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How atoms differ

Atomic Number

Moseley discovered that each element had a unique charge (+) in its nucleus Number of p+

Atomic Number = # of p+ = # of e-

Isotopes

All atoms of an element have the same number of: p+

e-

Number of n0 changesIsotopes – the same element with a different

number of n0 but the same number of p+ and e-

Thus, they have a different mass

Mass number

To identify isotopesNumber after element’s nameEx:

Carbon-14 Potassium-39

Short hand: 39

19K 14

6C Top number is sum of n0 and p+

Bottom number is the atomic number

Finding the number of n0

n0 = mass # - atomic #

Mass of individual atoms

p+ = 1.67 x10-24gn0 = 1.67 x10-24ge- = 1/1840

th of 1.67 x10-24g

So small scientist came up with Atomic mass units (amu) Gave c-12 an exact mass of 12.0000 1 amu = 1/12

th the mass of C-12

Practice

p+ & e- n0 Isotope Symbol

B Calcium-46

C Oxygen-17

D Iron-57

E Zinc-64

F Mercury-204

Atomic mass

Weighted average of the isotopes of that elementAverage atomic mass = (isotope mass1 x %1) + (isotope

mass2 x %2) + (isotope mass3 x %3)….

Cl 75% Cl-35 25% Cl-37

(35 x 0.75) + (37 x 0.25) = 35.526.25 + 9.25

Hydrogen H-1 Abundance 99.9851% H-2 Abundance 0.0151%

Helium He-3 abundance 0.0001373% He-4 abundance 99.9998633%

Application of average atomic mass

The closer an element is to a whole number for its average atomic mass the more likely it is that it’s most abundant isotope has that atomic mass Ex: F has an atomic mass of 18.998amu F-19 (99%)

There are exceptions Br has an atomic mass of 79.904amu Br-79 (51%) & Br-81 (49%) Br-80 doesn’t exist