bonding. why do elements bond? bonding results in greater stability through sharing electrons-...
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Bonding
Why do elements bond?
Bonding results in greater stability Through sharing electrons- covalent Through transfer of electrons- ionic
Compounds have different properties than elements Sodium Chloride vs. Sodium and Chlorine
Why do elements bond?
Ex. Sodium is a soft, slivery metal that combines with chlorine, a greenish-yellow gas, to form sodium chloride which is a white crystalline solid
Hydrogen is a flammable gas that combines with oxygen, another gas, to form water, a liquid
Bonding & Stability
Octet Rule- elements will bond to reach a stable number of valence electrons- 8
Exceptions: H, He, Li, Be, B stable with 2 S and P often have more than 8 Period 3 and beyond sometimes have more
than 8 Noble gases don’t form compounds because
they have 8 valence electrons already They are stable alone
Covalent Bonds
Covalent Bonds- sharing of electrons to become stable Results in a molecule- 2 or more atoms covalently
bonded Occurs between 2 nonmetals Have relatively low melting points so often liquid or gas
at room temperature No ions, so do not conduct if dissolved in water
Single Covalent bond- sharing of 2 electrons Longest and weakest of the 3 covalent bonds
Double Covalent bond- sharing of 4 electrons Triple Covalent bond- sharing of 6 electrons
Shortest and strongest
Covalent Bonds
Covalent Bonds
Covalent Bonds
Covalent Bonds
http://www.visionlearning.com/library/module_viewer.php?mid=55
Covalent Bonds- Polarity Atoms may share electrons equally- nonpolar covalent
Atoms have no partial charges Molecules that are nonpolar have weak attractions
for each other (called London Dispersion Forces) Electronegativity difference of 0.4 or less
Atoms may share electrons unequally- polar covalent Atoms have partial charges Molecules that are polar have a stronger attraction
for each other (called Dipole-Dipole forces) Electronegativity difference of over 0.4 within a
covalent bond
Polar vs. Nonpolar
http://www.school-for-champions.com/chemistry/polar_molecules.htm
Chemical Formulas
Chemical formula- used to tell us how many of each element are in a compound Consists of element symbols and subscripts
Water has 2 hydrogen atoms and 1 oxygen atom
Chemical Formulas
SiO2
silicon- 1 Oxygen-2 C12H22O11
Carbon- 12 Hydrogen-22 Oxygen-11 N2O
Nitrogen- 2 Oxygen-1
Writing Lewis Dot Structures for covalent compounds Count up total number of valence electrons Determine the central atom
C is often the central atom For now, the single atom will be the central atom
Bond atoms to satisfy the octet rule first Fill in valence electrons If all atoms are not stable, you’ll need to try double or
triple bonds If you have extra electrons, put them on the central
atom For more detailed rules:
http://antoine.frostburg.edu/chem/senese/101/bonds/faq/simple-lewis-structures.shtml
STOP HERE TO PRACTICE
Ionic Compounds
Ionic compound - composed of cations and anions Metal (cation) & nonmetal (anion) OR metal &
polyatomic ion Polyatomic Ions- groups of atoms that are covalently
bonded and collectively have a charge Examples of polyatomic ions: SO4
2- PO43-
Ionic Compounds
Ionic compound - composed of cations and anions Metal (cation) & nonmetal (anion) OR metal &
polyatomic ion Neutral overall
Total positive charge = total negative charge Solid crystals at room temperature Melt only at very high temperatures
Ionic Bond: The electrostatic forces that hold ions together in ionic compounds
Properties of Ionic Compounds
The orderly arrangement of component ions produces the beauty of crystalline solids.
7.2
Ionic Bond Formation
Sodium Chlorine Sodium Chloride
http://www.dac.neu.edu/physics/b.maheswaran/phy1121/data/ch09/anim/anim0904.htmCLICK FOR ANIMATION OF IONIC and COVALENT BONDING
Compare and Contrast Ionic and Covalent Bonding
IONIC COVALENT
High melting point Low melting point
Brittle solids at room temp. Often liquids or gases at room temp
Combination of metal and nonmetal
Combination of 2 nonmetals
Smallest unit: Formula unit (ratio of ions)
Smallest unit: Molecule
Named without prefixes Named using prefixes
2 elements always have the same structure
2 elements can often bond in several ways
Conduct when melted or dissolved
Not conductive at all