Atoms: The Smallest Unit of Matter

Definition and Structure

Atoms are the fundamental building blocks of all matter, both living and non-living. They are the smallest unit of an element and retain the properties of that element. Matter is anything that takes up space and has mass, including organisms, rocks, and water.

• Matter: Anything that occupies space and has mass.

• Chemical Element: A pure substance made of only one type of atom.

• Atom: The smallest unit of an element, composed of subatomic particles.

• Atoms: Make up both living and non-living matter.

• Example: Atoms are the smallest units of matter, forming the basis for all chemical substances.

Atomic Structure

Subatomic Particles

Atoms are composed of three main subatomic particles, each with distinct properties:

• Protons: Positively charged particles found in the nucleus; each has a mass of 1 atomic mass unit (AMU).

• Neutrons: Neutral particles (no charge) also located in the nucleus; each has a mass of 1 AMU.

• Electrons: Negatively charged particles with almost no mass, orbiting the nucleus in energy shells.

• Example: Negatively charged particles of atoms with almost no mass are called electrons.

Elements of Life

Periodic Table and Biological Elements

Of all known elements, only a small subset is found in living organisms. The periodic table arranges elements based on their chemical properties. Approximately 97% of the mass of most life is composed of Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS).

• Bulk Elements: Required for life in large amounts.

• Trace Elements: Required in small amounts for specific biological functions.

• Example: The periodic table highlights the elements most abundant in living systems.

Atomic Properties

Atomic Number, Mass Number, and Atomic Mass

Each atom of an element has unique properties:

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

• Mass Number: Total number of protons and neutrons in the nucleus.

• Atomic Mass: Weighted average mass of all isotopes of an element.

• Example: The atomic number of carbon is 6, and its most common isotope has a mass number of 12.

Electron Orbitals and Energy Shells

Electron Distribution and Valence Shells

Electrons occupy three-dimensional regions called orbitals, envisioned as energy shells. Shells closer to the nucleus are lower in energy, while distant shells are higher in energy. Valence electrons are found in the outermost energy shell and are critical for chemical bonding.

• 1st shell: Holds up to 2 electrons.

• 2nd shell: Holds up to 8 electrons.

• Valence Electrons: Electrons in the outermost shell, involved in chemical reactions.

• Example: Energy shells for C, H, N, O, P, S show the electron distribution in these elements.

Octet Rule

Stability of Atoms

The octet rule states that atoms are more stable (less reactive) when their valence shells are fully occupied, typically with eight electrons. Atoms are most reactive when their outer shells are not full.

• 1st shell: Maximum of 2 electrons.

• 2nd shell: Maximum of 8 electrons.

• Example: Neon is unreactive because its valence shell is full.

Isotopes

Variation in Neutron Number

Isotopes are atoms of the same element that differ in the number of neutrons. They have the same atomic number but different mass numbers. Atomic mass is the average mass of all isotopes.

• Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.

• Radioactive Isotopes: Unstable isotopes that break down and emit energy; used in medicine and dating fossils.

• Half-life: The time it takes for half of the radioactive atoms in a sample to decay.

Summary Table: Subatomic Particles

Additional info: These foundational chemical principles are essential for understanding the molecular basis of microbiology, including cell structure, metabolism, and genetic information.