CHEMICAL BONDS

Chemical Bond Mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. Ionic Bond Bonding that results from the electrical attraction between large numbers of cations and anions A large difference in electronegativity between two atoms in a bond will result in ionic bonding

Covalent (Molecular) Bond Sharing electrons pairs between two atoms A small difference in electronegativity between two atoms in a bond will result in covalent bonding. Electronegativity A measure of the ability of an atom in a chemical compound to attract electrons.

Valence Electrons Electrons in the highest occupied energy level of an elements’ atom Determines the chemical properties of an element Examples:

The Octet Rule Atoms tend to gain, lose, or share enough electrons to become surrounded by eight valence electrons Metallic elements lose electrons Non-metallic elements gain electrons or share them

Electron Dot Notation An electron configuration notation in which only the valence electrons of an atom of a particular element are shown Indicated by dots placed around the element’s symbol Example:

Learning Check A. X would be the electron dot formula for  A. X would be the electron dot formula for 1) Na 2) K 3) Al   B.  X  would be the electron dot formula 1) B 2) N 3) P

Dot notation can be used to represent molecules Example: (H : H) represents a shared electron pair An unshared pair or lone pair is a pair of electrons that is not involved in bonding and belongs exclusively to one atom Example: Lone Pair

Lewis Structures Electron distribution is depicted with Lewis electron dot structures Electrons are distributed as shared or Bond Pairs or unshared or Lone Pairs

Steps: Write electron dot notation for each atom in molecule. Determine total number of valence electrons. Arrange atoms to form skeleton structure for molecule. If Carbon is present it will go in the center. Otherwise least electronegative atoms will be in the center. Hydrogen never is in the center

Steps Continued Connect atoms by electron-pair bonds Add unshared pairs of electrons so each nonmetal is surrounded by 8 electrons Count the electrons to see it matches the number of valence electrons If too many electrons create double or triple bonds.

Lewis Structure Bond Formula Electrons are shared and represented by a dash lone electrons are represented by dots. To determine how many bonds exist in a molecule, use the following formula: N-A = # of Bonds 2

N-A = # of Bonds 2 Where: N = # of needed electrons, ( 8 for all elements but H, which is 2.) A = # of available electrons (the number of valence electrons.)

The least electronegative element is the central atom.

Example: Write the Lewis structure of NH3 The total number of valence electrons is: The number of electrons needed is: A = 8 1 x 8 = 8 N = 14 3 x 2 = 6

The Skeleton structure is: N – A 2 14 – 8 2 = 3 Bonds

Connect the atoms with electron pairs Connect the atoms with electron pairs. Remember 8 electrons are needed to obey the octet rule Finish the structure by using the remaining electrons as lone pairs Check that the final Lewis structure has the correct number of valence electrons (8) except Hydrogen (2)

Write the Lewis structure of H2CO Needed (N) Available (A) 2 (H) 2 x 1 = 2 2 (H) 2 x 2 = 4 1 (C) 1 x 8 = 8 1 (C) 1 x 4 = 4 1 (O) 1 x 6 = 6 1 (O) 1 x 8 = 8 A = 12 N = 20

Write the Lewis structure of H2CO cont. H | C= O | H N – A 2 20 – 12 2 = 4 Bonds

This is the chlorine molecule, Cl Cl This is the chlorine molecule, Cl2 or Single Covalent Bond Covalent bond produced by the sharing of one pair of electrons between two atoms

Multiple Covalent Bonds Double Bond Covalent bond produced by the sharing of two pairs of electrons between two atoms Shown by either two side-by-side pairs of dots or by two parallel dashes

O O = For convenience, the double bond can be shown as two dashes.

Triple Bond Covalent Bond produced by sharing of three pairs of electrons between two atoms Carbon forms a number of compounds containing triple bonds Lewis structures for molecules that contain carbon, oxygen or nitrogen, remember that multiple bonds between pairs are possible

If too many electrons have been used, subtract one or more pairs until the total number of valence electrons is correct. Then move one or more lone electron pairs to existing bonds between non-hydrogen atoms until the outer shells of all atoms are completely filled.

Ionic Bonding & Ionic Compounds Composed of positive (cations) and negative (anions) ions that are combined so that the numbers of positive and negative charges are equal.

Most ionic compounds exist as crystalline solids. A crystal of any ionic compound is a 3-D network of positive and negative ions mutually attracted to each other. Formation of an ionic bond can be viewed as a transfer of electrons

Ionic Bonds: One Big Greedy Thief Dog!

Crystal Lattice In an ionic compound, the ions minimize their potential energy by combining in an orderly arrangement. The distance between ions and their arrangement in a crystal represents a balance among all forces.

Properties of Ionic Compounds High melting points Solids at room temperature Soluble in polar solvents, insoluble in Nonpolar solvents Molten compounds & aqueous solutions conduct electricity

Metallic Bonding Chemical bonding is different in metals than it is in ionic, molecular or covalent network compounds

Metallic Bonding Chemical bonding that results from the attraction between metal atoms and the surrounding sea of electrons An attraction of the free-floating valence electrons for the positively charged metal ions.

Metallic Properties Good conductors of electricity and heat Malleability Ability of a substance to be hammered or beaten into thin sheets Ductility Ability of a substance to be drawn, pulled, or extruded through a small opening to produce a wire.