Download Notes 4.3 filled in

4.3. Distinguishing Among Atoms
A. Information in a Periodic Table Square
Atomic Number
Element Symbol
Element Name
Average Atomic Mass
1
H
Hydrogen
1.008
B. Atomic Number (AN)
 Equals the number of protons
 Is the only reliable number that identifies an
element!!!
 In the Periodic Table, elements are sorted by
increasing atomic number
 In a neutral atom, AN equals number of electrons
(#e-)
 Number of neutrons(# no) is similar but often not
equal to the number of protons
Q: how many protons does Potassium have?
A: 19
Q: What is the atomic number of Phosphorus?
A: 15
C. Mass number (MN) of an atom ( in amu’s)
 MN is not indicated on the PT.
 To calculate the MN, simply add up #p+ and # no
each having the mass of 1 amu.
Q: an atom has 5 protons and 7 neutrons, calculate the MN.
A: 5 amu + 7 amu = 12 amu
Q: Which element is that? (Look in the PT)
A: 5 protons, it’s Boron
D. Isotopes
 Every element exists as multiple versions, called isotopes
 Isotopes of the same element are identical with
regard to their
1. AN
2. # of p+
3. # of e different with regard to their
1. # no
2. MN
Example: Carbon - C
Comes in form of three different isotopes with either 6, 7
or 8 neutrons
Q: How many protons does Carbon have?
A: 6 protons – all carbon isotopes have 6 protons!!!
Q. Calculate the mass number of the three Carbon
isotopes?
Isotope
A
B
C
#p+
6
6
6
# no
6
7
8
MN
12 amu
13 amu
14 amu
E. Writing Conventions for Isotopes
Shorthand A) uses only the mass number MN
 Carbon-12, Carbon-13, Carbon-14
 C-12, C-13, C-14
Shorthand B) indicates MN on top!!!, AN on bottom
12
13
14
6C
6C
6C
F. Calculating the Number of Neutrons
 MN minus AN
Q: How many neutrons does N-15 have?
A: AN of Nitrogen is 7 so MN – AN = 15 – 7 = 8 no
Q. Write the #p+, no, e- for N-15
A: 7 p+, 8 no, 7 eQ: Calculate the p+, no , e- for P-30 and 2411Na
P-30
24
11Na
p+
15
11
no
15 (30-15)
13 (24-11)
e15
11
4.3 Average Atomic Mass -AAM
 Bottom number of a Periodic Table square
Average Atomic
Mass
 This is a calculated, weighed average mass composed of
a) All the element’s existing isotopes and their masses
b) their natural abundance (frequency in nature)
Elements all have varying number of isotopes….need list!
Calculating Average Atomic Mass
Example: Carbon
Isotope
Natural
Atomic mass
Abundance
Carbon-12 98.89%
12.000 amu
Carbon-13 1.11%
13.003 amu
How to do it
1. Convert the natural abundance % into decimals
(divide by 100 or slide decimal 2 x to the left)
98.89/100 = 0.9889
1.11/100 = 0.0111
2. Multiply the mass of each isotope by its natural
abundance and round to three decimals: this is
called Relative Mass
0.9889 x 12.000 = 11.867
0.0111 x 13.003 = _0.144
12.011
3. Add all the Relative Masses
See above
4. Round to two decimals (our periodic table has 2
decimals)
12.011 → 12.01
5. Compare with the MN found in the periodic table.
Do both numbers match? Yes: 12.01 amu
Practice: Calculate the Average Atomic Mass for
1. Chlorine
1. Chlorine-35

75.77%/100 = 0.7577

34.969 amu
2. Chlorine-37

24.23%/100 = 0.2423

36.966 amu
0.7577 x 34.969= 26.496
0.2423 x 36.966= _8.957
35.453 round 35.45 amu
2. Oxygen
1. Oxygen-16

99.759%/100 = 0.99759

15.995 amu
2. Oxygen-17

0.037%/100= 0.00037

16.995 amu
3. Oxygen-18

0.204%/100 = 0.00204

17.999 amu
0.99759 x 15.995 = 15.956
0.00037 x 16.995 = 0.006
0.00204 x 17.999 = _0.037
15.999 round to 16.00 amu