Chemical Bonding

Chemical Bonding

Chemical Bonding Covalent Bonding and Molecular Compounds A molecule is a neutral group of atoms that are held together by covalent bonds. It can consist of two or more atoms of the same element (like oxygen) or the atoms of two or more elements combined (like

water). Any compound whose simplest units are molecules is called a molecular compound.

A molecular formula shows the types and numbers of atoms combined in a single molecule of a molecular compound. H2 O C6H12O6 A diatomic molecule is a molecule containing only two atoms.

There are 7 elements which form diatomic molecules: Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine Bonding occurs because most atoms have lower potential energy when they are

bonded to other atoms. A covalent bond is formed when potential energy is at a minimum That minimum occurs where there is a balance between attractive and repulsive forces Characteristics of covalent

bonds The distance between two bonded atoms is the bond length Bond energy is the energy required to break a chemical bond In general, shorter bond lengths have higher bond energy.

The Octet Rule Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level. As with most rules, there are exceptions to

the octet rule. Hydrogen forms bonds in which it is surrounded by only 2 electrons Boron tends to form bonds in which it is surrounded by 6 electrons

Sometimes an element can be surrounded by more than 8 electrons In these cases of expanded valence, bonding involves electrons in d orbitals as well as the s and p orbitals Some examples are PF5 and SF6 Electron Dot Notation Covalent bond formation usually involves

only the valence electrons Electron dot notation shows just the valence electrons of an atom with the electrons pictured as dots surrounding the symbol: Lewis Structures Electron dot notation can be used to represent molecules

A pair of dots between two elements is the shared electron pair of the covalent bond The bond can also be represented as a dash. Each dash counts as two electrons

Lewis structures are formulas in which: atomic symbols represent nuclei and inner-shell electrons dot-pairs or dashes between two symbols represent electron pairs in covalent bonds dots adjacent to only one atomic symbol

represent unshared electrons Lewis structures can be drawn if you know the composition of the molecule and which atoms are bonded to each other. Drawing Lewis structures

1. Determine the type and number of atoms in the molecule 2. Count up the total number of valence

electrons in the molecule 3. Arrange the atoms and draw covalent bonds If carbon is present, it always goes in the middle Hydrogen never goes in the middle If carbon is not present, the least electronegative element goes in the middle

4. Add unshared pairs of electrons so that each is surrounded by an octet (except H which only gets 2 electrons) 5. Count the electrons to be sure that the number in the structure is the same as the

total number of valence electrons that you calculated in step 2 If not, you need a double or triple bond in your structure H2O

Cl2 CH3I A double bond is a covalent bond in which

two pairs of electrons are shared between two atoms. A double bond is represented by two parallel dashes A triple bond is a covalent bond in which three pairs of electrons are shared between two atoms

A triple bond is represented by three parallel dashes Double and triple bonds are called multiple bonds

Multiple bonds have shorter bond lengths and higher bond energies than single bonds They are most commonly found in molecules that contain carbon, nitrogen, or

oxygen O2 CO2

N2 Resonance Structures Some molecules and ions cannot be represented by just one Lewis structure These molecules have a multiple bond

which can be placed in two or more places in the molecule. Example - ozone O3

Chemists used to think that ozone spent its time alternating between the two structures, or resonating between them. Now chemists know that the bonds in ozone are identical to each other Ozones structure is actually an average of the two possible Lewis structures

Together, the structures are called resonance structures or resonance hybrids To represent resonance, draw both Lewis structures and put a double-headed arrow between them.

SO2

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