Chapter 3 - Chemical Bonds

Chemical bond

- An attractive force that holds together the atoms, ions, or groups of atoms in a molecule or compound. - results in a lower energy than the total energy of the separate atoms.

Kekule structure

- Kekule determined the structure of benzene. It consisted of conjugated double and single bonds in a circle of six carbons resonance explains the different properties of benzene ... 1. Reactivity: Benzene does not undergo reactions typical of compounds with double bonds 2. Bond lengths: All the carbon-carbon bonds in benzene are the same length 3. Structural evidence: One 1,2 dichlorobenzene (in which chlorine atoms are attached to two adjacent carbon atoms) exists - an important consequence of resonance is that is stabilizes a molecule by lowering its total energy. This stabilization makes benzene less reactive than expected for a molecules with three carbon-carbon double bonds

- effective nuclear charge

- The effective nuclear charge is the net charge an electron experiences in an atom with multiple electrons. Higher energy electrons can have other lower energy electrons between the electron and the nucleus, effectively lowering the positive charge experienced by the high energy electron.

Resonance

- a blending of all three Lewis structures, with each bond intermediate in properties between a single and double bond. This blending of structures, which is called resonance, is depicted by double-headed arrows. The blended structure is a resonance hybrid of the contributing Lewis structures. - Resonance is a mental exercise and method within the Valence Bond Theory of bonding that describes the delocalization of electrons within molecules. It compares and contrasts two or more possible Lewis structures that can represent a particular molecule. Resonance structures are used when one Lewis structure for a single molecule cannot fully describe the bonding that takes place between neighboring atoms relative to the empirical data for the actual bond lengths between those atoms. The net sum of valid resonance structures is defined as a resonance hybrid, which represents the overall delocalization of electrons within the molecule. A molecule that has several resonance structures is more stable than one with fewer. Some resonance structures are more favorable than others.

hypervalent compounds

- a compound that contains an atom with more atoms attached to it than is permitted by the octet rule

formal charge

- a measure of the redistribution of electrons is the formal charge on an atom in a given Lewis structure - the charge it would have if the bonding were perfectly covalent in the sense that the atom had exactly a half-share in the bonding electrons - FC is assigned by establishing the "ownership" of the valence electrons of each atom in a molecule and comparing that ownership with the number of valence electrons on the free atom. An atom own one electron of each bonding pair attached to it and owns its lone pairs completely. The most plausible Lewis structure will be the one in which the formal charges of the atoms are lowest

biradical

- a molecule with two unpaired electrons. The unpaired electrons are usually on different atoms - relative orientations of the two electron spins are random

incomplete octet

- an unusual feature of the Lewis structure of the colorless gas boron trifluoride is that the boron atom has an incomplete octet: its valence shell consists of only six electrons - because fluorine has a high ionization energy, it is unlikely to exist with a positive formal charge. Experimental evidence, such as the short B-F bond lengths, suggests that the true structure of BF3 is a resonance hybrid of both types of Lewis structures, with the singly bonded structure making the major contribution.

multiple bonds

- double and triple bonds are collectively called multiple bonds

Delocalized

- electrons that are shown in different positions in a set of resonance structures are said to be delocalized. Delocalization means that a shared electron pair is distributed over several pairs of atoms and cannot be identified with just one pair of atoms. As well as delocalizing electrons over the atoms, resonance also lowers the energy below that of any single contributing structure and helps to stabilize the molecule. This lowering of energy occurs for quantum mechanical reasons. Broadly speaking, the wave function that describes the resonance structure is a more accurate description of the electronic structure of the molecule than the wave function for any single structure alone, and the more accurate the wave function, the lower the corresponding energy. rules for writing appropriate resonance structures 1. In each contributing structure, the nuclei are in the same positions; only the locations of lone pairs and bonding pairs are changed. 2. Structures with the same energy (so called "equivalent structures") contribute equally to the resonance 3. Low-energy structures contribute more to the resonance mixture than high energy structures

variable covalence

- elements that can expand their valence shell commonly show variable covalence, the ability to form different numbers of covalent bonds. Elements that have variable covalence can form one number of bonds in some compounds and a different number in others

Covalent bond

- if the lowest energy can be achieved by sharing electrons, then the atoms link through a covalent bond and discrete molecules are formed.

