CHEMICAL BOND

Combining Atoms Through Chemical Bonding Chemical bonding is the joining of atoms to form new substances. An interaction that holds two atoms together is called a chemical bond. When chemical bonds form, electrons are shared, gained, or lost.

Forming Ionic Bonds An ionic bond is a bond that forms when electrons are transferred from one atom to another atom. Charged Particles An atom is neutral because the number of electrons in an atom equals the number of protons. So, the charges cancel each other out. But when an atom gains or loses electrons, it becomes a charged particle called an ion.

Forming Positive Ions Metal Atoms and the Loss of Electrons Atoms of most metals have few valence electrons and tend to lose these valence electrons and form positive ions. The Energy Needed to Lose Electrons Energy is needed to pull electrons away from atoms. The energy needed comes from the formation of negative ions.

Forming Negative Ions Nonmetal Atoms Gain Electrons The outer energy level of nonmetal atoms is almost full. So, nonmetal atoms tend to gain electrons and become negative ions. The Energy of Gaining Electrons Energy is given off when nonmetals gain electrons. An ionic bond will form between a metal and a nonmetal if the nonmetal releases more energy than is needed to take electrons from the metal.

Ionic Compounds When ionic bonds form, the number of electrons lost by the metal atoms equals the number gained by the nonmetal atoms. The ions that bond are charged, but the compound formed is neutral because the charges of the ions cancel each other.

Ionic Compounds When ions bond, they form a repeating three-dimensional pattern called a crystal lattice, such as the one shown below. Properties of ionic compounds include brittleness, high melting points, and high boiling points.

Covalent Bonds A covalent bond forms when atoms share one or more pairs of electrons. Substances that have covalent bonds tend to have low melting and boiling points and are brittle in the solid state. Covalent bonds usually form between atoms of nonmetals, such as the atoms shown on the next slide.

Covalent Bonds Covalent Bonds and Molecules Substances containing covalent bonds consist of particles called molecules. A molecule usually consists of two or more atoms joined in a definite ratio. The models on the next slide show two ways to represent the covalent bonds in a water molecule.

Covalent Bonds One way to represent atoms and molecules is to use electron-dot diagrams. An electron-dot diagram shows only the valence electrons in an atom.

Covalent Compounds and Molecules A molecule is the smallest particle into which a covalently bonded compound can be divided and still be the same compound. The Simplest Molecules are made up of two bonded atoms. Molecules made up of two atoms of the same element are called diatomic molecules.

Covalent Compounds and Molecules More-Complex Molecules Carbon atoms are the basis of many complex molecules. Each carbon atom can form four covalent bonds. These bonds can be with atoms of other elements or with other carbon atoms, as shown at right.

Metallic Bonds A metallic bond is a bond formed by the attraction between positively charged metal ions and the electrons in the metal. Movement of Electrons Throughout a Metal Bonding in metals is a result of the metal atoms being so close to one another that their outermost energy levels overlap. This overlapping allows valence electrons to move throughout the metal.