Key Concepts on Atoms, Bonds, and Water

ATOMS

  • Definition: All matter is composed of atoms, which are tiny particles consisting of:

    • Protons: Mass = 1, Charge = +1
    • Neutrons: Mass = 1, Charge = 0
    • Electrons: Negligible mass, Charge = -1

  • Volume:

    • Most of an atom's volume is empty space. For scale, if an atom were the size of a stadium, the nucleus would be about the size of a pea.

DETERMINANTS OF ATOM IDENTITY

  • Protons: The number of protons in the nucleus determines the identity of the atom.
    • Example: 1 proton = hydrogen, 2 protons = helium
  • Electrons: The number of electrons determines how an atom bonds with others.
  • Neutrons: The number of neutrons affects the atomic mass.
    • Protons contribute to the mass as well.

PERIODIC TABLE

  • Comprises 94 naturally occurring elements plus 25 synthetic elements.
  • Organized by the number of protons (atomic number) and electrons.

ELEMENTS

  • Definition: A substance made of only one type of atom.
    • Example: Aluminum is composed solely of aluminum atoms.
  • Each element has unique physical and chemical properties due to the different atomic compositions.

CHNOPS ELEMENTS

  • The six elements that make up 96% of the mass of all living organisms:
    • Carbon
    • Hydrogen
    • Nitrogen
    • Oxygen
    • Phosphorus
    • Sulfur

ATOMIC MASS

  • Calculation: Atomic mass is determined by the number of protons and neutrons in an atom.
    • Example: Carbon has 6 protons and 6 neutrons, mass = 12.
  • Notation: Mass number = number of protons + neutrons.
  • Atomic number: Number of protons.

ISOTOPES

  • Definition: Atoms with the same number of protons but different numbers of neutrons.
    • Example: Carbon with 6 protons and 6 neutrons = carbon-12; with 8 neutrons = carbon-14 (radioactive).

ELECTRONS AND BONDING

  • The number of electrons determines how atoms interact and bond.
  • Electrons occupy orbitals outside the nucleus:
    • First shell: 1 orbital, holds 2 electrons.
    • Second shell: 4 orbitals, holds 8 electrons.
    • Valence shell: Outermost shell determines reactivity and bonding.

CHEMICAL BONDS

  • Definition: The attractive forces that hold atoms together to form molecules.
  • Types:
    • Ionic Bonds: Transfer of electrons, typically between metals and nonmetals. Results in charged ions.
    • Covalent Bonds: Sharing of electrons between nonmetals. Strong and difficult to break.
    • Hydrogen Bonds: Weak attraction between a partial positive hydrogen in one molecule and a more electronegative atom (O, N, or F) in another.

POLARITY AND ELECTRONEGATIVITY

  • Electronegativity (EN): Measure of how strongly an atom attracts electrons.
    • Higher EN in top right of periodic table, lower in bottom left.
    • Large differences in EN indicate very polar bonds.
    • Nonpolar when there’s equal sharing of electrons.

WATER PROPERTIES

  • Hydrogen Bonding: Responsible for water's unique properties:

    • Ice floats due to lower density.
    • High specific heat capacity; stabilizes temperature.
    • High heat of vaporization; contributes to cooling effects during evaporation.
    • Cohesion (water sticks to itself) and adhesion (water sticks to other substances), crucial for plant water transport.

  • Solvent Properties:

    • Water is a great solvent for polar molecules and ions ("like dissolves like").
    • Biological reactions often occur in aqueous environments.

IONIC BONDS IN WATER

  • Ionic compounds dissolve in water as the polar molecules interact with charged ions (e.g., Na+ and Cl- in NaCl).