Biology » Chemical Foundation of Life » Atoms, Isotopes, Ions and Molecules: The Building Blocks

Summarizing the Chemical Building Blocks

Summary of Lessons So Far

Matter is anything that occupies space and has mass. It is made up of elements. All of the 92 elements that occur naturally have unique qualities that allow them to combine in various ways to create molecules, which in turn combine to form cells, tissues, organ systems, and organisms.

Atoms, which consist of protons, neutrons, and electrons, are the smallest units of an element that retain all of the properties of that element. Electrons can be transferred, shared, or cause charge disparities between atoms to create bonds, including ionic, covalent, and hydrogen bonds, as well as van der Waals interactions.

Glossary of Words


negative ion that is formed by an atom gaining one or more electrons


the smallest unit of matter that retains all of the chemical properties of an element

Atomic mass

calculated mean of the mass number for an element’s isotopes

Atomic number

total number of protons in an atom

Balanced chemical equation

statement of a chemical reaction with the number of each type of atom equalized for both the products and reactants


positive ion that is formed by an atom losing one or more electrons

Chemical bond

interaction between two or more of the same or different atoms that results in the formation of molecules

Chemical reaction

process leading to the rearrangement of atoms in molecules

Chemical reactivity

the ability to combine and to chemically bond with each other


substance composed of molecules consisting of atoms of at least two different elements

Covalent bond

type of strong bond formed between two of the same or different elements; forms when electrons are shared between atoms


ion necessary for nerve impulse conduction, muscle contractions and water balance


negatively charged subatomic particle that resides outside of the nucleus in the electron orbital; lacks functional mass and has a negative charge of –1 unit

Electron configuration

arrangement of electrons in an atom’s electron shell (for example, 1s22s22p6)

Electron orbital

how electrons are spatially distributed surrounding the nucleus; the area where an electron is most likely to be found

Electron transfer

movement of electrons from one element to another; important in creation of ionic bonds


ability of some elements to attract electrons (often of hydrogen atoms), acquiring partial negative charges in molecules and creating partial positive charges on the hydrogen atoms


one of 118 unique substances that cannot be broken down into smaller substances; each element has unique properties and a specified number of protons


steady state of relative reactant and product concentration in reversible chemical reactions in a closed system

Hydrogen bond

weak bond between slightly positively charged hydrogen atoms to slightly negatively charged atoms in other molecules

Inert gas

(also, noble gas) element with filled outer electron shell that is unreactive with other atoms


atom or chemical group that does not contain equal numbers of protons and electrons

Ionic bond

chemical bond that forms between ions with opposite charges (cations and anions)

Irreversible chemical reaction

chemical reaction where reactants proceed uni-directionally to form products


one or more forms of an element that have different numbers of neutrons

Law of mass action

chemical law stating that the rate of a reaction is proportional to the concentration of the reacting substances

Mass number

total number of protons and neutrons in an atom


anything that has mass and occupies space


two or more atoms chemically bonded together


uncharged particle that resides in the nucleus of an atom; has a mass of one amu

Noble gas

see inert gas

Nonpolar covalent bond

type of covalent bond that forms between atoms when electrons are shared equally between them


core of an atom; contains protons and neutrons

Octet rule

rule that atoms are most stable when they hold eight electrons in their outermost shells


region surrounding the nucleus; contains electrons

Periodic table

organizational chart of elements indicating the atomic number and atomic mass of each element; provides key information about the properties of the elements

Polar covalent bond

type of covalent bond that forms as a result of unequal sharing of electrons, resulting in the creation of slightly positive and slightly negative charged regions of the molecule


molecule found on the right side of a chemical equation


positively charged particle that resides in the nucleus of an atom; has a mass of one amu and a charge of +1


isotope that emits radiation composed of subatomic particles to form more stable elements


molecule found on the left side of a chemical equation

Reversible chemical reaction

chemical reaction that functions bi-directionally, where products may turn into reactants if their concentration is great enough

Valence shell

outermost shell of an atom

Van der Waals interaction

very weak interaction between molecules due to temporary charges attracting atoms that are very close together

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