### April 18 Atomic Mass and Isotopes

When you look at the periodic table of elements, you see that each element is in a box with numbers and a symbol.  It goes from top to bottom as element name, atomic number, symbol, and atomic mass.  The atomic number is the amount of protons that the element has and the atomic number is the mass of the atom.  In order to get the mass, you add the amount of protons and neutrons.  The reason why you add these two is because the nucleus of the atom contains only of protons and neutrons.  You don't add electrons because they only orbit the atom.  The protons are already given to you from the atomic number so all you need to do is subtract the atomic mass with the atomic number to get the number of neutrons.  The atomic mass is usually shown as a decimal number in periodic tables because that the average mass of the atom.  From the picture above, you can just say that Helium has an atomic mass of 4. That means it has 2 neutrons because it has 2 protons from the atomic number.

In Copper, its atomic number is 29.  This means that there is 29 protons.  The atomic mass is 63.5.  You can make it 64.  We then subtract 64 and 29.  The result is 35 neutrons.  From Titanium, its atomic number is 22.  That means there are 22 protons.  With a atomic mass of 48, we subtract the two numbers and get 26 neutrons.

What does the word isotope mean? An isotope is a variation of an atom with more or less neutrons.  Isotopes are atoms that have lost/gain neutrons and also electrons.  For example, Krypton has an atomic mass of 84.   It can also be called Krypton-84.  Sometimes you might see it written as Krypton-83 or Krypton-85.  This means it lost and gained an electron.  From Krypton-84 to Krypton-83 it lost a neutron.  And from Krypton-84 to Krypton-85 it gained an electron.  Sometimes you might see a number and a positive or negative symbol beside the atomic mass number.  If there is no number or symbol beside the atomic mass number, this means the isotope is neutral.  If you see a positive symbol and a number beside it, it means it has lost the number of electrons from the isotope making it positive by the number of charge because it lost that number of electrons.  This means the isotope has more protons than electrons.  And same goes for a negative number beside the atomic mass number.  It means that the isotope gained the amount of electrons making the isotope negative because it has more electrons than protons.  If there is just a negative or positive symbol, it means it has 1 less/more electron.

Examples:
How many electrons are in Ca2+?
For Ca, there is 20 protons.  Since it is 2+, this means that the isotope is positively charged.  The amount of electrons would be 18.
How many protons and electrons are in Cl-
Cl has 17 protons.  This isotope is negatively charged.  Remember that if it just has a negative sign, it means it lost 1 proton.  That means there is 16 electrons in this isotope.

This is pretty much straight forward.  No need to video... After doing the worksheets you should be fine with the stuff.