Atomic Number (Z)  Identity of each element is determined by the number of protons in the nucleus.  The number of protons in the nucleus is the atomic number.  In a normal atom, the number of electrons equals the number of protons.

Atomic Mass (A)  …is equal to the number of protons and the number of neutrons in the nucleus.

Isotopes 1. Atoms of the same element having different atomic masses. 2. Atoms of the same element having different numbers of neutrons.  Hydrogen has three isotopes: Protium – 0 neutrons Deuterium – 1 neutron Tritium – 2 neutrons

Isotopes - Naming  It is often important to identify one isotope from another. This is done in two ways.  Hyphen notation. This gives the element name (which you can use to determine the atomic number) followed by the atomic mass which you can use to determine the number of neutrons.  Hydrogen – 2

 It is often important to identify one isotope from another. This is done in two ways.  Nuclide or nuclear symbol. Atomic number (Z) Atomic mass (A) Isotopes - Naming 4242 He

Some Important Isotopes  Carbon – 14  Carbon - 12

Average Atomic Mass  An average sample of hydrogen in nature contains % protium, 0.015% deuterium and a miniscule amount of tritium.  Protium has an atomic mass of amu.  Deuterium has an atomic mass of amu.

Average Atomic Mass  Multiply the masses of each isotope by its percentage. These are weighted values.  Add all of these values together. (Do NOT divide by the number of isotopes. This is unnecessary.)

Average Atomic Mass  amu is a weighted average because most of the hydrogen is protium * = amu protium * = amu deuterium = amu

Average Atomic Mass  amu is a weighted average because most of the hydrogen is protium.  Multiply the masses of each isotope by its percentage. These are weighted values.  Add all of these values together. (Do NOT divide by the number of isotopes. This is unnecessary.)

Average Atomic Mass  The normal way to determine the average mass of something would be to add the two masses and simply divide by two.

Average Atomic Mass  The normal way to determine the average mass of something would be to add the two masses and simply divide by two = / 2 = amu

Sample Problem  What is the atomic mass of silicon if 92.21% of its atoms have mass u, 4.70% have mass u and 3.09% have mass ?  28.1 u .9221 x = u .0470 x = 1.36 u .0309 x = u

Sample #2  Neon has two isotopes. Neon-20 has a mass of u and neon-22 has a mass of u. In any sample of 100 neon atoms, 90 will be neon-20 and 10 will be neon-22. Calculate the average atomic mass of neon.  u  (.90 x ) + (.10 x ) 