Bonding Of the Chemical Kind

Bonding and Properties u Compounds are formed by fixed ratios of atoms forming chemical bonds (Dalton) u When elements form chemical bonds, their chemical and physical properties are changed u The properties of the resulting compounds depend on the elements making up the compound and the kind of bonds formed.

Forming Bonds u In order to react (form a bond), atoms must collide. u When collisions occur, the valence electrons touch first. u Valence electrons (outside shell) determine the type and number of bonds.

Octet Rule uEuEvery atom “strives” to have eight electrons in its valence shell. AAtoms or ions are more stable (less reactive) with eight valence electrons because of symmetry AA full valence shell can be had by:

u Stealing enough of someone else’s electrons to make eight valence electrons – nonmetals take from metals u Giving away all valence electrons to expose a full shell underneath – metals give to nonmetals u These both happen in ionic bonding.

Octet rule and Bonding u Sharing electrons. Shared electrons “count” for both atoms – happens between nonmetals. u Shared electrons make a covalent bond (2e - per bond)

Ionic Bonding u When metals lose valence electrons, a positive ion called a cation is formed. u Metals usually lose all their valence electrons. u The resulting positive charge is equal to the number of electrons lost.

Ionic Bonding u When nonmetals gain extra valence electrons, a negative ion called an anion is formed. u Nonmetals gain enough electrons to fill their valence shell. u The negative charge that results is equal to the number of electrons gained.

Ionic Bonding u Ionic bonds are formed by the electrostatic attraction between oppositely charged ions. Na + + Cl - NaCl u Compounds containing ionic bonds are called salts. u Salts are composed of a metal (+) and a nonmetal (-). u Ionic compounds form a crystal lattice. u Ionic bonds are broken when the salt is dissolved in water or melted.

Properties of Ionic Compounds u High melting point This is because the ionic bonds must be broken in order to melt the salt. u Molten salts conduct electricity u Many are soluble in water u All water solutions of salts conduct electricity

Representing Ions uTuThe Lewis dot structure of positive metal ions is the same as the atom minus the valence electrons, and a charge is added. uNuNa + Mg +2 Al +3 Sn +4 uTuThe Lewis dot structure of nonmetal ions is the same as the atom except that the gained valence electrons and a negative charge are added.

u Examples of nonmetal (negative) ions: F: : : : - O: -2 : : : uFor most purposes the dots can be left off.

Formulas of Ionic Compounds u Ionic compounds are electrically neutral u Ionic compounds are made of charged ions uThe sum of the negative charges must equal the sum of the positive charges

Formulas of Ionic Compounds

Names of Ionic Compounds u Metal ions are named after the metal – Na + is the sodium ion u Nonmetal ions are named after the nonmetal, with the ending changed to “-ide” – Cl - is the chloride ion u Salts are named with the metal first, then the nonmetal with the “-ide” ending – CdS is called “cadmium sulfide”.

Covalent Bonds u 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. u In Lewis structures a single bond (two electrons) is represented by a line. H-H (same as H 2 )

Covalent Bonds u A double bond is two lines, and a triple bond is three lines. O=O : : : : O 2, or oxygen gas :N  N: N 2, or nitrogen gas

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

Octet Rule Lewis Structures u The electrons in the bonds can be replaced with lines. uThe extra electrons on oxygen are called “lone pairs”.

Properties of covalent compounds u Covalent compounds form molecules – small groups of atoms bonded together u When solid molecular compounds (having covalent bonds) melt, boil or dissolve, the covalent bonds do not break. u Instead, the molecules separate from each other.

Properties of covalent compounds uMuMolecules are stuck to each other with intermolecular forces. uTuThese are much weaker than chemical bonds uCuCovalent molecular compounds tend to have low melting points uTuThey are insulators, and water solutions will not conduct electricity