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