Atoms, Elements, and the Chemical Basis of Life: Study Notes for General Biology

Atoms, Elements, and the Chemical Basis of Life

Introduction to Matter and Life

All living organisms are composed of matter, which is anything that has mass and occupies space. Understanding the structure and properties of matter is fundamental to biology, as it underpins the composition and function of all biological systems.

• Matter: Anything that takes up space and has mass.

• Organisms: Complex systems built from atoms and molecules, organized into cells, tissues, organs, and higher levels of biological organization.

• Hierarchy of Biological Organization: Atoms → Molecules → Cell Organelles → Cells → Tissues → Organs → Organisms → Populations → Ecosystems → Biosphere.

• Example: A human is composed of trillions of cells, each made up of molecules, which in turn are made of atoms.

Atoms: The Building Blocks of Matter

Structure of Atoms

Atoms are the smallest units of matter that retain the properties of an element. They consist of a dense nucleus surrounded by a cloud of electrons.

• Nucleus: Contains positively charged protons and neutral neutrons.

• Electrons: Negatively charged particles that move in a cloud around the nucleus.

• Atomic Number: Number of protons in the nucleus; defines the element.

• Atomic Mass: Sum of protons and neutrons in the nucleus.

• Scale: If the nucleus were the size of a tennis ball, the electron cloud would extend several football fields away, illustrating that atoms are mostly empty space.

• Example: A hydrogen atom has 1 proton and 1 electron; a helium atom has 2 protons, 2 neutrons, and 2 electrons.

Why Can't We Walk Through Walls?

• Although atoms are mostly empty space, the electron clouds repel each other due to their negative charge, preventing matter from passing through other matter under normal conditions.

Elements and the Periodic Table

Elements

An element is a substance that cannot be broken down into other substances by chemical means. Each element is defined by its atomic number.

• There are 92 naturally occurring elements on Earth.

• Periodic Table: Organizes elements by increasing atomic number and similar chemical properties.

• Element Symbol: One- or two-letter abbreviation (e.g., H for hydrogen, O for oxygen).

• Atomic Mass: Approximately equal to the sum of protons and neutrons.

Isotopes

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons.

• Stable Isotopes: Do not change or decay over time (e.g., Carbon-12, Carbon-13).

• Radioactive Isotopes: Unstable; decay spontaneously, emitting radiation (e.g., Carbon-14).

• Applications: Radioactive isotopes are used as tracers in medicine and for radiometric dating in geology and archaeology.

• Example: Carbon-12 (6 protons, 6 neutrons), Carbon-13 (6 protons, 7 neutrons), Carbon-14 (6 protons, 8 neutrons; radioactive).

Table: Major Elements in the Human Body

The following table summarizes the main elements that make up the human body, including their symbols and approximate percentage of body mass (including water):

Element	Symbol	Percentage of Body Mass
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Calcium	Ca	1.5%
Phosphorus	P	1.0%
Potassium	K	0.4%
Sulfur	S	0.3%
Sodium	Na	0.2%
Chlorine	Cl	0.2%
Magnesium	Mg	0.1%
Trace Elements	Various	<0.01%

Additional info: Trace elements include boron, chromium, cobalt, copper, fluorine, iodine, iron, manganese, molybdenum, selenium, silicon, tin, vanadium, and zinc.

Origin of Elements

The elements found on Earth and in living organisms originated from cosmic processes.

• Big Bang Nucleosynthesis: Produced hydrogen, helium, and small amounts of lithium and beryllium.

• Stellar Nucleosynthesis: Heavier elements (carbon, oxygen, etc.) were formed in stars and distributed into space by supernovae.

• "We are made of star stuff": The atoms in our bodies were formed in ancient stars.

Electron Structure and Chemical Properties

Electron Shells and Orbitals

Electrons are arranged in shells around the nucleus, each with a specific energy level and capacity.

• First Shell: Holds up to 2 electrons.

• Second Shell: Holds up to 8 electrons.

• Third Shell: Holds up to 8 electrons (for main group elements).

• Orbitals: Regions within shells where electrons are likely to be found; each orbital holds up to 2 electrons with opposite spins.

• Octet Rule: Atoms tend to fill their outermost shell with 8 electrons for stability (except for hydrogen and helium, which are stable with 2).

Valence Electrons and Chemical Reactivity

The chemical behavior of an atom is determined by the number of electrons in its outermost shell, known as valence electrons.

• Valence Shell: The outermost electron shell of an atom.

• Atoms with full valence shells (e.g., noble gases) are chemically inert.

• Atoms with incomplete valence shells are reactive and tend to gain, lose, or share electrons to achieve stability.

Compounds and Emergent Properties

Compounds

A compound is a substance consisting of two or more elements combined in a fixed ratio. Compounds have properties different from their constituent elements, known as emergent properties.

• Example: Sodium (a soft, reactive metal) and chlorine (a toxic gas) combine to form sodium chloride (table salt), which is safe to eat.

• Emergent Properties: New characteristics that arise when elements combine to form compounds; the whole is greater than the sum of its parts.

Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The main types of chemical bonds are:

• Ionic Bonds: Formed when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons.

• Only valence electrons are involved in bond formation.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Ionic	Transfer of electrons; attraction between ions	NaCl (sodium chloride)
Covalent	Sharing of electron pairs	H2O (water)
Hydrogen	Weak attraction between polar molecules	Between water molecules

Additional info: Hydrogen bonds are important in the structure of DNA and proteins.

Key Equations and Concepts

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

• Isotope Notation: $^{A}_{Z}\text{X}$, where $A$ is mass number, $Z$ is atomic number, and $X$ is the element symbol.

• Half-life (Radioactive Decay): $N(t) = N_0 \times (1/2)^{t/t_{1/2}}$ where $N(t)$ is the remaining quantity, $N_0$ is the initial quantity, $t$ is time, and $t_{1/2}$ is the half-life.

Conclusion

Understanding atoms, elements, and their interactions is essential for studying the chemistry of life. The unique properties of elements and the compounds they form underlie all biological structure and function.