According to Bohr… Examples of Bohr diagrams

Quantized light As an atom is increasingly excited (by heat, electricity,etc) it is raised to a higher energy level. The same amount of energy will be released on the way back down! The colors emitted do not change, just the brightness of the light changes. The energy emitted by one atom is called a photon

Line Spectra Atoms emit only certain colors when excited. These are expressed as colored lines when observed through spectroscopes. Niels Bohr proposed that this was because electrons could occupy only certain energy levels

Emission Spectra of Hydrogen When excited hydrogen atoms return to a lower energy state, they emit photons of certain energies and thus certain color (Specific wavelengths in nanometers are emitted of the spectrum which correspond to a specific color of band we see)

Line spectra emitted by excited atoms = Evidence of electron activity Low frequency light (red) High frequency light (violet, UV light) Ground State = Electrons unexcited, close to nucleus, no light emitted When atoms are energized (by heat, electricity) electrons begin “hopping”.

1.Must absorb the exact amount of energy in order to make a “hop”. 2.When electron returns to ground state, it emits the exact energy (color) that is absorbed. (Quantums of energy that are shown by a particular color of light) 3. dex.plhttp:// dex.pl

What did you discover in the periodic properties lab? Which substance was the most volatile? Which substances had the lowest melting point? Which substances conducted electricity? Which substances dissolved in water? Hexane?

Chemical Bonds Definition: The force that holds two atoms together. Why does a bond form? So that an atom: 1. becomes more stable 2. takes on a noble gas configuration

What qualities allow you to determine the type of bond Electronegativity: measure of how strongly an atom attracts the electrons that are shared in a bond

NaCl Fe(NO 3 ) 2 MgO Cs 2 SO 4 All these substances contain Ionic Bonds What rules could you determine about ionic bonds from examining these compounds?

Types of Bonds 1. Ionic The attraction between oppositely charged ions Atoms become ions by adding or losing electrons They form these charges to reach a noble gas configuration in their outer energy level Usually a Metal and a Non Metal Difference in Electronegativity of>1.7

These compounds have covalent bonds. What rules could you determine about covalent bonds? CO 2 H 2 O CH 4 SiO 2

2. Covalent Bonds A sharing of a pair of electrons between two atoms Individual atoms attain a noble gas configuration with the shared electrons in their outer energy level Difference in electronegativity lessthan 1.7>0

Lewis Dot Drawings Show the sharing or transfer of electrons Also called “electron dot” drawings. Involve only valence electrons (those in the outermost energy level) Show the type of bond formed (either ionic or covalent) All atoms will satisfy the “octet” rule (except for hydrogen (duet rule) and metals)

Elementvalence electronsLewis dot N O F C

Lewis dot drawings for 1.Ionic bonds Show electrons being transferred Include brackets and charges on ions examples: H and F Na and Cl

Lewis Structure for Covalent bonds Technique: Place the atom with the largest number of unpaired electrons in the middle. (Never put H in the middle of a molecule!!) Determine how the electrons will be shared so that all atoms are stable (Octet Rule\Duet Rule) H 2 O CH 4 SCl 2

Double and Triple Bonds Example: HCN Make a table: atomhaveneed H 1 2 C 4 8 N 5 8 total Difference: 18-10=8 divide by 2 = 4 You need 4 bonds in this structure Sharing 4 or 6 electrons (Double or Triple bonds allow this to happen)

Examples C 3 H 6 SO 2

Electron dot drawings for polyatomic ions Always include brackets and charges, but have covalent bonds inside the ion Count the number of valence electrons for each and the add or subtract and electron to make the correct charge NH 4 + OH - SO 4 2- Draw NH 4 OH

Exceptions to the octet rule 1.Metals MgH 2 BH 3 2. Molecules with an odd number of electrons NO NO 2

3. Some Nonmetal atoms because of their size, they can have more than an octet of electrons (due to the presence of empty “d” orbitals which can be used for bonding). SF 6 PCl 5 DON’T FOCUS ON THESE BUT KNOW THEY OCCUR!

