Bonding Homework: Bond Types Worksheet Quiz on Friday Test next Friday

Valence Electrons Valence electrons are the electrons in the highest occupied energy level of an element’s atoms. The number of valence electrons largely determines the chemical properties of an element. To find the number of valence electrons in an atom you look at the last number in the electron configuration.

The Octet Rule Octet rule: In forming compounds, atoms tend to achieve the electron configuration of a noble gas. -Metals tend to lose their electrons in order to achieve a complete octet -Non-metals tend to gain or share electrons in order to complete an octet

The electron configuration of the sodium ion is the same as that of a neon atom. Na Na Ne 2-8 Atoms want to get the electron configuration of the nearest Noble Gas Formation of Cations

Cations of Group 1 elements always have a charge of 1+. Cations of group 2 elements always have a charge of 2+.

Formation of Anions Both a chloride ion and the argon atom have an octet of electrons in their highest occupied energy levels. Cl Cl Ar 2-8-8

Energy and Chemical Bonds Chemical bonds are forces that hold compounds together Energy is needed to overcome these attractive forces and separate the atoms in a compound –Breaking a chemical bond is Endothermic –Formation of a bond is Exothermic The less energy released when a bond is broken the more stable the bond.

Example: –B + C  BC joules –X + Y  XY joules Which bond is more stable?. –The first bond is more stable Why? –It is more stable because it releases less energy.

Electronegativity Electronegativity – the attraction an atom has for electrons –Higher electronegativity the greater the attraction for electrons –Lower electronegativity the less attraction for electrons Electronegativity differences in bond –Greater the difference the more ionic in character, until no longer sharing electrons, but electrons have been transferred Difference of 1.7 or more = Ionic bond Difference of 1.6 or less = covalent bond

Distinguishing Bond Types To tell whether substance contains metallic, covalent, or ionic bonds one has to look at the properties (conductors, melting/boiling points, hardness) and remember: –Metallic bonds = Metal –Covalent = Nonmetals –Ionic = Metal and Nonmetal

Conductivity Bond TypeMelting/ Boiling Point HardnessSolidLiquidAqueous MetallicHighHardYes CovalentLowSoftNo IonicHighHardNoYes

Formation of Ionic Compounds Compounds composed of cations and anions are called ionic compounds. –Although they are composed of ions, ionic compounds are electrically neutral.

Formation of Ionic Compounds Aluminum metal and the nonmetal bromine react to form an ionic solid, aluminum bromide.

Formation of Ionic Compounds –Ionic Bonds The electrostatic forces that hold ions together in ionic compounds are called ionic bonds.

Ionic Bonding Ionic bond is formed when ions bond together because of the electrostatic attraction of oppositely charged ions Key phrases to remember: –Transfer of electrons –Metal and Nonmetal –Ions –Electronegativity difference of 1.7 or greater

Ion Formation Metals –Lose electrons –Postively charged ions –Octet of electrons –Radii decreases Nonmetals –Gain electrons –Negatively charged ions –Octet of electrons –Radii increases

Formation of Ionic Compounds –Animation 8 Take an atomic-level look at the formation of KCl.Animation 8

Formation of Ionic Compounds –Formula Units –A chemical formula shows the kinds and numbers of atoms in the smallest representative unit of a substance. –A formula unit is the lowest whole- number ratio of ions in an ionic compound.

Properties of Ionic Compounds –Most ionic compounds are hard crystalline solids at room temperature. –Ionic compounds generally have high melting and boiling points.

Properties of Ionic Compounds The orderly arrangement of component ions produces the beauty of crystalline solids.

Properties of Ionic Compounds –Simulation 5 Simulate the formation of ionic compounds at the atomic level.Simulation 5

Properties of Ionic Compounds –Ionic compounds can conduct an electric current when melted or dissolved in water because electrons are able to move.

Properties of Ionic Bonds High melting points and high boiling points Hard Conduct heat and electricity when in the liquid and aqueous phase –Electrons are able to move