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: SO42- 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 7.2 Properties of Ionic Compounds The orderly arrangement of component ions produces the beauty of crystalline solids. The beauty of crystalline solids, such as these, comes from the orderly arrangement of their component ions.

Ionic Bond Formation

Sodium Chlorine Sodium Chloride http://www.dac.neu.edu/physics/b.maheswaran/phy1121/data/ch09/anim/anim0904.htm CLICK FOR ANIMATION OF IONIC and COVALENT BONDING

Compare and Contrast Ionic and Covalent Bonding 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