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S
UNIT 4
Bonding and Stereochemistry
Stable Electron Configurations
All elements on the periodic table (except for Noble Gases) have incomplete outer energy levels Valence electrons- electrons in outer energy level of atom
Elements will gain, lose, or share electrons to get full outer levels (octet rule) Eight electrons = STABLE!!!
Electron dot diagrams help to visualize valence electrons Symbol represents nucleus and inner electrons Dots represent valence electrons Group # = valence electrons
Drawing Electron Dot Diagrams
Determine number of valence electrons from periodic table
Draw the symbol for the element
Place dots around the symbol, one per side, until all valence electrons are accounted for
Example- Aluminum with 3 valence electrons
Al
Ions
Charged atoms where the number of protons and electrons is not equal
Charge indicates how many electrons are added or subtracted Negative charge- ADD electrons Positive charge- SUBTRACT electrons Example Sodium ion
Na atomic number 11 = 11 electrons Na+ subtract one electron = 10 electrons
Chemical Bonds
Forces that hold groups of atoms together and make them function as a unit.
A bond will form if the energy of the pairing is lower than that of the separate atoms.
Some elements have stronger attractions to e- when bonded ELECTRONEGATIVITY (EN)
Relative attraction an atom has for shared electrons in a covalent bond
Unit- paulings Arbitrary number used for comparison purposes
F is 4.0, Cs/Fr 0.7
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Increase from left to right in a period- nonmetals higher than metals
Decrease from top to bottom in a group Metals on left side and nonmetals on right side most
reactive (alkali metals and halogens) Electrons attracted to the higher EN element Using EN to predict bonds
Ionic Bonds- Metal + nonmetal Covalent Bonds (nonmetal + nonmetal)
Polar- EN is different Nonpolar- EN is same value
Types of Chemical Bonds
Ionic Bonds Some elements achieve stable configurations by transferring
electrons Example- sodium and chlorine Sodium 1 valence electron Chlorine 7 valence electrons Both want to be stable Sodium will lose the one electron, and chlorine will gain that electron, forming
IONS (atoms that have gained or lost electrons)
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Na+ Cl-
Charge on ion represented by + or – sign Positive ion- cation Negative ion- anion (use suffix –ide) Na+Cl- is sodium chloride (NaCl) Groups 1, 2, and 3 will lose electrons Groups 5, 6, and 7 will gain electrons Group 4 will go either way- usually share
though
Ionic compounds- compounds that contain ionic bonds Can be made with single
elements or polyatomic ions
Empirical formula- shows ratios of ions contained in the bond Na+Cl- one to one NaCl Mg2+Cl- one to two MgCl2
Crystal Lattices Each ionic compound
makes specific shape based on arrangement
Crystal- solid whose particles are arranged in a lattice structure (NaCl- cubes, ruby- hexagonal)
Properties of ionic compounds High melting point, boiling
point Poor conductor when
solid, good when molten/dissolved
Crystal structure- shatters when hit
Covalent Bonds Nonmetals have high ionization energy
Don’t usually form ions-share electrons to get to stable energy level
Covalent bond-chemical bond in which two atoms share a pair of valence electrons May share one (single bond), two
(double bond), or three pairs (triple bond)
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Form molecules Neutral group of atoms that are joined together
by one or more covalent bonds May exist as diatomic molecules
Made of 2 atoms of same element
May form single or multiple bonds
Molecular Formula- expression of the number and type of atoms that are present in a single molecule of a substance.
Subscript tells how many of each element are present N2O- 2 atoms of N, 1 atom of O
When atoms share electrons, they rarely share equally One element will “attract” electrons more than the
others Polar Covalent Bond- a covalent bond in which electrons
are not shared equally Atom with greater attraction gets a partial negative charge (δ-),
lesser attraction partial positive charge (δ+)
Metallic “bonding”
Attraction of metal atoms and the sea of electrons surrounding them
Gives metals their properties Malleability Good conductors
Writing Lewis Diagrams for Molecules
Steps Count all valence e- Draw skeleton structure Put a pair of e- between all atoms to show a covalent
bond All should have 8 (exc H which has 2)
Distribute lone pairs around atoms (exc H) If an atom needs more e- then move pairs between
atoms to get 8 Most multiple bonds are in C, N, and O Double bond (share two), triple bond (share three)
A double covalent bond, or simply a double bond, is a covalent bond in which two pairs of electrons are shared between two atoms.
Double bonds are often found in molecules containing carbon, nitrogen, and oxygen.
A double bond is shown either by two side-by-side pairs of dots or by two parallel dashes
A triple covalent bond, or simply a triple bond, is a covalent bond in which three pairs of electrons are shared between two atoms.
• example 1—diatomic nitrogen:
• example 2—ethyne, C2H2:
Stereochemistry- VSEPR Theory
All molecules have 3D shape Stereochemistry- study of shapes of molecules
VSEPR theory Valence Shell Electron Pair Repulsion Helps to understand and predict molecuar geometry
(from Lewis Dot diagrams) Developed by Gillespie and Nyholm in 1956-57 Rules based on the idea that the arrangement in
space of the covalent bonds formed by an atom depends on the arrangement of valence e- e- try to push each other far away while still bonding to central atom
Restricted VSEPR rules Valence e- pairs (both shared and lone) arrange
themselves around the central atom in a molecule in such a way as to minimize repulsion (as far away from each other as possible)
When predicting molecular geometry, double and triple bonds act like single bonds
Lone pairs of e- occupy more space than bonding e-
Steps to draw VSEPR molecules Draw Lewis Diagram Determine the central atom (lowest EN) Count the number of bonding and lone pairs surrounding the
central atom Multiple bonds count as one pair
Shape molecule in order to minimize repulsion Find on VSEPR chart
EXAMPLES
Water, H2O
2 bond pairs
2 lone pairs The molecular geometry is
BENT.
H O H••
••
Carbon Dioxide CO2
As a Lewis Dot diagram:
Has two electron pairs - two sets of double bonds – two bonding pairs
Creates a LINEAR shape according to VESPR