Introduction to Matter and Chemical Elements

Definition of Matter

Matter is defined as anything that takes up space and has mass. This includes all physical substances, such as organisms, rocks, oceans, and air.

• All matter consists of at least one chemical substance.

Chemical Elements

Chemical elements are pure substances made of only one type of atom. Each element is defined by its unique atomic structure.

• Atom: The smallest unit of an element that retains the properties of that element.

• Atoms make up both elements and compounds.

Example: Atoms are the smallest units of matter. For instance, a diamond is made entirely of carbon atoms, while honey contains glucose molecules composed of carbon, hydrogen, and oxygen atoms.

Atomic Structure

Subatomic Particles

Atoms are made of subatomic particles, each with a characteristic charge, mass, and location within the atom.

• Protons and neutrons are located in the nucleus.

• Electrons orbit the nucleus in defined energy levels or shells.

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

Elements of Life

Essential Elements

Of all the known elements, only a small subset is found in living organisms. The periodic table of elements arranges all known elements based on their chemical properties.

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

• Other elements, called trace elements, are required for life in small amounts.

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

Atomic Properties

Atomic Number and Mass Number

Each atom of an element has unique properties:

• Atomic number (Z): Number of protons in the nucleus (also determines the element).

• Mass number (A): Total number of protons and neutrons in the nucleus.

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

Formulas:

• Atomic number:

• Mass number:

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 Arrangement

Electrons are arranged around a nucleus in energy shells or orbitals. These shells are envisioned as concentric layers around the nucleus.

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

• Valence electrons are found in the outermost energy shell and are important for chemical bonding.

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

Example: Carbon has 4 valence electrons, while oxygen has 6.

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 8 electrons.

• The first energy shell holds up to 2 electrons; the second and third shells can hold up to 8 electrons each.

• Atoms are less reactive when their outer valence shells are full.

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

Application: Oxygen needs 2 more electrons to fill its valence shell and achieve stability according to the octet rule.

Summary Table: Key Atomic Properties

Additional info: These notes provide foundational concepts in atomic structure, essential elements for life, and the principles governing electron arrangement and stability in atoms, which are critical for understanding chemical bonding and reactivity in General Chemistry.