganese Mn 25 30 55

11. W 76 128 45 41 57 150 Symbol Protons Electrons Neutrons Atomic #
Mass # W 76
128 45 41 57
150

12. III. Isotopes A. Def – same atom but different # of neutrons.
1. This alters the mass #
2. Chemically the same
3. Ex: Hydrogen
 3 isotopes = H-1, H-2, H-3

13. 4. Drawing an isotope
1st put in # of protons
2nd use mass # to determine the # of neutrons
5. Draw the 3 isotopes of hydrogen

15. 6. Draw the 3 isotopes of Neon

16. 7. Draw the 2 isotopes of Chlorine (35 and 37)

17. IV. Atomic Mass
A. Def – a weighted average of all the isotopes in a sample of a given element.
1. reflects mass
2. reflects relative abundance
B. Atomic Mass Unit
1. Def – 1/12th of a carbon-12 atom
2. Prevents using scientific notation
ex: x g = fluorine
3. Why is the atomic mass not a whole number?
isotopes
4. There are isotopes for almost every element

18. 5. Ex: Chlorine
2 isotopes (Cl-35 and Cl-37)
What would the atomic mass be?
 right? Wrong
we need the relative abundance

19. 6. Cl-35 has an abundance of 75.77%. Cl-37 has an abundance of 24.23%.
35 x = 26.52
37 x = 8.965 

20. 7. Hydrogen has 3 isotopes, H-1, H-2, H-3. H-1 occurs 99
7. Hydrogen has 3 isotopes, H-1, H-2, H-3. H-1 occurs 99.99% of the time, H-2 occurs 0.015% of the time, and H-3 is negligible (can only be made in the lab). What is the atomic mass?
 1.030