Atoms to Molecules

The chemical basis of life is rooted in the fundamental unit of matter, the atom. Atoms serve as the building blocks of all substances, including living organisms. Molecules form when two or more atoms chemically bond together. The atoms in a molecule are held together by chemical bonds, which can be covalent, ionic, or metallic. Diatomic molecules, such as oxygen (O2), nitrogen (N2), and chlorine (Cl2), are made up of two atoms of the same element. CO, on the other hand, is classified as a heteronuclear diatomic molecule since it consists of two different elements. Other examples of heteronuclear diatomic molecules include carbon monoxide (CO), hydrogen chloride (HCl), and nitric oxide (NO). Triatomic molecules consist of three atoms, such as water (H2O) and carbon dioxide (CO2). Multiatomic molecules contain four or more atoms, such as methane (CH4) and glucose (C6H12O6). The properties of a molecule are determined by the types of atoms it contains and the nature of the chemical bonds between them.

Hydrogen and oxygen are two elements that play a crucial role in the chemistry of life. When two hydrogen atoms combine with one oxygen atom, they form a molecule of water, which is essential to life. The formation of water occurs through a covalent bond between the hydrogen and oxygen atoms. Oxygen is more electronegative than hydrogen, meaning it attracts electrons more strongly. This creates a partial negative charge on the oxygen atom and a partial positive charge on the hydrogen atoms. The polar nature of the water molecule makes it an excellent solvent, allowing it to dissolve many substances, including salts, sugars, and amino acids. Water also plays a vital role in temperature regulation, acting as a coolant through the process of evaporation and helping to maintain homeostasis in the body.

The combination of hydrogen and oxygen atoms to form water is just one example of how the chemical properties of atoms are essential to life. The chemical basis of life is complex, and understanding the properties and behavior of atoms is fundamental to understanding the chemistry of living organisms. The number of protons in an atom's nucleus is known as the atomic number, which defines the element. For example, all carbon atoms have six protons, and all oxygen atoms have eight protons. The number of neutrons in the nucleus can vary, giving rise to isotopes of the same element. Isotopes have the same atomic number but a different atomic mass due to a different number of neutrons.

The chemical behavior of an element is determined by the number of electrons in its outermost shell, known as the valence shell. The electrons in the valence shell participate in chemical reactions and determine the atom's ability to form chemical bonds with other atoms.

Chemical bonds are formed when two or more atoms share or transfer electrons. The most common types of chemical bonds are covalent, ionic, and hydrogen bonds. Covalent bonds form when two or more atoms share electrons, while ionic bonds form when electrons transfer from one atom to another. Hydrogen bonds are a special type of weak bond that forms between hydrogen atoms and other electronegative atoms such as oxygen or nitrogen.

The human body comprises four major elements: carbon, hydrogen, oxygen, and nitrogen. These elements are essential to forming the organic molecules that make up the body, including carbohydrates, lipids, proteins, and nucleic acids. This brings us to the topic of organic molecules.


Organic Molecules

Living things are made up of just four classes of organic molecules. All organic compounds contain carbon. While all organic molecules contain carbon, not all carbon-containing molecules are considered organic. To be considered organic, a molecule must contain carbon atoms bonded to hydrogen atoms and they usually have carbon to carbon covalent bonds. Organic molecules are involved in many essential biological processes, such as energy production, genetic information storage, and the structure and function of cells.

Organic molecules play a vital role in the structure and function of living organisms. They are the building blocks of carbohydrates, lipids, proteins, and nucleic acids, which are essential components of all living cells. These molecules can form complex structures, and they are the basis of life on Earth. The unique properties of carbon, such as its ability to form covalent bonds with other carbon atoms and other elements, allow for the creation of a vast number of organic molecules with diverse properties and functions. These can range from simple molecules such as methane (CH4) to highly complex molecules such as proteins, carbohydrates, and nucleic acids.

There are four major types of organic compounds: carbohydrates, lipids, proteins, and nucleic acids.


Carbohydrates are composed of carbon, hydrogen, and oxygen atoms and serve as a source of energy for the body. They are found in foods such as bread, pasta, and potatoes and can exist as monosaccharides (single sugar molecules), disaccharides (two sugar molecules), or polysaccharides (many sugar molecules). Polysaccharides examples include starch, glycogen, and cellulose.



Proteins are complex molecules composed of amino acids that play a crucial role in the body. They provide structural support, transport of molecules, and catalysis of chemical reactions. Proteins are composed of long chains of amino acids that fold into specific shapes, allowing them to carry out specific functions. Enzymes, a type of protein, catalyze chemical reactions in the body.


Lipids are insoluble in water and include fats, oils, and waxes. They are essential components of cell membranes and are composed of carbon, hydrogen, and oxygen atoms. Lipids also serve as a source of energy for the body and provide insulation and protection for organs.

Nucleic acids

Nucleic acids store and transmit genetic information. DNA is the genetic material that carries instructions for the development and function of all living organisms. RNA is involved in the synthesis of proteins and serves as a messenger between DNA and the protein-synthesis machinery.


Metabolic reactions, which occur in living organisms, involve the transformation of molecules into new compounds through the breaking and forming of chemical bonds. These reactions require energy, which is provided by the breakdown of carbohydrates, fats, and proteins.