Overview
This lecture introduces the Bohr model of the atom, the periodic table, and the main types of chemical bonds.
The Periodic Table
- The periodic table organizes elements by shared chemical properties, atomic mass, and atomic number.
- Developed by Dmitriy Mendeleev in 1869 to display recurring element characteristics.
- Elements are classified by physical state at room temperature (solid, liquid, gas) and reactivity.
- Atomic number (number of protons) is shown above, atomic mass below the element symbol.
Bohr Model of the Atom
- Proposed by Niels Bohr in 1913, describing atoms with a nucleus (protons, neutrons) and electrons in specific circular orbitals (energy shells).
- Electron shells are filled in order; electrons fill lower energy shells first.
- The innermost shell holds 2 electrons; the next two shells hold up to 8 electrons each (octet rule).
- Atoms are most stable when their outer (valence) shell is full.
- Group 18 elements (noble gases) have full valence shells and are very stable.
- The Bohr model is limited; it doesn’t account for the uncertainty principle or complex electron behavior.
Modern Atomic Theory & Electron Orbitals
- Electrons exist in regions called orbitals, not fixed paths, behaving as both particles and waves.
- Quantum mechanics predicts the probability of an electron’s location within an orbital.
- Electron shells contain subshells (designated s, p, d, f); s is spherical, p is dumbbell-shaped.
- Valence electrons (outer shell) determine chemical reactivity and bonding.
Chemical Bonds and Reactions
- Chemical bonds form through interactions of valence electrons.
- Chemical reactions have reactants (starting materials) and products (resulting substances) and can be reversible or irreversible.
- Four main types of bonds: ionic, covalent (polar and nonpolar), hydrogen, van der Waals.
- Ionic bonds: formed by transfer of electrons between oppositely charged ions (e.g., NaCl).
- Covalent bonds: formed by sharing electron pairs; can be polar (unequal sharing, e.g., H2O) or nonpolar (equal sharing, e.g., CH4).
- Hydrogen bonds: weak bonds between a hydrogen atom and a more electronegative atom (important in water).
- Van der Waals interactions: weak attractions due to temporary changes in electron density.
Key Terms & Definitions
- Atomic Number — Number of protons in an atom’s nucleus.
- Atomic Mass — Total number of protons and neutrons in an atom.
- Valence Electrons — Electrons in the outermost shell, involved in bonding.
- Octet Rule — Atoms are most stable with eight electrons in the valence shell.
- Orbital — Probable region for an electron around the nucleus.
- Ionic Bond — Bond from electron transfer between atoms.
- Covalent Bond — Bond from sharing electrons between atoms.
- Polar Covalent Bond — Unequal sharing of electrons in a covalent bond.
- Nonpolar Covalent Bond — Equal sharing of electrons in a covalent bond.
- Hydrogen Bond — Weak bond between hydrogen and electronegative atom.
- Van der Waals Interaction — Weak attraction from momentary electron distribution.
Action Items / Next Steps
- Review the periodic table and identify element groups, atomic numbers, and masses.
- Read about quantum mechanics and electron orbitals in textbook.
- Practice drawing Bohr and modern electron configurations for sample atoms.