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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 + 0 Mass p = mass n 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 = + o # of p + # of n o Then # of n = + mass number – # of p Gold Atomic number 79 Element symbol Au 196.967 Atomic Mass How many + 0 p ? e? n ? 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 + 0 Ne-20: 10p & 10n + 0 Ne-21: 10p & 11n + 0 Ne-22: 10p & 12n 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 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 6.015 amu and is 7.5% abundant X-7 has a mass of 7.016 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)