Bonding

Of the Chemical Kind

Bonding and Properties Compounds are formed by fixed ratios of

atoms forming chemical bonds (Dalton) When elements form chemical bonds,

their chemical and physical properties are changed

The properties of the resulting compounds depend on the elements making up the compound and the kind of bonds formed.

Forming Bonds In order to react (form a bond),

atoms must collide. When collisions occur, the valence

electrons touch first. Valence electrons (outside shell)

determine the type and number of bonds.

Octet Rule Every atom “strives” to have eight

electrons in its valence shell. Atoms or ions are more stable (less

reactive) with eight valence electrons because of symmetry

A full valence shell can be had by:

Octet Rule Stealing enough of someone else’s

electrons to make eight valence electrons – nonmetals take from metals

Giving away all valence electrons to expose a full shell underneath – metals give to nonmetals

These both happen in ionic bonding.

Octet rule and Bonding Sharing electrons. Shared

electrons “count” for both atoms – happens between nonmetals.

Shared electrons make a covalent bond (2e- per bond)

Ionic Bonding When metals lose valence

electrons, a positive ion called a cation is formed.

Metals usually lose all their valence electrons.

The resulting positive charge is equal to the number of electrons lost.

Ionic Bonding When nonmetals gain extra

valence electrons, a negative ion called an anion is formed.

Nonmetals gain enough electrons to fill their valence shell.

The negative charge that results is equal to the number of electrons gained.

Ionic Bonding Ionic bonds are formed by the electrostatic

attraction between oppositely charged ions.Na+ + Cl- NaCl

Compounds containing ionic bonds are called salts.

Salts are composed of a metal (+) and a nonmetal (-).

Ionic compounds form a crystal lattice. Ionic bonds are broken when the salt is

dissolved in water or melted.

Properties of Ionic Compounds

High melting point• This is because the ionic bonds must

be broken in order to melt the salt. Molten salts conduct electricity Many are soluble in water All water solutions of salts conduct

electricity

Representing Ions The Lewis dot structure of positive

metal ions is the same as the atom minus the valence electrons, and a charge is added.

Na+ Mg+2 Al+3 Sn+4

The Lewis dot structure of nonmetal ions is the same as the atom except that the gained valence electrons and a negative charge are added.

Representing Ions Examples of nonmetal (negative)

ions:

F:

:

:: - O:-2::

:For most purposes the dots can be left off.

Formulas of Ionic Compounds

Ionic compounds are electrically neutral Ionic compounds are made of charged ions

K+ + Br- KBrThe sum of the negative charges must equal the sum of the positive charges

Formulas of Ionic Compounds

Na+ + O-2 ?

2 Na+ + O-2 Na2O

(two positive charges, two negativecharges)

Names of Ionic Compounds

Metal ions are named after the metal – Na+ is the sodium ion

Nonmetal ions are named after the nonmetal, with the ending changed to “-ide” – Cl- is the chloride ion

Salts are named with the metal first, then the nonmetal with the “-ide” ending – CdS is called “cadmium sulfide”.

Covalent Bonds Covalent bonds are formed by atoms (two

nonmetals) sharing two more valence electrons.• A single bond contains two electrons (one

pair), a double bond consists of two pair of electrons, and a triple bond is made of three pairs of electrons.

In Lewis structures a single bond (two electrons) is represented by a line.

H-H (same as H2)

Covalent Bonds A double bond is two lines, and a

triple bond is three lines.

O=O::

::O2, or oxygen gas

:NN:N2, or nitrogen gas

Octet Rule Atoms share enough electrons to fulfill the

octet rule. Shared electrons count for both bonded atoms.

H. O:

H.

:

.. H:O:

H.

:

. H:O: H

::

Octet RuleLewis Structures

Lewis dot structure for water – every atom has a complete valence shell (8 electrons for O and 2 electrons for H)

Octet RuleLewis Structures

The electrons in the bonds can be replaced with lines.

The extra electrons on oxygen are called “lone pairs”.

Properties of covalent compounds

Covalent compounds form molecules – small groups of atoms bonded together

When solid molecular compounds (having covalent bonds) melt, boil or dissolve, the covalent bonds do not break.

Instead, the molecules separate from each other.

Properties of covalent compounds

Molecules are stuck to each other with intermolecular forces.

These are much weaker than chemical bonds

Covalent molecular compounds tend to have low melting points

They are insulators, and water solutions will not conduct electricity