 # Unit 3: Atomic Structure Atomic Number, Mass Number, and Isotopes.

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Unit 3: Atomic Structure Atomic Number, Mass Number, and Isotopes

After today, you will be able to… Find the atomic mass, atomic number, and symbols of elements off of the Periodic Table Calculate the number of neutrons in a particular atom Explain what an isotope is

Atomic Number The number of protons in the element. Always a whole number Example: Oxygen’s atomic number is 8, so there are 8 protons in Oxygen’s nucleus

Atomic Number For neutral elements (no charge) the # of p + = # of e - The number of protons will never change for an element It is like our social security number, it identifies them.

Atomic Mass Read right off the Periodic Table. The number will have a decimal. It is the average (weighted) of all the elements isotopes.

Atomic Number vs. Atomic Mass 8 O Oxygen 15.999 Atomic number Atomic mass

Mass Number Number of protons & neutrons combined Always a whole number

Finding the Number of Neutrons Neutrons = mass # - atomic # Example: Oxygen’s mass number is 16 and its atomic number is 8. The number of neutrons will be 16-8 = 8.

Isotopes Atoms with the same number of protons, but different number of neutrons. They are chemically alike Have different mass numbers Written like this: Element–Mass Number

Examples: Neutral hydrogen isotopes Hydrogen-1 Hydrogen-2Hydrogen-3 1 proton 1 proton 1 proton 1 electron 1 electron 1 electron 0 neutrons1 neutron2 neutrons

Summary Atomic # = protons = electrons (neutral atoms) Mass # = protons + neutrons = rounded atomic mass Neutrons = mass # - atomic #

Practice makes perfect… Element Symbol Atomic Number Mass Number # of Protons # of Neutrons # of electrons S 16 714 17 18 4020 163216 N Cl Ca 777 35 17 20

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