 Deals with letters/symbols from the periodic table to describe substances instead of words EXAMPLE: CO 2 = carbon dioxide O 2 = oxygen NaCl = sodium chloride Molecular formulas Words Molecular Formulas

Molecular Formulas:  Recall elements can combine to form compounds  But when atoms combine, they do so in specific ratios  For example: Water, H 2 O Combines 2 Hydrogens for every 1 Oxygen

 An atom wants to be stable  To be stable, an atom must have a full outer electron shell (8 electrons) Octet Rule  Atoms combine together, forming bonds, to fill their outer electron shell Molecular Formulas:

 The number of bonds that an atom can form is termed the valence Molecular Formulas: Atoms in FamilyValence Number of bonds each atom can make

How many bonds would an element in the families below make? a)Family 2 b)Family 5 c)Family 7 d)Family 8 2 bonds 3 bonds 1 bond none

 To determine a molecule’s formula, use the atoms valences & the cross over rule  Ex.Water – why H 2 O? Molecular Formulas: Family 1Family 6 Valence 1 Valence 2 Makes 2 bondsMakes 1 bond So, each H needs & makes 1 bond with O So, the O needs & makes 2 bonds – 1 with each H

Cross Over Rules: Write symbols for the elements, with the metal first Write the valence of each atom Cross over valences If needed, simplify the valences Eliminate any “1”s 21 Not needed here

Write symbols for the elements, with the metal first Write the valence of each atom Cross over valences If needed, simplify the valences Eliminate any “1”s 11 Not needed here What is the molecular formula when Li & Br combine?

Write symbols for the elements, with the metal first Write the valence of each atom Cross over valences If needed, simplify the valences Eliminate any “1”s 21 Not needed here What is the molecular formula when Be & Cl combine?

Write symbols for the elements, with the metal first Write the valence of each atom Cross over valences If needed, simplify the valences Eliminate any “1”s 32 Not needed What is the molecular formula when Al & O combine?

Write symbols for the elements, with the metal first Write the valence of each atom Cross over valences If needed, simplify the valences Eliminate any “1”s 22 Both “2”s cancel each other out 11 What is the molecular formula when Mg & O combine?

Write symbols for the elements, with the metal first Write the valence of each atom Cross over valences If needed, simplify the valences Eliminate any “1”s 22 Both “2”s cancel each other out 11 What is the molecular formula when Ca & S combine?

Atom “X” from family II combines with atom “Y” from family VII. What is the compound’s molecular formula? Atoms in FamilyValence

Given the compound, BxOyBxOy What would be the numerical value of “x” & “y”? Why? B is from family 3, so it forms 3 bonds O is from family 6, so it forms 2 bonds Apply cross-over rule,x = 2 & y = 3 B2O3B2O3

Naming Molecules:  Scientists need a standardized way to name molecules.  Two methods: 1.For molecules with a metal & a non-metal 2.For molecules with 2 non- metals

Naming Molecules: Steps to naming molecules with a metal & non-metal: Identify the metal & the non-metal metalnon-metal Name the metal sodium Name the non-metal but change ending to “ide” chloride

Name Al 2 O 3 Identify the metal & the non-metal metalnon-metal Name the metal aluminum Name the non-metal but change ending to “ide” oxide

Name CaF 2 Identify the metal & the non-metal metalnon-metal Name the metal calcium Name the non-metal but change ending to “ide” fluoride

Naming Molecules: Steps to naming molecules with a 2 non-metals: Name the first non- metal Name the second non-metal, change ending to “ide” carbon Add prefixes to each non- metal oxide # of Atoms Prefix 1mono 2di 3tri 4tetra 5penta 6hexa monodi “mono” never placed on 1 st non-metal!!!

Steps to naming molecules with a 2 non-metals: Name the first non- metal Name the second non-metal, change ending to “ide” phosphorus Add prefixes to each non- metal oxide # of Atoms Prefix 1mono 2di 3tri 4tetra 5penta 6hexa dipenta Name P 2 O 5

Steps to naming molecules with a 2 non-metals: Name the first non- metal Name the second non-metal, change ending to “ide” carbon Add prefixes to each non- metal bromide # of Atoms Prefix 1mono 2di 3tri 4tetra 5penta 6hexa tetra Name CBr 4

Name the following compounds: a)CF 4 b)MgCl 2 c)Na 2 O d)N 2 S 3 carbon tetrafluoride magnesium chloride sodium oxide dinitrogen trisulfide

 Elements combine to form molecules in specific ratio to become stable (fill outer electron shell)  Octet Rule  Use Cross Over Rule & Valences to determine a molecule’s formula Atoms in FamilyValence

 Naming compounds with a metal & non- metal: name of metal + name of non-metal – ide  Naming compounds with 2 non-metals: prefix-name of non-metal + prefix-name of non-metal – ide

Activities:  436 Worksheet #3