# How Atoms Differ/ Isotopes

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How Atoms Differ/ Isotopes
Notes #5

I. Parts of an atom Protons – positive charge Neutrons – no charge
Electrons – negative charge 1. in constant motion around nucleus of atom; travel in orbitals, or shells

II. Determining # of each subatomic particle
Atomic number - # of protons in the nucleus of an atom Atomic mass - # of protons and neutrons in the nucleus Example for Scandium atom atomic # = # of p = ___ # p+ = # e = ___ atomic mass – atomic # = n0 = ___ 21 Sc 45.0

II. Determining # of sub. Part.
Atomic number - # of protons in the nucleus of an atom Atomic mass - # of protons and neutrons in the nucleus Example for Sc atom atomic # = # of p = 21 # p+ = # e = 21 atomic mass – atomic # = n0 = 24 21 Sc 45.0

III. Atomic Number Identity of each element is determined by the number of protons in the nucleus. In a neutral atom, the number of electrons equals the number of protons. Practice problems pg. 99

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

IV. Isotopes C. Naming It is often important to identify one isotope from another. This is done in two ways. 1. 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

2. Nuclide or nuclear symbol notation
Atomic mass 4 2 He Atomic number

Practice problems find p+, e-, n0
1. Symbol notation 107Ag 2. Hyphen notation Potassium – 39 Potassium – 40 Potassium – 41 Solve practice problems pg. 99,101 #’s For #14 add on hyphen notation and symbol notation

V. Average Atomic Mass 3.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 1.007825 * .99985 = 1.0076738 amu protium
* = amu deuterium = amu amu is a weighted average because most of the hydrogen is protium.

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 4. 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 5. 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 )