Atoms found in substances are often joined to other atoms via chemical bonding. This results in the formation of a molecule, which is a group of two or more atoms bonded together. These atoms can be the same element or different elements. When they are different elements, they form a compound, which is a substance made of atoms of two or more elements bonded together in a certain ratio. For example, water (H2O) is a compound formed when two hydrogen atoms form chemical bonds with an oxygen atom. Methane (CH4) is a compound formed when a carbon atom forms bonds with four hydrogen atoms. If the ratio of atoms changes, the compound itself is different: hydrogen peroxide (H3O) is not water (H2O). Thus, all compounds are molecules, but not all molecules are compounds. For example, oxygen gas (O2) is a molecule, because it contains two atoms bonded together, but it is not a compound, because it contains only a single element. Molecules made up of two or more atoms of a single element are often referred to with the adjective molecular to distinguish them from the elemental form. Like compounds, molecules formed from only one element bond in specific ratios; oxygen gas (O2) is different from ozone (O3).
Molecules and compounds have properties which arise from the atoms making them up. For example, water is polar, meaning it has a positive charge on one side (the hydrogen side) and a negative charge on the other side (the oxygen side). This allows it to interact with other molecules in very specific ways, such as dissolving solids. Atoms are the smallest units that can take part in chemical reactions.
Different Types of Chemical Bonds
Atoms can join together by forming weak or strong chemical bonds. Atoms make these connections in order to reach their lowest-energy state. According to the octet rule, atoms tend to fill their valence shell with eight electrons, which represents the most stable state of the atom. The formation of a chemical bond often makes an atom stable. Atoms will either share, gain, or lose electrons through chemical bonding in order to satisfy the octet rule. Atoms can form several different chemical bonds.An ionic bond is a chemical bond that forms when valence electrons are transferred between atoms. Ionic bonds form from the electrostatic forces (forces based on charge) occurring between oppositely charged ions. Often, atoms will form ionic bonds when they need to acquire one or two electrons to fill their valence shell. During valence-electron transfer to form an ionic bond, the atom losing electrons becomes a cation, while the atom gaining electrons becomes an anion. Atoms classified as nonmetals (such as hydrogen and fluorine), alkali metals (such as sodium and potassium), and alkaline earth metals (such as calcium and magnesium) commonly participate in ionic bonding. For example, sodium chloride (NaCl) is a compound formed with ionic bonding. For this reason, it is known as an ionic compound. A covalent bond is a chemical bond that forms when atoms share electrons. This is different from ionic bonding where the electron is fully lost or gained. The number of valence electrons an atom has determines how many covalent bonds the atom can form. For example, carbon, which has four valence electrons, can form four covalent bonds, while nitrogen, which has five valence electrons, can form three covalent bonds. The octet rule explains why nitrogen forms fewer covalent bonds than carbon does: fewer electrons are needed for nitrogen's outer shell to reach eight electrons.
- A polar covalent bond forms from an unequal sharing of electrons between atoms. Because of this unequal distribution, partial charges are generated for each atom. Water (H2O) is an example of a compound with a polar covalent bond. The nucleus of the oxygen atom is strongly attracted to the electrons of the hydrogen atoms because oxygen has a high electronegativity. Electronegativity is the tendency of an atom to attract electrons. Because of this unequal sharing, the oxygen atom carries a partial negative charge and the hydrogen atoms both carry a partial positive charge. Bonds formed in this manner are weaker than ionic and standard covalent bonds.
- A nonpolar covalent bond forms when two atoms share electrons equally. They can form between two atoms of the same element or between atoms of different elements. An example includes the hydrogen gas (H2) molecule—two hydrogen atoms equally share electrons to form this molecule. The molecule methane (CH4) is another molecule formed from nonpolar covalent bonds. Nonpolar covalent bonds are considered standard covalent bonds and are thus strong bonds.
Hydrogen Bonding and Dispersion Forces
Ionic and covalent bonds are the strongest types of chemical bonds. Weaker, or more temporary, bonds include hydrogen bonds and London dispersion forces (also called van der Waals forces).A hydrogen bond is a weak bond between two molecules that results from an attraction between a proton in one molecule and an electronegative atom in the other. Hydrogen bonds do not form when a hydrogen atom bonds to another atom. Rather, there is an attractive force between a hydrogen atom in a molecule and a negatively charged atom in another molecule. Hydrogen bonding in water molecules is of vital importance to biology. Water molecules form polar covalent bonds, with the oxygen having a slight negative charge and the hydrogens having a slight positive charge. This allows hydrogen bonds to form between the hydrogens of one water molecule and the oxygen of another water molecule. These hydrogen bonds give water very important properties, such as adhesion (the ability to stick to other substances), cohesion (the ability to stick to itself), and high heat of vaporization, meaning it takes a lot of heat to change water from a liquid to a gas. A London dispersion force is the weakest intermolecular force that forms due to attraction between molecules. Intermolecular force refers to the attraction between molecules. Unlike hydrogen bonds, London dispersion forces occur between atoms and molecules of any kind. Electrons are in constant motion. Thus, at any moment, electrons may be located more toward one side of a molecule than the other. If electrons are concentrated on one end of a molecule, this end of the molecule becomes slightly negative. The other end is slightly positive, and the molecule forms a temporary dipole. A dipole is a pair of equally and oppositely charged particles.
This transient dipole in one molecule encourages nearby molecules to also create transient dipoles. Then, the van der Waal's forces can occur between oppositely charged dipoles. When the motion of electrons creates temporary dipoles between molecules, a London dispersion force is formed. Because of this interaction between molecules, the dispersion force is also called an induced dipole-dipole attraction. Typically, the attractive forces between nonpolar substances rely on London dispersion forces. For example, benzene is a nonpolar substance that is liquid at room temperature. It is a 6-carbon compound that is used as a solvent in paints and varnishes.