Chapter 2: Atoms & Chemical Bonds

Matter

• Matter is anything that takes up space and has mass.

• Organisms are composed of matter, which is made up of elements.

Elements (C, O, H, N)

• Carbon (C), Oxygen (O), Hydrogen (H), and Nitrogen (N) make up 96% of living matter.

• Trace elements are required in minute quantities for biological processes.

Atomic Structure

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

• Subatomic Particles:

  • Protons: Positively charged, found in the nucleus.

  • Neutrons: No charge, found in the nucleus.

  • Electrons: Negatively charged, orbit around the nucleus.

Atomic Number vs. Mass Number

• Atomic Number: The number of protons in an element’s nucleus (also equals the number of electrons in a neutral atom).

• Mass Number: The sum of protons and neutrons in the nucleus.

• Atomic Mass: The atom’s total mass, approximated by the mass number.

Isotopes

• Isotopes are atoms of the same element that differ in the number of neutrons.

• Radioactive isotopes decay spontaneously, giving off energy.

• Applications: Dating fossils, tracing atoms in metabolism, diagnosing medical disorders.

Valence Electrons & Shell

• Valence Electrons: Electrons in the outermost shell.

• Valence Shell: The outermost energy level of an atom.

• Atoms with a full valence shell are chemically inert.

• Atoms with incomplete valence shells tend to form chemical bonds.

Compounds vs. Molecules

• Compound: A substance composed of two or more elements in a fixed ratio (e.g., NaCl).

• Molecule: Two or more atoms held together by covalent bonds.

Chemical Bonds

Chemical bonds form due to interactions between valence electrons.

1. Covalent Bonds (Strongest)

   • Covalent Bond: The sharing of a pair of valence electrons by two atoms.

   • Polar Covalent Bond: Electrons are unequally shared (e.g., water H₂O).

   • Non-Polar Covalent Bond: Electrons are equally shared (e.g., O₂).

2. Hydrogen Bonds (Weak)

   • Forms between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

   • Example: Water molecules interact through hydrogen bonding.

3. Ionic Bonds (Medium Strength)

   • Ionic Bond: Formed when an atom transfers electrons to another atom, creating charged ions.

   • Cation: Positively charged ion (loses electrons).

   • Anion: Negatively charged ion (gains electrons).

   • Example: NaCl (table salt) forms via ionic bonding between Na⁺ (cation) and Cl⁻ (anion).