# Masses of Atoms. Atomic Mass  Remember that Protons and Neutrons are much bigger than an Electron.  Because of this, the nucleus contains the majority.

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Masses of Atoms

Atomic Mass  Remember that Protons and Neutrons are much bigger than an Electron.  Because of this, the nucleus contains the majority of the mass in any atom.  The mass of a Proton and Neutron are approximately the same.

Add em up!  The mass of one Proton or one Neutron is considered to be 1 atomic mass unit. 1 amu =  By counting the number of Protons in an atom we get the atomic number.  Atoms of the same element will all have the same atomic number.

Mass Number  If we add up the number of Protons and Neutrons in an atom we get the mass number of the atom. Some books call this the atomic mass. Number of Protons + Number of Neutrons = Mass Number Number of Protons = Atomic Number Atomic Number ≠ Mass Number

Practice Hydrogen Helium Lithium Carbon

Isotopes  Elements are defined by how many protons are in the nucleus of the atom.  Atoms that come from the same element but have a different number of Neutrons are called Isotopes.  Do Isotopes of the same element have the same atomic number?  Do Isotopes of the same element have the same mass number?

Example All threes isotopes have the same number of … PROTONS! All three isotopes have a different number of… NEUTRONS! All three isotopes have the same atomic number but different mass numbers.

Isotope Examples

Average Atomic Mass  Because most elements have more than one type of isotope, the masses of each isotope are used to make a weighted- average. Average atomic mass  Isotopes of Carbon Carbon-12 99% Carbon-13 1% Carbon 14 0. 0000000001%

Weighted-Average  Take the mass of each isotope and multiple it by the % of that isotope that exists in nature.  Next, add them together.  The average atomic mass of Carbon is… 12.01 amu (12)X(0.99)(13)x(0.01)(14)x(0.000000000001) + +

Weight lose  Atoms with small nuclei are very stable. So, what is the most stable element in the universe?  Atoms with large nuclei become less stable.  Atoms with very large nuclei are unstable. If a nucleus is unstable it will begin to lose Protons and Neutrons. These particles will shoot off as radiation leaving a smaller nucleus behind. Basically the atom looses weight!

Remember, Protons define the element.  If an atom is losing protons that means it is turning into another type of element. This process is known as radioactive decay.  Different elements and their isotopes will decay at different rates. Some are very fast…. Microseconds Some are very slow…. Billions of years

Half-Life  NO! not the game!  The time that it takes for half of a substance to disappear due to radio-active decay is called the materials half-life. Number of half-livesFraction remaining% remaining 01100% 11/250% 21/425% 31/812.50% 41/166.25%

Half-Lives of certain isotopes

Excellent!  Find a partner.  You will work together to identify different atoms.

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