Note: Not all covalent bonds have equal sharing of electrons… There are electron hogs!!! Elements that hold on to the electrons more tightly than others You can determine if a bond is ionic,covalent and if there is an electron hogs, through looking at a characteristic property.

To determine the type of bond Electronegativity: measure of how strongly an atom attracts the electrons that are shared in a bond The difference of electronegativity will determine the type of bond

What would you predict are the trends in electro negativity? in families? in periods? What family has the highest electronegativity? What family has the lowest electronegativity? What period has the highest electronegativity? What family has the highest electronegativity

Electronegativity Allows you to predict the nature of the bond between two atoms To determine where the electrons tend to spend the most time in the molecule

To determine the type of bond When the difference in electronegativity (ΔE.N.) is 2.0 or greater, the bond is ionic Examples: NaCl KF Where are these atoms on the periodic table in relation to one another?

When the ΔE.N. is less than 2.0, the bond is covalent Examples: H 2 O NO 2 This means the electrons spend more time around one of the elements giving it a partial charge Draw a picture of how you think the electrons would be distributed for each of these molecules. When the electrons are shared equally ex: H 2 NCl 3 the bond is pure covalent and has no partial charge Why do you think there would not be a partial charge on these bonds? Which covalent bond do you think is stronger? H 2 or HCl

These bonds are called intramolecular forces Have various strengths –Ionic (STRONGEST) –Polar Covalent (NEXT STRONGEST) –Covalent (STRENGHTH DEPENDS ON ELECTRONEGATIVITY DIFFERENCE)

Shapes of Molecules/Compounds 1.Determined by presence of electrons… 2.Electrons will repel each other 3.Ionic substances don’t form molecules Ions stack together, anions alternating with cations

The structure of Ionic solids

What types of shapes do you think molecules can have? Each balloon represents a bonded pair Non bonded pairs repel bonded electrons

Possible Shapes Linear Ex: Trigonal Planar Tetrahedral Triganol bipyramidal octahedral

2. Covalent compounds The shape of the molecule is determined by the repulsion between the electrons that the atoms share Example: BH 3 When it forms a molecule, the H atoms are 360/3= 120º apart

A Lewis dot drawing cannot accurately show 3-dimensional shapes

VSEPR Theory Valence Shell Electron Pair Repulsion The shape of a molecule is determined by the repulsion between the electron of the bonded atoms

Molecules containing a central atom Lone pairs of electrons on a central atom influence shape They are more repulsive than bonded electrons because they flare (take up more space) Their influence must be taken into consideration when determining the shape of the molecule

Different shapes 1.Molecules with only two atoms will always be linear 2.Molecules with three atoms can have two different shapes Example: BeCl 2 H2OH2O

This molecule is linear. The H 2 O molecule is “bent” or “angular”

Effect of the lone pairs on shape or H 2 O

Molecules with four atoms Can have two shapes Ex: NH 3 BeF 3

This one is trigonal pyramidal because of the lone pair of electrons This one is trigonal planar due to the absence of lone pairs on the central atom

Molecules with 5 atoms If there are no lone pairs on the central atom: Ex: CH 4 This is called tetrahedral

Molecules with 6 atoms (and no lone pairs on the middle atom) Ex.: PCl 5 This is called trigonal bipyramidal

Molecules with 7 atoms and no lone pairs on the central atom Ex.: SF 6 This is called octahedral

Predict the shapes of these molecules AlBr 3 PH 3 SO 2 CO 2

Draw and predict shape: CO 3 2- NO 3 - SO 3

Practice Make electron dot drawings of the following substances and predict the shape: CaCO 3 Mg 3 (PO 4 ) 2

Trigonal bypyramidal

Polarity of water

The atom with the smallest E.N. has a partial positive charge on it This is represented by the symbol δ+ The atom with the largest EN has a partial negative charge which is represented by the symbol What type of bond is formed between Na and F Ca and N C and I