Atoms, Molecules, and Chemical Bonds: Foundations for General Biology

Atoms, Molecules, and the Basics of Matter

Introduction to Matter and Its Units

All matter is composed of fundamental units called atoms. Atoms combine to form molecules, which are the building blocks of all substances. Understanding the structure and properties of atoms and molecules is essential for studying biological systems.

• Atom: The smallest unit of an element that retains the properties of that element.

• Molecule: A group of two or more atoms held together by chemical bonds.

• Element: A pure substance consisting of only one type of atom (e.g., He for Helium).

• Compound: A substance formed when two or more different elements combine in fixed proportions (e.g., H2O).

Example: Water (H2O) is a molecule and a compound, composed of two hydrogen atoms and one oxygen atom.

Atomic Structure

Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in orbitals. The number of protons defines the atomic number, while the sum of protons and neutrons gives the atomic mass.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Electron: Negatively charged particle orbiting the nucleus.

• Atomic Number (Z): Number of protons in the nucleus.

• Atomic Mass (A): Total number of protons and neutrons.

Formula:

$\text{Atomic Mass} = \text{Number of Protons} + \text{Number of Neutrons}$

Classification: Atoms, Molecules, Compounds, and Elements

Comparison Table

The following table summarizes the differences between atoms, molecules, compounds, and elements:

Type	Description	Example
Atom	Smallest unit of an element	H, O, He
Molecule	Two or more atoms bonded together	O2, H2O
Element	Substance made of only one type of atom	O2, He
Compound	Substance made of atoms of different elements bonded together	H2O, CO2

Chemical Bonds

Introduction to Chemical Bonds

Chemical bonds are forces that hold atoms together in molecules and compounds. The main types of chemical bonds in biology are covalent bonds and ionic bonds.

• Chemical Bond: An attraction between atoms that allows the formation of chemical substances.

• Valence Electrons: Electrons in the outermost shell of an atom, involved in bond formation.

Why Do Atoms Form Bonds?

Atoms form bonds to achieve a stable electron configuration, often resembling the nearest noble gas. This usually means filling their outermost electron shell.

• To attain a stable electron configuration (octet rule).

• To lower their potential energy.

Covalent Bonds

Covalent bonds involve the sharing of pairs of electrons between atoms. These are the most common bonds in biological molecules.

• Single Covalent Bond: Sharing of one pair of electrons.

• Double Covalent Bond: Sharing of two pairs of electrons.

• Triple Covalent Bond: Sharing of three pairs of electrons.

Example: In a water molecule (H2O), each hydrogen atom shares one electron with oxygen, forming two single covalent bonds.

Types of Covalent Bonds: Polar and Non-Polar

The sharing of electrons in covalent bonds can be equal or unequal, leading to non-polar or polar covalent bonds, respectively.

• Non-Polar Covalent Bond: Equal sharing of electrons between atoms (e.g., O2).

• Polar Covalent Bond: Unequal sharing of electrons, resulting in partial charges (e.g., H2O).

Comparison Table: Non-Polar vs. Polar Covalent Bonds

Bond Type	Electron Sharing	Charge Distribution	Example
Non-Polar Covalent	Equal	No partial charges	O2, N2
Polar Covalent	Unequal	Partial positive and negative charges	H2O, NH3

Electronegativity

Electronegativity is the tendency of an atom to attract shared electrons in a chemical bond. Differences in electronegativity between atoms determine whether a bond is polar or non-polar.

• Atoms with higher electronegativity attract electrons more strongly.

• Common scale: Pauling scale (e.g., O = 3.5, H = 2.1).

Example: In HCl, chlorine is more electronegative than hydrogen, so the shared electrons are closer to chlorine, making the bond polar.

Partial Charges and Polarity

When electrons are shared unequally, the more electronegative atom gains a partial negative charge (δ−), and the less electronegative atom gains a partial positive charge (δ+). This creates a dipole, making the molecule polar.

• Partial Charge: A small electrical charge due to unequal electron sharing.

• Polar Molecule: A molecule with regions of partial positive and negative charge (e.g., water).

Summary Table: Key Vocabulary

Term	Definition	Example
Atom	Smallest unit of an element	H, O
Element	Substance made of one type of atom	He, O2
Compound	Substance made of atoms of different elements	H2O, CO2
Molecule	Group of atoms bonded together	O2, H2O

Learning Outcomes

• List the components of an atom.

• Draw an atom and calculate its atomic mass and atomic number.

• Explain why atoms create chemical bonds.

• List and describe the types of chemical interactions.

• Analyze atoms and decide what type of chemical interaction the atoms could form.

• Create analogies for chemical interactions.