Chemistry of Life

Introduction to Matter and Elements

Understanding the chemical basis of life is essential in biology. All living and non-living things are composed of matter, which is made up of elements and atoms.

• Matter: Anything that takes up space and has mass (e.g., organisms, rocks, oceans, air).

• Chemical Elements: Pure substances consisting of only one type of atom. Examples include carbon (C), hydrogen (H), and oxygen (O).

• Atoms: The smallest units of matter that retain the properties of an element.

Example: Both diamonds and honey bees are made of atoms, but their properties differ due to the arrangement and types of atoms present.

Atomic Structure

Atoms are composed of subatomic particles, each with specific properties and locations within the atom.

• Protons determine the atomic number and identity of an element.

• Neutrons contribute to the atomic mass and can vary in number, resulting in isotopes.

• Electrons are involved in chemical bonding and reactions.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Elements of Life

Essential Elements

Of the 92 naturally occurring elements, only a small subset is found in living organisms. The periodic table organizes all known elements by their chemical properties.

• About 97% of the mass of most living organisms is composed of Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P), and Sulfur (S)—often abbreviated as CHNOPS.

• Trace elements are required in minute amounts but are essential for life (e.g., iron, iodine).

Example: The periodic table highlights the main elements and trace elements required for life.

Atomic Properties

Each atom of an element has unique properties:

• Atomic Number: Number of protons in the nucleus.

• Mass Number: Total number of protons and neutrons.

• Atomic Mass: The average mass of all atoms of an element, accounting for isotopes.

Example: Carbon has an atomic number of 6 and a mass number of 12.

Electron Configuration and Energy Shells

Electron Orbitals & Energy Shells

Electrons are arranged in shells or energy levels around the nucleus. The arrangement of electrons determines how atoms interact and bond with each other.

• Shells closer to the nucleus are lower in energy; shells farther away are higher in energy.

• Valence Electrons: Electrons in the outermost shell, important for chemical bonding.

• The first shell holds up to 2 electrons; the second shell holds up to 8 electrons.

Example: The electron configuration for carbon (atomic number 6) is 2 electrons in the first shell and 4 in the second shell.

Octet Rule

The octet rule states that atoms are most stable when their outermost electron shell is full, typically with 8 electrons (except for the first shell, which is full with 2 electrons).

• Atoms with incomplete valence shells are more reactive.

• Atoms achieve stability by gaining, losing, or sharing electrons to fill their valence shell.

Example: Neon (Ne) is unreactive because its valence shell is full with 8 electrons.

Isotopes and Radioactivity

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

• Stable Isotopes: Do not change or decay over time.

• Radioactive Isotopes: Unstable; they decay and emit energy in the form of radiation.

Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon. Carbon-14 is radioactive and used in radiometric dating.

Radioactive Decay and Half-Life

Radioactive isotopes decay at a predictable rate, measured by their half-life—the time it takes for half of a sample to decay.

• Used in dating fossils and rocks (e.g., Carbon-14 dating).

• Applications in medicine (e.g., cancer treatment, diagnostic imaging).

Example: Carbon-14 has a half-life of 5,730 years.

Summary Table: Subatomic Particles

Key Equations

• Atomic Number:

• Mass Number:

• Number of Neutrons:

Practice Questions (Examples)

• What is the atomic number of an element with 8 protons? Answer: 8

• How many neutrons are in an oxygen atom with a mass number of 16? Answer: 8

• Why is neon unreactive? Answer: Its valence shell is full.

• What is the half-life of Carbon-14? Answer: 5,730 years

Additional info: These notes cover foundational chemistry concepts essential for understanding biological molecules and processes, including atomic structure, isotopes, and the chemical elements of life.