Matter, Elements, Atoms, and Chemical Bonding

Matter, Elements, and Atoms

Definition and Classification

Matter is anything that has mass and occupies space. In biology, understanding matter is fundamental because all living organisms are composed of matter, which is organized into elements and compounds.

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

• Element: A pure substance consisting of only one type of atom (e.g., Oxygen, Carbon).

• Compound: A substance formed when two or more elements are chemically bonded (e.g., H2O).

Key Elements in Biology: Six elements make up ~95% of the human body: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S) (often abbreviated as CHNOPS).

Atomic Structure

An atom is the smallest unit of an element that retains its chemical properties. Atoms are composed of subatomic particles:

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus.

Atomic Number: Number of protons in an atom. Mass Number: Sum of protons and neutrons.

Example: Carbon-12 has 6 protons and 6 neutrons (mass number = 12).

Isotopes

Definition and Applications

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers.

• Stable Isotopes: Do not decay over time.

• Radioactive Isotopes: Unstable; decay and emit radiation.

Half-life: The time required for half of a radioactive isotope to decay. Useful for dating fossils and tracing biological processes.

Example: Carbon-14 is used in radiocarbon dating.

Electrons and Energy Levels

Electron Arrangement

Electrons are arranged in energy levels (shells) around the nucleus. The arrangement determines an atom's chemical properties.

• Electron Shells: Energy levels where electrons reside.

• Maximum Electrons per Shell: 2n2, where n is the shell number.

Valence Electrons: Electrons in the outermost shell; determine chemical reactivity.

Example: Atoms with incomplete valence shells are more chemically reactive.

Chemical Bonding

Electron Interactions

Atoms interact by gaining, losing, or sharing electrons to achieve stable electron configurations.

• Ionic Bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions (cations and anions).

• Covalent Bonds: Formed by the sharing of electron pairs between atoms. Can be single, double, or triple bonds.

• Polar Covalent Bonds: Electrons are shared unequally, leading to partial charges.

• Nonpolar Covalent Bonds: Electrons are shared equally.

Example: NaCl forms via ionic bonding; H2O forms via polar covalent bonding.

Electronegativity and Hydrogen Bonds

Electronegativity: The ability of an atom to attract electrons in a bond. Differences in electronegativity lead to polar bonds.

Hydrogen Bonds: Weak attractions between a hydrogen atom (covalently bonded to an electronegative atom) and another electronegative atom. Important in water and biological molecules.

Example: Water molecules form hydrogen bonds, contributing to water's unique properties.

Summary Table: Types of Chemical Bonds

Bond Type	How Formed	Example	Relative Strength
Ionic	Electron transfer	NaCl	Strong (in solid state)
Covalent	Electron sharing	H2O, O2	Very strong
Hydrogen	Attraction between H and electronegative atom	Between water molecules	Weak (individually)

Additional info: Understanding these basic chemical principles is essential for studying biological molecules and processes, as all life is based on chemical interactions.