Biology Concepts: Atoms, Elements, Compounds, Chemical Bonds and Mixtures

An atom, the smallest particle that retains the properties of an element, consists of the subatomic particles of protons, neutrons, and electrons. Protons have a positive charge and are found in the atomic nucleus. Neutrons have no charge and are found also inside of the atomic nucleus. Electrons have a negative charge and are found outside the nucleus, shown on orbitals.

Elements, compounds, and molecules differ from each other. An element is a fundamental form of matter that takes up space and has mass. A molecule is the outcome of two or more atoms covalently bonding together. A compound consists of two or more different elements in proportions that never vary. A molecule is only covalent bonding. A molecule can be atoms of the same elements, but a compound has to have two different elements. Atoms of elements make up both compounds and molecules.

The subatomic particles that determine the identity of an atom are protons, for they determine the atomic number. The subatomic particles that determine the chemical reactivity of an atom are electrons, because they are used in chemical bonding. The subatomic particles that determine the isotopes of the elements are neutrons, because isotopes of an element have the same amount of protons but vary in neutrons.

A chemical bond is the union between the electron structures of an atom. Two kinds of chemical bonds are ionic bonds and covalent bonds. An ionic bond is an association of two ions that have opposing charges. A covalent bond is a sharing of a pair of electrons. In an ionic bond, there is a transfer of electrons as the atoms either gain or lose electrons, thus charging the atom. In a covalent bond, however, there is a sharing of electrons so that both have a full outer energy level. An example of ionic bonding is NaCl, known as Sodium Chloride. Sodium has eleven electrons, with one on its outer energy level. Chlorine has seven electrons on its outer energy level. The one electron from Sodium is lost and Chlorine gains it. Both have a full outer energy level. The atom of sodium's charge is now positive one and chlorine's is negative 1. An example of a covalent bond is water, or H₂O. Oxygen has an outer energy level of 6, and Hydrogen has 1 electron in its outer energy level. The Oxygen and hydrogen atom each share two electrons. Now, the Oxygen has 8, and both Hydrogen have 2, filling each energy level.

A compound consists of two or more different elements in proportions that never vary. A mixture consists of two or more elements that are simply intermingling in proportions that can vary (and usually do). The difference between the two is that a compound is a chemical combination that never varies, while a mixture is a physical combination that does vary.

Mixtures can be heterogeneous or homogenous. Homogenous mixtures are called solutions where they are same throughout, and an example would be plasma. If water is the solvent in a homogenous mixture, it is referred to as an aqueous solution. Salt water, in which salt is the solvent and salt is the solute, is an example of an aqueous solution. A heterogeneous mixture is not the same throughout. Suspensions are a type of heterogeneous mixture where the suspended particles will settle out over time. Blood is an example of a suspension. The other type of heterogeneous mixture is a colloid, where the suspended particles remain suspended and do not settle out over time, such as in cytoplasm.

Sources:
Zuhmdahl, Chemistry: 5th ed.
Campbell & Reece, Biology: 7th ed.