 Protons are positive!!!  Protons have a +1 charge  Protons have a mass of 1 amu  Neutrons are neutral!!!  Neutrons have a neutral or no charge  Neutrons have a mass of 1 amu  Electrons are negative!!!  Electrons have a -1 charge  Electrons have a mass very close to 0 amu Protons and neutrons are found in the nucleus, the center of the atom. An amu is an atomic mass unit, an easier unit to use than grams.

 Each element has a symbol in order to make them easier to write out. The symbol for chlorine is Cl

 The atomic number tells two things about the atom of that element.  The number of protons  The number of electrons in a neutral atom The atomic number is given at the top of an element’s entry.

 The mass number tells you the (average) number of protons and neutrons in the nucleus.  To figure out the number of neutrons…  Round off the mass number  Subtract the atomic number (which is the number of protons)  Number of neutrons = mass number – atomic number The mass number is usually found at the bottom of an element’s entry

 Scientists will note elements in different ways. Here are some examples.   19 F: 19 is the atomic mass, they assume you can figure out the atomic number  F-19 or Fluorine-19: 19 is the atomic mass, they assume you can figure out the atomic number

 Number of protons = atomic number  Number of electrons = atomic number  Number of neutrons = mass number – atomic number

 Why is there the same number of protons and electrons in an atom of an element?  An atom has a neutral charge so it must have equal numbers of positive and negative charges.  Why is the atomic mass number not a whole number since neutrons have a mass of 1 and so do protons?  The mass number is an average since there are often different forms of an element (isotopes) with different numbers of neutrons.

 Some periodic tables offer some added information in the form of electron configurations  Chlorine has…  2 electrons on the first energy level  8 electrons on the second energy level  7 electrons on the third energy level Here’s the electron configuration

 Element name?  Sodium  Number of protons  11  Number of electrons  11  Number of neutrons  = 12

 Element name?  Gold  Number of protons  79  Number of electrons  79  Number of neutrons  = 118

 Element name?  Molybdenum  Number of protons  42  Number of electrons  42  Number of neutrons  96 – 42 = 54