A chemical bond is a mutual attraction between nuclei and valence electrons of different atoms that binds the atoms together. A chemical bond forms when two atoms at relatively high potential energy come in close proximity to one another and react to minimize the potential energy. This results in a more stable arrangement of matter.
Types of Chemical Bonds
It may result from a transfer of one or more electrons from one atom to another, causing ions to be formed. This is called an ionic bond. Positive ions - Cations are formed when an atom loses one or more electrons. Negative ions – Anions are formed when an atom gains one or more electrons. It may result from the sharing of an electron pair by two atoms. This is called a covalent bond. Pure covalent – equal sharing Polar covalent – unequal sharing Coordinate covalent – one atom donates both electrons Metallic – a sea of electrons flows among atoms
See Figure 6.1 on page 162 for examples of each type of chemical bond.
Types of Chemical Bonds
Ionic bonds
Result from the attraction of cations for anions. Occur between metals and nonmetals Occur when the electronegativity difference between atoms is greater than 1.7. Form formula units as their representative particles. Have high melting points, high densities, tend to be solids
Table of Electronegativities
Predicting Bond Type from Electronegativity Difference
Covalent bonds
Result from sharing of an electron pair by two atoms. Occur between nonmetals Occur when the electronegativity difference is between 0.3 and 1.7. Form molecules as their representative particles.
A molecule is a neutral group of atoms that is held together by covalent bonds. It is capable of existing on its own. A molecular formula indicates the number and kinds of atoms combined in a molecule of a molecular compound. A diatomic molecule is one consisting of only two atoms. A chemical compound whose simplest units are molecules is called a molecular compound.
See fig. 6.2 on page 162
Formation of a Covalent Bond
Nature favors chemical bonding because most atoms are at lower potential energy when bonded to other atoms. An example is with hydrogen-hydrogen bonding. Two hydrogens are separated by a distance large enough to prevent them from influencing one another. The potential energy is at zero. See Figure 6.5 on page 165. If these atoms are brought close to one another, their charged subatomic particles begin to interact.
Approaching nuclei and electrons are attracted to one another, which corresponds to a decrease in the total potential energy of the atoms.
Characteristics of the Covalent Bond
Bond length is the distance between two bonded atoms at their minimum potential energy, or the average distance between the two bonded atoms. Atoms generally release energy as they change from individual atoms to parts of a molecule. The amount of energy released equals the difference between the potential energy at the zero level and that at the bottom of the valley. The amount of energy required to break a chemical bond and form neutral isolated atoms is called bond energy. The electron clouds of the two atoms overlap and allow the sharing of the electron pair.
The Octet Rule
Chemical compounds tend to form so that each atom, by gaining, losing, or sharing electrons, has an octet of electrons in its highest occupied energy level.
Exceptions to the Octet Rule
Most main-group elements tend to form covalent bonds according to the octet rule. Hydrogen and boron tend to form compounds and they will not have complete octets. Elements with “d” subshells available will form compounds by creating expanded octets, using electrons from the s, p, and d subshells to form bonds.
Electron Dot Notation
Electron dot notation is an electron notation showing only the valence electrons in the atom. These are indicated by dots around the atomic symbol. All elements in a family will have the same dot notation because they have the same number of valence electron. The only difference will be in the elemental symbol used.
Lewis Structures
Electron dot notation can be used to represent molecules. A pair of dots represents a pair of electrons. If the dots are between the atoms, they represent a shared pair. If the dots are not between the atoms, they represent an unshared pair, or a lone pair.
Ionic Bonding and Ionic Compounds
Most rocks and minerals consist of positive and negative ions held together by ionic bonding. An ionic compound is composed of positive and negative ions that are combined so that the numbers of positive and negative charges are equal.
Ionic Bonding and Ionic Compounds
Most ionic compounds are crystalline substances that have a crystal lattice. A crystal lattice is an orderly arrangement of the ions which minimizes their potential energy. The attractive forces are those which include those between oppositely charged ions and those between the nuclei and electrons of adjacent ions. The repulsive include those between like-charged ions and those between electrons of adjacent ions.
Ionic Bonding and Ionic Compounds
The smallest particle of an ionic compound that maintains the identity of the compound is the formula unit. Lattice energy is the energy released when one mole of ionic crystalline compound is formed from gaseous ions. Lewis structures for ionic compounds.
A Comparison of Ionic and Molecular Compounds
Ionic compounds usually have very high melting points, high boiling points, and great hardness. They are brittle. The forces holding ionic and covalent compounds together are very strong but the forces holding adjacent particles together is much weaker in covalent compounds. In the molten state, ions are free to move so ionic compounds are conductors of electricity. In solution, some ionic compounds are very soluble and hence will conduct. They are called electrolytes.
Intermolecular Forces
The forces of attraction between molecules are the intermolecular forces. They vary in strength but are weaker than bonds because they do not involve the transfer or sharing of electrons. IMF help give molecules their properties. Ionically bonded compounds and metallically bonded atoms tend to have higher boiling points due to attractive forces involved.
Intermolecular Forces
The strongest IMF occur between polar molecules. Polar molecules act as dipoles because of the unequal distribution of electron density.
Hydrogen Bonds
Very strong IMF that occur between atoms of hydrogen and one of the following elements
Fluorine Oxygen Nitrogen
London Dispersion Forces
Between nonpolar molecules Weak Result of instantaneous dipoles
