Section 4.3 How atoms differ

Atomic Number Represents three things in a neutral atom: 1. What element it is 2. The number of protons in each atom 3. The number of electrons in each atom

Atoms  Must have neutral charge so positive charge = negative charge  This means: # p + = # e -

Finding Mass Number  Mass p + = mass n 0  A proton or neutron has a mass of 1 amu  Mass e - ~ 0

Mass number  Will be equal to the number of protons + number of neutrons  If not given to you, round the atomic mass # off to a whole #  Atoms of the same element can have different mass #s

Neutrons  Since mass number = # of p + + # of n o  Then # of n o = mass number – # of p +

Au Atomic Mass Atomic number Element symbol How many p + ? e - ? n 0 ? Gold

Isotopes  Atoms of the same element with a different number of neutrons (different mass #)  Still have same # of protons (that determines the element)  Same atomic number

Neon(atomic # = 10) 3 isotopes  Ne-20: 10p + & 10n 0  Ne-21: 10p + & 11n 0  Ne-22: 10p + & 12n 0

Isotopes of H  H-1 hydrogen  H-2 deuterium  H-3 tritium

Atomic Mass  Different from mass #  A weighted average of all isotopes of that element  The mass given on the periodic table

Atomic Mass  Isotopes occur in different amounts or % - not in equal amounts  So atomic mass has to be a weighted average

Finding a weighted average atomic mass (% abundance isotope 1 x mass isotope 1 ) (% abundance isotope 1 x mass isotope 1 ) + (% abundance isotope 2 x mass isotope 2 ) + (% abundance isotope 3 x mass isotope 3 ) + …….. Continue for all the isotopes you have

Finding a weighted average atomic mass  Element X has 2 isotopes: X-6 has a mass of amu and is 7.5% abundant X-7 has a mass of amu and is 92.5% abundant  Find the average atomic mass for element X

Abundance  Which isotope must be more abundant Cu-63 or Cu-65?  How did you decide?

The Standard  C-12 is the standard that atomic mass is based on  C = 12 amu (atomic mass units)