The Molecules of Life: Elements, Compounds, and Chemical Bonds

Chapter 2: The Molecules of Life

Key Concepts

• Matter consists of chemical elements in pure form and combinations called compounds.

• An element’s properties depend on the structure of its atoms.

• The formation and function of molecules and ionic compounds depend on chemical bonding between atoms.

• Chemical reactions make or break chemical bonds.

Matter, Elements, and Compounds

Definitions and Classifications

Matter is anything that occupies space and has mass. Chemistry is the study of matter, its properties, and the changes it undergoes.

• Element: A substance made up of only one type of atom. Elements cannot be broken down into other substances by chemical reactions. Examples include iron (Fe), copper (Cu), gold (Au), and hydrogen (H).

• Compound: A substance made of two or more elements combined in a fixed ratio. Compounds have properties different from their constituent elements. Example: $\text{Na} + \text{Cl} \rightarrow \text{NaCl}$ (table salt).

• Mixture: A physical combination of two or more substances where each retains its own properties.

Type	Description	Example
Element	Only one type of atom present	O2, H2
Compound	Two or more elements chemically combined in fixed ratio	H2O, NaCl
Mixture	Two or more substances physically combined	Air, salt water

Atoms: Structure and Properties

Subatomic Particles

Atoms are the basic units of matter, composed of subatomic particles:

• Protons: Positively charged particles found in the nucleus. The number of protons determines the atomic number and the identity of the element.

• Neutrons: Neutral particles found in the nucleus. The number of neutrons can vary, resulting in different isotopes of an element.

• Electrons: Negatively charged particles that orbit the nucleus in energy levels (shells).

Atomic Number and Atomic Mass

• Atomic Number (Z): The number of protons in an atom’s nucleus. It defines the element.

• Atomic Mass (A): The total number of protons and neutrons in the nucleus.

For example, carbon has 6 protons (atomic number 6). Its most common isotope has 6 neutrons, so its atomic mass is 12.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons. They have the same chemical properties but different physical properties.

• Stable Isotopes: Do not undergo radioactive decay.

• Radioactive Isotopes: Nucleus decays spontaneously, emitting particles and energy.

Example: Carbon has three isotopes—$^{12}\text{C}$ (stable), $^{13}\text{C}$ (stable), and $^{14}\text{C}$ (radioactive).

Electron Configuration and Energy Levels

Electron Shells and Orbitals

Electrons are arranged in shells around the nucleus. Each shell represents a different energy level:

• 1st shell: Holds up to 2 electrons ($1s$ orbital)

• 2nd shell: Holds up to 8 electrons ($2s$ and $2p$ orbitals)

• 3rd shell: Holds up to 8 electrons ($3s$ and $3p$ orbitals)

The outermost shell is called the valence shell. Electrons in this shell are called valence electrons and are involved in chemical bonding.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds are forces that hold atoms together in molecules and compounds. The main types are:

• Covalent Bonds: Atoms share pairs of electrons. Can be single, double, or triple bonds depending on the number of shared electron pairs.

• Ionic Bonds: One atom transfers electrons to another, resulting in oppositely charged ions that attract each other.

• Hydrogen Bonds: Weak attractions between a hydrogen atom covalently bonded to a highly electronegative atom (like O or N) and another electronegative atom.

• Van der Waals Interactions: Weak attractions due to transient local charges.

Covalent Bonding

• Single Bond: One pair of electrons shared ($\text{H-H}$)

• Double Bond: Two pairs of electrons shared ($\text{O=O}$)

• Triple Bond: Three pairs of electrons shared ($\text{N\equiv N}$)

Bond strength and length vary: single bonds are longest and weakest, triple bonds are shortest and strongest.

Ionic Bonding

Occurs when atoms have very different electronegativities. One atom loses electrons (becomes a cation), and another gains electrons (becomes an anion). The resulting electrostatic attraction forms an ionic bond.

Example: $\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$

Electronegativity and Bond Polarity

Electronegativity is an atom’s ability to attract electrons in a bond. The difference in electronegativity between atoms determines bond type:

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., $\text{H}_2$, $\text{O}_2$).

• Polar Covalent Bond: Electrons are shared unequally, creating partial charges (e.g., $\text{H}_2\text{O}$).

• Ionic Bond: Electrons are transferred, creating ions (e.g., $\text{NaCl}$).

Electronegativity increases across a period (left to right) and up a group (bottom to top) in the periodic table.

Biological Importance of Elements and Compounds

Essential Elements

Living organisms require certain elements for survival. The most common are:

• Carbon (C)

• Oxygen (O)

• Hydrogen (H)

• Nitrogen (N)

Trace elements like iron (Fe), iodine (I), fluorine (F), and zinc (Zn) are also vital for health.

Elemental Deficiencies

• Iron deficiency: Can lead to anemia.

• Iodine deficiency: Can cause goiter.

• Nitrogen deficiency: Affects plant growth.

• Fluorine deficiency: Increases risk of tooth decay.

Summary Table: Types of Chemical Bonds

Bond Type	Mechanism	Strength	Example
Covalent	Electron sharing	Strong	H2O, CH4
Ionic	Electron transfer	Strong (in solid state)	NaCl
Hydrogen	Attraction between H and electronegative atom	Weak	Between water molecules
Van der Waals	Transient charge interactions	Very weak	Between nonpolar molecules

Key Vocabulary

• Matter

• Element

• Compound

• Atom, Proton, Electron, Neutron

• Atomic Mass, Atomic Number

• Isotope

• Covalent Bond, Ionic Bond

• Electronegativity

• Polar, Nonpolar

Example Problems

• Draw an electron distribution diagram for Argon (Ar). Is it chemically reactive? Why or why not? Argon has a full valence shell (8 electrons in the outer shell), making it chemically inert (nonreactive).

• Define these chemical bonds:

  • Hydrogen bond: A weak attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

  • Nonpolar covalent bond: Electrons are shared equally between atoms.

  • Polar covalent bond: Electrons are shared unequally, resulting in partial charges.

  • Van der Waals interaction: Weak attractions due to transient local charges.

Additional info: These notes expand on the provided slides and handwritten content to ensure completeness and clarity for General Chemistry students.