Chemistry (university) · Part 01
Atomic structure and periodic trends
How electrons are arranged in atoms and why the periodic table has patterns.
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- Atomic emission spectrumWhen atoms are heated, they give off light at only a few exact colours. Each element has its own set of colours. This shows that electron energies are fixed steps, not a smooth range.
- Photon energyLight comes in tiny packets called photons. A photon's energy is E = hc/λ, where h is Planck's constant, c is the speed of light and λ is the wavelength. Shorter wavelength means more energy.
- Wave-particle dualityLight behaves like a wave and like a stream of particles. Matter such as electrons also behaves like a wave. The de Broglie wavelength is λ = h/mv, so heavy, fast objects have wavelengths too small to notice.
- Quantum numbersFour numbers describe each electron in an atom. n is the shell (size and energy), l is the subshell shape, mₗ is the orbital direction and mₛ is the spin, +½ or −½.
- Atomic orbitalAn orbital is a region around the nucleus where an electron is most likely to be found. An s orbital is a sphere. A p orbital is shaped like a dumbbell. Each orbital holds at most two electrons.
- Electron configurationThe electron configuration lists which orbitals an atom's electrons fill. Electrons fill the lowest-energy orbitals first, in the order 1s, 2s, 2p, 3s, 3p, 4s, 3d. This is called the Aufbau principle.
- Pauli exclusion principle and Hund's rulePauli: no two electrons in an atom can have all four quantum numbers the same, so an orbital holds at most two, with opposite spins. Hund: in equal-energy orbitals, electrons spread out singly before pairing up.
- Effective nuclear chargeEffective nuclear charge (Zeff) is the pull an outer electron actually feels from the nucleus. Inner electrons shield some of the nucleus's positive charge, so Zeff is smaller than the full charge. Roughly, Zeff = Z − shielding.
- Atomic and ionic radiusAtomic radius shrinks across a period because Zeff grows, and grows down a group because new shells are added. A cation is smaller than its atom. An anion is larger than its atom.
- Ionization energyIonization energy is the energy needed to pull one electron off a gas-phase atom. It rises across a period and falls down a group. A sudden big jump in successive values shows that the next electron comes from an inner shell.