Comparing Properties of Ionic and Molecular Compounds

Properties of Matter Macroscopic properties of matter vary greatly due to the type of bonding

What is a chemical bond? An attractive force that holds two atoms together Can form by – The attraction of positive ion to a negative ion or – The attraction of the positive nucleus of one atom and the negative electrons of another atom

Three Types of Bonding Metallic Covalent Ionic

Formula Unit The smallest whole number ratio of ions in an ionic compound. Ions surround each other forming a crystal lattice

Ionic Solids Ionic solids are solids composed of ionic particles (ions). These ions are held together in a regular array by ionic bonding. Ionic bonding results from attractive interactions from oppositely charged ions. In a typical ionic solid, positively charged ions are surrounded by negatively charged ions and vice-versa. The close distance between these oppositely charged particles results in very strong attractive forces. The alternating pattern of positive and negative ions continues in three dimensions. Forms a crystal lattice.

Properties of Ionic Compounds Crystalline solids (made of ions) High melting and boiling points Conduct electricity when melted or dissolved in water Many are soluble in water but not in non- polar liquid

Comparison of Conductivity

Common Ionic Compounds – NaCl - sodium chloride - table salt – KCl - potassium chloride - present in "light" salt (mixed with NaCl) – CaCl 2 - calcium chloride - driveway salt – NaOH - sodium hydroxide - found in some surface cleaners as well as oven and drain cleaners – CaCO 3 - calcium carbonate - found in calcium supplements – NH 4 NO 3 - ammonium nitrate - found in some fertilizers

Drawing ionic compounds Draw the Lewis dot diagram for the atom Add or remove the number of electrons to make the atom an ion If the atom loses electrons, the outer shell will be empty, and no electrons will be shown around the symbol If the atom gains electrons, the outer shell will be a full octet, and the symbol should be surrounded by all 8 electrons Put square brackets around the symbol and electrons (if applicable) Write the ionic charge at the top right of the symbol Arrange the ions with opposite symbols surrounding each other, and show the correct number of each ion for the formula unit. Ex. AlCl 3

Properties of Covalent Compounds Gases, liquids, or solids (made of molecules) Low melting and boiling points Poor electrical conductors in all phases Many soluble in non- polar liquids but not in water

Molecular (Covalent) Substances

Drawing Molecular Compounds Begin with the Lewis dot diagram for the central atom (usually the first element in the chemical formula) Arrange the other elements around the unpaired electrons of the central atom Pair up the single electrons to form bonds Example: CBr 4

Drawing Covalent Compounds

Properties of Covalent Network Solids Covalent because combinations of nonmetals Interconnected very hard and brittle Insoluble Extreme melting and boiling points Diamond

Polar Covalent Bonds Covalent Bonds in which the sharing of the electron pair is unequal the electrons spend more time around the more nonmetallic atom In such a bond there is a charge separation with one atom being slightly more positive and the other more negative……. will produce a dipole moment.

Types of Covalent bonds Pure Covalent (also called non- polar covalent) bonds are ones in which both atoms share the electrons evenly By evenly, we mean that the electrons have an equal probability of being at a certain radius from the nuclei of either atom. Polar covalent bonds are ones in which the electrons have a higher probability of being in the proximity of one of the atoms Determined by Electronegativity Difference

Pure covalent or Non-polar covalent bond Electronegativity difference of 0.3 or less in between the two atoms. A pure covalent bond can form between two atoms of the same element (such as in diatomic oxygen molecule) or atoms of different elements that have similar electronegativies (such as in the carbon and hydrogen atom in methane).

Polar Covalent Bond A is a pair of electrons shared between two atoms with significantly different electronegativities (from 0.3 to 1.7 difference). These bonds tend to form between highly electronegative non-metals and other non-metals, such as the bond between hydrogen and oxygen in water.

Ionic Bonds In compounds that have elements with very different electronegativities (greater than 1.7 difference), the electrons can be considered to have been transferred to form ions.

Many of the properties of a compound, such as solubility and boiling point, depend, in part, on the degree of the polarity of its bonds.

Bond Character Nonpolar-Covalent bonds (H 2 ) Nonpolar-Covalent bonds (H 2 ) – Electrons are equally shared – Electronegativity difference of 0 to 0.3 Polar-Covalent bonds (HCl) Polar-Covalent bonds (HCl) – Electrons are unequally shared – Electronegativity difference between 0.3 and 1.7 Ionic Bonds (NaCl) Ionic Bonds (NaCl) – Electrons are transferred – Electronegativity difference of more than 1.7

Diatomic Molecules hydrogen gas H 2 the halogens: – chlorine Cl 2 – fluorine F 2 – bromine Br 2 – iodine I 2 Nitrogen N 2 Oxygen O 2 Pneumonic Device to remember the diatomic molecules: Professor BrINClHOF

Metals and Metallic Bonding Typical Properties of Metals – Malleable – Ductile – Good Conductors of Heat and Electricity – Generally high melting and boiling points

Metallic Bonds The properties of metals suggest that their atoms possess strong bonds yet the ease of conduction of heat and electricity suggest that electrons can move freely in all directions in a metal The general observations give rise to a picture of "positive ions in a sea of electrons" to describe metallic bonding.

Metallic Bonding Electron Sea Model Explained by the Electron Sea Model the atoms in a metallic solid contribute their valence electrons to form a “sea” of electrons that surrounds metallic cations. delocalized electrons are not held by any specific atom and can move easily throughout the solid. A metallic bond is the attraction between these electrons and the metallic cation.

Metallic Bonding the Electron Sea Model The more delocalized electrons the stronger the bond

Ionic CompoundsCovalent CompoundsMetallic Compounds -Formed from a combination of metals and nonmetals. -Electron transfer from the cation to the anion. -Opposite charged ions attract each other. -Formed from a combination of nonmetals. -Electron sharing between atoms. -Formed from a combination of metals -“sea of electrons”; electrons can move among atoms Solids at room temperature Can be solid, liquid, or gas at room temperature. Solids at room temperature High melting pointsLow melting pointsVarious melting points Dissolve well in water Do not dissolve in water (Sugar is an exception) Do not dissolve in water. Electrolytes: Conduct electricity when dissolved in water, or when molten Do not conduct electricity; non electrolytes Conduct electricity in solid form. Brittle, hardSoftMetallic compounds range in hardness. Group 1 and 2 metals are soft; transition metals are hard. Metals are malleable, ductile, and have luster.