Honors Chemistry 1: Study Guide for Unit 3 (Periodic Trends and Electron Structure)

The Quantum Model and Light

The Bohr model correctly proposed that electrons exist in specific energy levels and can't be found between them. When electrons absorb energy (from flame or electricity), they jump to higher levels—an excited state. When they drop back to their ground state, they emit energy, usually as light.

The color of light depends on the energy transition. Red light comes from level 3→2 transitions, green from 4→2, blue from 5→2, and violet from 6→2. This is why different elements produce different colored flames!

Quantum theory changed our understanding of light itself. Light travels as both waves AND as discrete bundles called photons. Higher frequency light has higher photon energy, calculated using Planck's constant: Energy = 6.626 × 10⁻³⁴ J-sec × frequency.

Think about it: If light shows both wave and particle properties, could other things do the same? This led to the discovery that electrons also behave as waves, particularly when they're moving—a concept called wave-particle duality.

Heisenberg's uncertainty principle tells us something fascinating—it's impossible to know both an electron's exact position AND velocity at the same time. This isn't about measurement limitations; it's a fundamental property of reality that Schrödinger mapped through probability calculations.

Electron Orbitals and Quantum Structure

Forget the neat electron orbits you've seen before! Electrons actually exist in orbitals—fuzzy, cloud-like regions where we're 90% certain to find them. These orbitals come in different shapes and correspond to specific energy levels.

The quantum mechanical model organizes electrons into a hierarchy. Each energy level $1-7$ contains sublevels (s, p, d, f), which contain orbitals, which each hold up to 2 electrons. The s sublevel has 1 orbital (2 electrons max), p has 3 orbitals (6 electrons max), d has 5 orbitals (10 electrons max), and f has 7 orbitals (14 electrons max).

When filling orbitals, three important rules apply. The Aufbau principle says electrons fill lowest energy levels first (s before p, etc.). The Pauli exclusion principle states each orbital holds maximum 2 electrons with opposite spins. Hund's rule requires all orbitals in a sublevel to get one electron before any pairs up.

Pro tip: Notice how the periodic table's organization reflects electron configurations! The s-block elements are groups 1-2, p-block are groups 13-18, d-block are transition metals, and f-block are lanthanides and actinides.

Electron Configuration and Valence Electrons

Writing out full electron configurations can be tedious, but there's a shortcut! You can use the periodic table to determine configurations—the row indicates the sublevel, and the group number helps identify electron count. For even greater efficiency, you can use noble gas notation by putting the previous noble gas symbol in brackets.

Valence electrons are the most important electrons for chemistry—they're in the outermost energy level and participate in bonding. Elements in the same group have the same number of valence electrons (usually 1-8). We represent these visually with Lewis dot structures that show valence electrons as dots around the element symbol.

Elements generally want to achieve a stable octet (8 valence electrons) like noble gases. When forming ions, atoms typically gain or lose the fewest electrons possible. Metals tend to lose electrons to form positive cations, while nonmetals gain electrons to form negative anions.

Remember: You can predict the charge of an ion by looking at its group number! Group 1 metals form 1+ ions, Group 2 form 2+ ions, and so on. Similarly, Group 17 (halogens) form 1- ions, and Group 16 form 2- ions.

Periodic Trends

Atomic radius increases down a group but decreases across a period. Why? Down a group, more energy levels mean electrons are further from the nucleus. Across a period, the increasing number of protons pulls electrons tighter despite having the same energy level.

When atoms form ions, their size changes. Anions (negative ions) are larger than their neutral atoms because extra electrons increase repulsion. Cations (positive ions) are smaller than their atoms because they've lost electrons, reducing repulsion.

Ionization energy (energy needed to remove an electron) increases across periods and decreases down groups. Smaller atoms hold their electrons more tightly! Each subsequent electron removal requires more energy, with a significant jump when removing core (non-valence) electrons.

Electronegativity measures an atom's ability to attract shared electrons. It follows the same pattern as ionization energy—increasing across periods and decreasing down groups. This helps explain reactivity patterns: metals (which lose electrons) become more reactive down groups, while nonmetals (which gain electrons) become more reactive across periods.

Exam tip: Remember these trends by thinking about "nuclear pull" versus "distance." The stronger the nuclear pull and the shorter the distance between the nucleus and outer electrons, the smaller the atom, the higher the ionization energy, and the greater the electronegativity.