Review 3' Isotopes and atomic mass

1
Review 3. Isotopes and atomic mass

• In this lesson you will review the notation for
  atomic structure and the calculation of average
  atomic mass

2
Atomic structure

• A nucleus of protons and neutrons surrounded by a
  cloud of electrons.
• Protons (p) have a 1 charge and weigh about 1
  atomic mass unit
• Neutrons (n0) have no charge and weigh slightly
  more than protons
• Electrons (e) have a 1 charge and weigh about
  0.00055 atomic mass unit

3
Atomic number, mass number, and charge

• Atomic number Z number of p
• Identifies the element found on periodic table
• Mass number A p n0
• Charge p e
• Atoms are neutral p e
• Atoms acquire a charge by losing or gaining e
• An atom with a charge is called an ION

4
Notation
charge p e
mass number p n0
A
?
E
Z
atomic number p
assume charge is zero if not shown
5
Give the structure (p, n0, e)of these atoms
19F

• 9 p, 10 n0, 9 e
• 11 p, 12 n0, 10 e
• 92 p, 143 n0, 92 e
• 16 p, 18 n0, 18 e

23Na1
235U
34S2
6
Give the symbol of each atom or ion

• 35 p, 44 n0, 36 e
• 79Br1
• 47 p, 62 n0, 46 e
• 109Ag1
• 26 p, 28 n0, 26 e
• 54Fe

7
Isotopes

• Isotopes are two atoms with the same atomic
  number but different mass numbers
• They are atoms of the same element but their
  nuclei differ
• same number of protons
• different numbers of neutrons
• Isotopes of Li 6Li (lithium-6) has 3 p, 3 n0
  and 7Li (lithium-7) has 3 p, 4 n0

8
Average atomic mass

• An elements atomic mass is the weighted average
  of the masses of the naturally occurring isotopes
  of that element
• atomic mass (abundance1 x mass1)
  (abundance 2 x mass2) . . .

9
How to calculate the average atomic mass

• Magnesium occurs as three isotopes 24Mg, mass
  23.98504 amu (78.99) 25Mg, mass 24.98584 amu
  (10.00) 26Mg, mass 25.98259 amu (11.01)
• The average atomic mass of Mg is (0.7899 x
  23.98504) (0.1000 x 24.98584) (0.1101 x
  25.98259) 24.31 amu
• Be careful about sigfigs!

10
Atomic mass

• Two natural isotopes of antimony exist. 57.3
  exists as 121Sb (mass 120.9038 amu), and the rest
  is 123Sb (mass 122.9041 amu). What is the atomic
  mass of antimony?
• The abundances must total 100, so you can figure
  out the abundance of 123Sb
• (0.573 x 120.9038) (0.427 x 122.9041) 121.8
  amu

11
Percent abundance

• Two natural isotopes of copper exist, 63Cu
  (62.9296 amu) and 65Cu (64.9278 amu). What is
  the abundance of each isotope?
• The abundances must total 100
• If x 63Cu abundance, then 1-x 65Cu
• The average mass is found on the periodic table
• x 62.9296 (1-x) 64.9278) 63.546
• x 0.6915 and 1-x 0.3085
• 63Cu 69.15 and 65Cu 30.85

12
The bottom line

• You should be able to define atomic number and
  mass number
• You should be able to describe the structure of
  any atom, given its complete symbol (or write the
  symbol, given its structure)
• You should be able to calculate average atomic
  mass, isotope mass, and percent abundance from
  the appropriate data

13
Where to get more information

• For more about atomic structure, see Chapter 2,
  section 6 in your text
• For more on isotope mass and atomic mass, see
  Chapter 2, section 8 in your text