Ionic bond

- if the lowest energy can be achieved by the complete transfer of one or more electrons from each atom of one element to those of another, then ions form and the compound is held together by the electrostatic attraction between them. This is called an ionic bond.

octet rule

- in covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs.

bond order

- is the number of bonds that link a specific pair of atoms

lone pairs

- pairs of valence electrons that do not take part in bonding

radicals

- some species have an odd number of valence electrons, and so at least one of their atoms cannot have an octet. Species having electrons with unpaired spins are called radicals. They are generally highly reactive. - radicals are responsible for the rancidity of foods and the degradation of plastics in sunlight. Damage from radicals can be delayed by an additive called an antioxidant, which reacts rapidly with radicals before the radicals have a chance to do their damage.

Lewis structure

- the Lewis structure of a molecule shows atoms by their chemical symbols, covalent bonds by lines, and lone pairs by pairs of dots.

variable valence

- the ability of an element to form ions with different charges

Coordinate covalent bond

- the boron atom in BF3 can complete its octet if an additional atom of ion with a lone pair of electrons forms a bond by providing both electrons. A bond in which both electrons come form one of the atoms is called a coordinate covalent bond.

Madelung constant

- the factor A is a numerical coefficient called the Madelung constant; its value depends on how the ions are arranged about one another,

valence

- the valence of an element is the number of bonds that its atoms can form

crystalline solids

-solids that consist of atoms, molecules or ions stacked together in a regular pattern.

Writing ionic compounds with Lewis Symbols

1. Represent the cation by removing the appropriate number of dots from the symbol for the metal atom 2. Represent the anion by transferring those dots to the Lewis symbol for the nonmetal atom to complete its valence shell 3. If necessary, adjust the numbers of atoms of each kind so that all the dots removed from the metal atom symbols are accommodated by the nonmetal atom symbols 4. Write the charge of each ion as a superscript in the normal way

polar covalent bond

A covalent bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive. Property applies to nonmetal elements.

ionic model

A model used to calculate theoretical lattice enthalpies which assumes that the only bonding in an ionic solid is the electrostatic forces between the ions. These forces depend on the ionic radii and charges.

Lewis symbols

Consists of a chemical symbol for the element plus a dot for each valence electron. The valence electrons in any representative element is the same as the group number of the element. For example, Oxygen and Sulfur have 6 dots.

expanded valence shell

Many molecules (and ions) have more than eight valance electrons around the central atom. That atom expands its valance shell to form more bonds, which release energy. These occur only with nonmetals from period 3 or higher because they have d orbitals available. The central atom may be bonded to more than four atoms or to four or fewer. - the electrons in such an expanded valence shell may be present as lone pairs or may be used by the central atom to form additional bonds

valence electrons

The electrons in the outermost shell (main energy level) of an atom; these are the electrons involved in forming bonds.

nonpolar covalent bond

a covalent bond in which the electrons are shared equally by the two atoms. Property applies to nonmetal elements.

ionic solid

a kind of crystalline solid; aggregates of positively and negatively charged ions; no discrete ions; high melting points, high boiling points, poor electrical conductivity in the solid phase (due to strong electrostatic interactions which also cause ions to be relatively immobile).

octet

atoms react by gaining or losing electrons so as to acquire the stable electron structure of a noble gas, usually eight valence electrons

duplet

duplet rule states that when an element gets 2 electrons in its valence or last shell it has achieved a stable electronic configuration . Hydrogen seems to be the only element obeying it, as well as Lithium, Berylllium.

Dimers

linked pairs of molecules. - example is Al2Cl6. This molecule exists because a Cl atom in one AlCl3 molecule uses one of its lone pairs to form a coordinate covalent bond to the Al atom in a neighboring AlCl3 molecule. This arrangement can occur in aluminum chloride but not boron trichloride because the atomic radius of Al is bigger than that of B.

lattice energy

the energy released when one mole of an ionic crystalline compound is formed from gaseous ions. - the difference in energy between the ions of a compound widely separated as a gas and packed together in a solid -- usually very large.