Valence Electrons  The number of electrons in the outermost energy level (shell) The currency of the atomic world Can be found from Bohr models/electron configuration Can be read from periodic table (Group #) 2e - 8e - 2e - Mg 2 valence electrons Mg is in group 2 so it has 2 valence electrons

Octet Rule  Atoms are most stable when they have a full outer shell. For most atoms, that means they will have 8 electrons in their valence shell Atoms gain, lose or share electrons to get a full outer shell. -Noble gases already have a valence shell. -Period 1 elements are an exception.

Warm-up: How many valence electrons do each of the following atoms have? O B Ar v.e. ____ v.e. ____v.e. ____ S H Ca v.e. ____ v.e. ____v.e. ____

Dot Diagrams Also called Lewis Dot Structures, after Gilbert Newton Lewis. A way of keeping track of valence electrons

Lewis Dot Diagrams (Don't need to write this down!) Mg Write the element’s symbol How many valence electrons? Draw dots to represent electrons Each side of the symbol gets one dot before any side gets two

Examples C F How many valence electrons? 47

Ion Formation Atoms will do whatever is “easiest” (takes the least amount of energy) to have a full valence shell Li F lithium loses 1 e - (becomes + charged) fluorine gains 1 e - (becomes - charged) + -

Ions Review: Cations are positively charged and form when atoms lose electrons. To name: add the word “ion” to the end of the element name. Anions are negatively charged and form when atoms gain electrons. To name: replace the element suffix with –ide and add the word “ion”.

Li + F - = =

Electron Configurations Atoms gain or lose electrons to achieve the electron configuration of the nearest noble gas F - Fluoride ion fluorine: 2e - 7e - gains 1e - fluoride ion: 2e - 8e - (same as Ne!)

Examples How many valence electrons? What noble gas is closest to Calcium? Argon Will Ca gain or lose electrons? What is the charge now? What is the ion’s name? Ca 2+ Calcium ion

How many valence electrons? What noble gas is closest to Oxygen? Neon Will Oxygen gain or lose electrons? What is the charge now? What is the ion’s name? O 2- Oxide ion

How many valence electrons? What noble gas is closest to Sodium? Neon Will Na gain or lose electrons? What is the charge now? What is the ion’s name? Na + Sodium ion

How many valence electrons? What noble gas is closest to Bromine? Krypton Will Bromine gain or lose electrons? What is the charge now? What is the ion’s name? Br - Bromide ion

Equations representing ion formation Equations must be balanced…this means: 1)Equal # of each element on both sides of the arrow. 2) Charges on both sides of the equation must be equal Mg Mg e -

Write the equation (using dot notation)for the ion that will form when a: Aluminum atom becomes an aluminum ion: Bromine atom becomes a bromide ion: Al Al e - Br + 1e - Br -

Write the equation for the formation of a: 1)potassium ion from a potassium atom: K K + + 1e - 2) oxide ion from an oxygen atom: 2e - + O O 2- 3) Write the formula for K+K+ O 2- K2OK2O + 2 as an example

Monatomic vs. Polyatomic Ions Monatomic ions: ions formed when single (mon-) atoms gain or lose electrons Ex: Mg 2+ P 3- O 2-

Polyatomic ions: ions formed when two or more (poly-) atoms, working as a group, gain or lose electrons. Ex: NO 3 - SO 4 2- NH 4 + N O O O -

Some transition metals form more than one ion: Fe can lose 2 or 3 electrons so it can form… Fe 2+ or Fe 3+ Because there are two possible charges, we add a roman numeral when naming them: Fe 2+ = iron (II) ion Fe 3+ = iron (III) ion

Ions Wrap-Up Metals lose electrons to form cations. To name: add the word “ion” to the end of the element name. Non-metals gain electrons to form anions. To name: change the element suffix to –ide and add the word ion. Transition metal ions may have more than one charge; a roman numeral after their name. iron (II) ion = Fe 2+ iron (III) ion = Fe 3+

The nitrate ion (NO 3 ) – has, in total, one more electron than protons N O O O 1–1–

When balancing charges in an ionic compound, the polyatomic ion acts as a single unit of charge: Mg(NO 3 ) 2 Mg 2+ N O O O 1-1- N O O O 1-1-

N O O O Mg N O O O the charges are balanced Mg(NO 3 ) 2