CHEM 121 Module 3

CHEM 121 Foundations of General Chemistry

ions

atoms or molecules bearing an electrical charge

cation

(a positive ion) forms when a neutral atom loses one or more electrons from its valence shell

anion

(a negative ion) forms when a neutral atom gains one or more electrons in its valence shell

ionic compounds

(or salts) compounds composed of ions

ionic bonds

electrostatic forces of attraction between oppositely charged cations and anions

ionic solids

exhibit a crystalline structure and tend to be rigid and brittle; they also tend to have high melting and boiling points, which suggests that ionic bonds are very strong, are also poor conductors of electricity, dissolve readily in water

binary ionic compounds

composed of just two elements

metal

which forms the cations

nonmetal

which forms the anions

ionic bonding

electrostatic attraction of oppositely charged ions that are typically produced by the transfer of electrons between metallic and nonmetallic atoms

covalent bonds

formed between two atoms when both have similar tendencies to attract electrons to themselves

bond length

determined by the distance at which the lowest potential energy is achieved

pure covalent bonds

equal probability of being near each nucleus.

polar covalent bond

unequal distribution of electrons, characterized by a partial positive charge on one atom and a partial negative charge on the other

electronegativity

measure of the tendency of an atom to attract electrons (or electron density) towards itself

electron affinity

measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol

< 0.4

pure covalent

between 0.4 and 1.8

polar covalent

> 1.8

ionic

nomenclature

ollection of rules for naming things, is important in science and in many other situations

binary compounds

those containing only two elements

–ide

the name of the metal followed by the name of the anion (the name of the nonmetallic element with its ending replaced by the suffix

hydrates

ionic compounds that contain water molecules as integral components of their crystals

binary acid

comprised of hydrogen and one other nonmetallic element

hydro-

The word “hydrogen” is changed to the prefix

-ic

the other nonmetallic element name is modified by adding the suffix

acid

the word — is added as a second word

oxyacids

compounds that contain hydrogen, oxygen, and at least one other element

Lewis symbol

consists of an elemental symbol surrounded by one dot for each of its valence electrons

Lewis structures

drawings that describe the bonding in molecules and polyatomic ions

lone pairs

electrons that are not used in bonding

octet rule

the tendency of main group atoms to form enough bonds to obtain eight valence electrons

double bond

forms when two pairs of electrons are shared between a pair of atoms

triple bond

forms when three electron pairs are shared by a pair of atoms

free radicals

molecules that contain an odd number of electrons

hypervalent molecules

Molecules formed from these elements

formal charge

atom in a molecule is the hypothetical charge the atom would have if we could redistribute the electrons in the bonds evenly between the atoms

# valence shell electrons (free atom) − # lone pair electrons − ½ # bonding electrons

formal charge

molecular structure

the arrangement of atoms in a molecule or ion

bond angle

the angle between any two bonds that include a common atom, usually measured in degrees

bond distance

(or bond length) is the distance between the nuclei of two bonded atoms along the straight line joining the nuclei

Valence shell electron-pair repulsion theory (VSEPR theory)

enables us to predict the molecular structure, including approximate bond angles around a central atom, of a molecule from an examination of the number of bonds and lone electron pairs in its Lewis structure

electron geometry

differentiate between these two situations by naming the geometry that includes all electron pairs

molecular geometry

the structure that includes only the placement of the atoms in the molecule