Ionic vs. Covalent Bonding and Intermolecular Forces Foothill Chemistry

Ionic vs. Covalent Bond Ionic Bond - Chemical bonding that results from the electrical attraction between cations and anions (opposites attract) Covalent Bond – Chemical bonding from the sharing of electron pairs between two atoms (atomic tug of war) Most bonding is somewhere between ionic and covalent, depending on the strength of the atoms for attracting electrons.

Using Electronegativity to Predict Bond Character You’ve seen it before, you’ll see it again Calculate the difference in electronegativity between atoms to determine bond character

Molecular Compounds (Covalently Bonded Compounds) The composition of a compound is given by its chemical formula. A chemical formula indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts. A molecular formula shows the types and numbers of atoms combined in a single molecule of a molecular compound.

Ionic Compounds An ionic compound is composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal. (The sum of the ionic charges = zero.) In contrast to a molecular compound, an ionic compound is not composed of independent, neutral units that can be isolated. The chemical formula of an ionic compound represents not molecules, but the simplest ratio of the compound’s ions. A formula unit is the simplest collection of atoms from which an ionic compound’s formula can be established.

Crystal Lattice – Ionic Compounds In an ionic crystal, ions minimize their potential energy by combining in an orderly arrangement known as a crystal lattice. Attractive forces exist between oppositely charged ions within the lattice. Repulsive forces exist between like-charged ions within the lattice. The combined attractive and repulsive forces within a crystal lattice determine: the distances between ions the pattern of the ions’ arrangement in the crystal

Crystal Lattices

Ionic Bond Strength Lattice Energy – Amount of energy released when one mole of an ionic crystalline compound is created from gaseous ions

Chemical Formulae Molecular Formula – Covalent Compound Formula Unit – Ionic Compound Chemical Formula – Umbrella Term for Both Other Types of Formulae (Formulas) Lewis Structure Structural Formula - Like a Lewis structure, but does not show electrons Empirical Formula – Shows the composition of a compound in terms of the simplest ratio of atoms

Intermolecular Forces Forces of attraction between molecules (not just atoms) Weaker than the bonding forces of molecular compounds (covalent bonds) Weaker than the bonding forces of ions in ionic compounds (attractive forces) Weaker than the bonding forces in metal atoms in metal solids (metallic bonds) The boiling points of various molecules give an estimation of the relative strength of the intermolecular force. The higher the boiling point, the stronger the intermolecular force.

Relative Boiling Points

Dipole The strongest intermolecular forces exist between polar molecules. Because of their uneven charge distribution, polar molecules have dipoles. A dipole is created by equal but opposite charges that are separated by a short distance. The direction of a dipole is from the dipole’s positive pole to its negative pole A dipole is represented by an arrow with its head pointing toward the negative pole and a crossed tail at the positive pole. The dipole created by a hydrogen chloride molecule is indicated as follows: H – Cl

Polar vs. Nonpolar Molecules

Dipole-Dipole Force The negative region in one polar molecule attracts the positive region in adjacent molecules. So the molecules all attract each other from opposite sides Dipole-dipole forces act at short range, only between nearby molecules.

Hydrogen Bonding A hydrogen atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.

London Dispersion Forces The weakest of the intermolecular forces. Occurs with all molecule types, even noble gases and nonpolar molecules. Result from the constant motion of electrons and the creation of instantaneous dipoles. The London dispersion forces increase as the number of electrons in that atoms or molecules increase (higher forces for bigger atoms).