2.3 Periodic Table and Atomic Theory

Bohr Diagrams Diagrams that shows how may electrons are in each shell of an atom Each shell can only hold a maximum number of electron Shell 1 = 2 Shell 2 = 8 (total 10) Shell 3 = 8 (total 18) **Must fill each shell completely before filling the next**

Bohr Diagram Stable Octet - has a full outer shell Valence Shell - Outer shell Valence electrons - electrons in outer shell

Drawing a Bohr Diagram: 1. Find out the # of protons, neutrons and electrons Remember… Atomic # = # of protons & electrons (in an atom) Mass # - Atomic # = # of neutrons 2. Start filling electrons in shells starting from the inner most working outward

Bohr Diagram # of protons = atomic # # of protons = 15 # of electrons = # of protons # of electrons = 15 # of neutrons = mass # - atomic # # of neutrons = 31-15 # of neutrons = 16 15P 16N

Practice Together… Bohr model for: You try… Sodium Chlorine Lithium Fluorine

Noble Gas Stability Noble gases are the most unreactive elements Their atoms have full valence shells (they contain the maximum possible number of electrons without going to the next shell) When elements combine, they are trying to gain a full outer shell of electrons by either giving away their electrons to another element, or taking electrons from another element. Noble gases are happy the way they are – they do not want to gain or lose electrons. This is why they do not react easily with other elements!

Noble Gas Stability Other elements either gain or lose electrons to try to become like the noble gas nearest to them on the periodic table. Non-metal atoms will gain electrons. Metal atoms will lose electrons.

How Atoms become Ions Atoms become ions by gaining or losing electrons. When this happens, the number of protons and electrons becomes unequal. Remember that a neutral atom (one with no charge) has an equal number of protons and electrons.

Example… Lithium tends to form a +1 ion. It has 1 valence electron that it loses to become like the closest noble gas on the periodic table, Helium A neutral lithium atom has 3 protons and 3 electrons An Li+ ion has 3 protons and 2 electrons Less electrons = less negatives = +1 charge

Example… Oxygen tends to form a -2 ion. It is missing 2 electrons in it’s valence shell, by gaining two electrons, oxygen becomes like the closest noble gas on the periodic table, Neon A neutral oxygen atom has 8 protons and 8 electrons. An O-2 atom has 8 protons and 10 electrons. It gained 2 electrons = more negatives = -2 charge