Atomic Mass = All the Isotopes “Atomic mass” is the weighted average of the mass numbers of an element’s isotopes Atomic mass depends in the relative abundance of the isotopes in nature Atomic mass is shown in the Periodic Table Isotope Mass Number Percent Weighted Mass Number Chlorine-35 35 75.78% 26.52 Chlorine-37 37 24.22% 8.96 “Chlorine” 35.48

Orbitals = Inside Energy Levels Within energy levels there are “orbitals” Orbitals can hold only two electrons s orbital – 2 e- p orbitals – 3 x 2 e- d orbitals – 5 x 2 e- f orbitals – 7 x 2e- Different energy levels have different orbitals 1 = s only; 2 = s, p; 3 = s, p, d; 4 = s, p, d, f; 5 = s, p, d, f; 6 = s, p, d; 7 = s, p

Orbitals & the Periodic Table Orbitals fill from low energy to high energy, but there is overlap between energy levels – this is what gives the Periodic Table its unique shape The Periodic Table tells us which orbital is being filled with the next electron: Columns 1 & 2 = the s orbital Columns 13 through 18 = the p orbitals Columns 3 through 12 = the d orbitals* (*of the energy level one less than the row number) The Rare Earth Metals = the f orbitals** (**of the energy level two less than the row number)

An Expanded Periodic Table

Octet Rule = Valence Electrons Notice that each row of the Periodic Table ends with an element that has 8 electrons in its outermost energy level (the s and p orbitals are filled) This is a chemically stable configuration (the noble gases!!) that all atoms seek to achieve This is known as the Octet Rule – the tendency of atoms to give, take, or share electrons so there are 8 electrons in its outermost energy level Thus it is the electrons in the outermost energy level that dictate the chemical behavior of an element – these electrons are called “valence electrons”

Valence Electrons & Columns For many elements, the number of valence electrons is indicated by its column in the periodic table: The number of valence electrons for the transition metals and rare earth metals varies – no general rule is possible Column 1 Column 2 Columns 3 - 12 Column 13 Column 14 Column 15 Column 16 Column 17 Column 18 1 2 (varies) 3 4 5 6 7 8

Electron Dot Diagrams When we are interested in how an atom will behave chemically rather than its entire structure, we can simplify the Bohr Diagram into an Electron Dot Diagram (Lewis Dot Diagram) We use the element symbol to represent the nucleus and all of the non-valence electrons And we show the valence electrons as dots – two each right, above, left, and below the symbol as appropriate to the number of electrons

Electron Dot Diagrams Elements 1 – 20 Hydrogen 1 H Electron Dot Diagrams Elements 1 – 20 Helium 2 He Lithium 3 Li Beryllium 4 Be Boron 5 B Carbon 6 C Nitrogen 7 N Oxygen 8 O Fluorine 9 F Neon 10 Ne Sodium 11 Na Magnesium 12 Mg Aluminum 13 Al Silicon 14 Si Phosphorous 15 P Sulfur 16 S Chlorine 17 Cl Argon 18 Ar Potassium 19 K Calcium 20 Ca  

Electron Dot Diagrams