Introduction to Atoms and Chemical Bonding

Atoms are stable when their outermost electron shell, or valence shell, is full.
- To achieve this, atoms can donate, accept, or share electrons.
- This process is known as chemical bonding.

Matter: Everything that has mass and occupies space, including nonliving things and living organisms.
Atoms: The basic units of all matter, both living and nonliving.
- Atoms are so small that billions can fit on the head of a pin.
Atomic Nucleus: The inner central region of an atom, which is not to be confused with the nucleus of a cell.
  - Subatomic Particles:
    - Protons: Positively charged particles found in the nucleus.
    - Neutrons: Neutral particles (no charge) also in the nucleus.
  - Each proton and neutron has an atomic mass unit of one.
  - Protons and neutrons together comprise nearly the entire mass of an atom but occupy less than 1% of its volume.
  - Electrons: Subatomic particles that are negatively charged and found in layers around the nucleus, called electron shells or energy levels.

Electron Shells/Energy Levels:
  - Each shell has a maximum number of electrons it can hold.
  - Electrons are sometimes depicted as a fuzzy electron cloud.
  - Charge Balance: Neutral atoms have equal numbers of protons and electrons, resulting in no net charge.

An atom consists of:
  - Protons (positive charge)
  - Neutrons (no charge)
  - Electrons (negative charge)
The number of protons in an atom defines its element.

Element: A pure substance made of only one type of atom (e.g., hydrogen, carbon, mercury).
The atomic number of an element is equal to the number of protons in its nucleus.
  - Example:
    - Hydrogen has 1 proton (atomic number = 1).
    - Carbon has 6 protons (atomic number = 6).
    - Mercury has 80 protons (atomic number = 80).
  - The total number of protons and neutrons is known as the mass number.
    - Example:
       - Hydrogen (1 proton, 0 neutrons) has a mass number of 1.
       - Carbon (6 protons, 6 neutrons) has a mass number of 12.
       - Mercury (80 protons, 121 neutrons) has a mass number of 201.

Isotopes: Atoms of the same element that have different numbers of neutrons.
- Example: Ordinary hydrogen (no neutrons), hydrogen-1 (one neutron), hydrogen-2 (two neutrons).
- Isotopes differ in mass number.

Chemical Compound: A substance made of two or more elements chemically bonded together in fixed proportions.
  - Examples include:
    - Water (H₂O): Two hydrogen atoms and one oxygen atom.
    - Sodium Chloride (NaCl): One sodium ion and one chloride ion.
Chemical formulas indicate the proportions:
- H₂O means two hydrogen atoms and one oxygen atom.
- A formula without a subscript means one atom of that element.
Compounds generally exhibit different physical and chemical properties than the individual elements from which they are made.

Ionic Bonds: Occur when a metal donates one or more electrons to a nonmetal, creating charged ions.
Covalent Bonds: Occur when electrons are shared between atoms.

Ions: Charged particles formed when atoms gain or lose electrons.
- A negatively charged ion (anion) is formed by gaining electrons.
- A positively charged ion (cation) is formed by losing electrons.
Example: Sodium (Na) loses one electron to become Na⁺, while chlorine (Cl) gains that electron to become Cl⁻, forming NaCl (table salt).

Covalent Bonds: Formed when atoms share pairs of electrons.
- Example: Hydrogen molecules (H₂) and carbon dioxide (CO₂) formed through shared electrons.
Water is an example of a compound formed by covalent bonding (H₂O).
- Oxygen has two single covalent bonds with the hydrogen atoms, resulting in the molecule being stable.

Ionic Bonds: Involves the transfer of electrons from one atom to another creating ions.
Covalent Bonds: Involves the sharing of electron pairs between atoms.

Can chemical reactions change one element into a different element?
- No, chemical reactions do not change an element's identity—only nuclear reactions can do this.

Continuing with deeper exploration into ionic bonds, covalent bonds, and hydrogen bonds.