CHEM 132 - Ch 3 Electronic Structure (Exam 1)

What is the relationship between wavelength and frequency of light?

They are inversely proportional: c = (wavelength)(frequency), where c is the speed of light, 2.998 x 10^8 m/s. Longer wavelength means lower frequency.

What is the equation relating a photon's energy to its frequency?

E = h x frequency, where h is Planck's constant, 6.626 x 10^-34 J.s. Energy is directly proportional to frequency.

How do you calculate photon energy from wavelength?

E = hc / wavelength. Energy is INVERSELY proportional to wavelength — shorter wavelength means higher energy.

Order the electromagnetic spectrum from LOWEST to HIGHEST energy.

Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray.

Within visible light, which colour has the highest energy and which the lowest?

Violet (about 400 nm) has the highest energy and shortest wavelength. Red (about 700 nm) has the lowest energy and longest wavelength.

What is the photoelectric effect and why does it matter?

Light striking a metal ejects electrons only above a threshold FREQUENCY, regardless of intensity. Einstein explained it by treating light as discrete packets (photons) — evidence that energy is quantised.

What does quantised mean?

Restricted to specific discrete values rather than any value on a continuum. Like stairs rather than a ramp.

Why do elements produce line spectra rather than continuous spectra?

Because electron energy levels are quantised. Electrons can only occupy specific energies, so only specific photon energies are emitted, producing discrete lines.

What happens when an electron ABSORBS a photon?

It jumps to a higher energy level (an excited state). The photon's energy must exactly match the gap between levels.

What happens when an electron falls to a lower energy level?

It EMITS a photon whose energy equals the difference between the two levels. This produces emission lines.

What was the key idea of the Bohr model?

Electrons orbit the nucleus in fixed, quantised energy levels. Energy is absorbed or emitted only when an electron moves between levels.

What is the Bohr energy equation for hydrogen?

E(n) = -2.18 x 10^-18 J / n^2. Energies are negative because the electron is bound; n = infinity corresponds to a free electron at zero energy.

What is the main limitation of the Bohr model?

It works only for hydrogen and other one-electron species. It fails for multi-electron atoms because it ignores electron-electron repulsion.

What is the de Broglie hypothesis?

Matter has wave properties: wavelength = h / mv. Significant only for very small, fast particles like electrons, not for baseballs.

What is the Heisenberg uncertainty principle?

You cannot simultaneously know both the exact position and the exact momentum of an electron. The more precisely you know one, the less precisely you know the other.

What is an atomic orbital?

A mathematical function from the Schrodinger equation describing a REGION OF PROBABILITY where an electron is likely to be found, not a fixed path or orbit.

What does the principal quantum number (n) describe, and what values can it take?

The energy level and overall size of the orbital. Values: 1, 2, 3, and so on. Higher n means higher energy and larger average distance from the nucleus.

What does the angular momentum quantum number (l) describe, and what values can it take?

The SHAPE of the orbital (subshell). Values: 0 to n-1. l = 0 is s, 1 is p, 2 is d, 3 is f.

What does the magnetic quantum number (m sub l) describe, and what values can it take?

The ORIENTATION of the orbital in space. Values: -l through 0 to +l.

What does the spin quantum number describe?

The intrinsic spin of the electron. Values: +1/2 or -1/2 only.

How many orbitals are in an s subshell, and how many electrons can it hold?

1 orbital, 2 electrons.

How many orbitals are in a p subshell, and how many electrons?

3 orbitals, 6 electrons.

How many orbitals are in a d subshell, and how many electrons?

5 orbitals, 10 electrons.

How many orbitals are in an f subshell, and how many electrons?

7 orbitals, 14 electrons.

What is the maximum number of electrons in a shell with principal quantum number n?

2n^2. So n=1 holds 2, n=2 holds 8, n=3 holds 18, n=4 holds 32.

What shape is an s orbital?

Spherical, centred on the nucleus.

What shape is a p orbital?

Dumbbell shaped, with two lobes and a node at the nucleus. The three p orbitals lie along the x, y and z axes.

What is the Pauli exclusion principle?

No two electrons in an atom can have the same set of all four quantum numbers. Practically: an orbital holds at most 2 electrons, and they must have opposite spins.

What is the Aufbau principle?

Electrons fill the lowest-energy orbitals first, building up from the ground state.

What is Hund's rule?

Within a set of equal-energy (degenerate) orbitals, electrons occupy them SINGLY with parallel spins before any orbital gets a second electron. This minimises repulsion.

What is the orbital filling order through 4p?

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Note 4s fills before 3d.

Why is chromium's configuration [Ar]4s1 3d5 rather than [Ar]4s2 3d4?

A half-filled d subshell is unusually stable, so one 4s electron shifts to 3d. Copper does the same to reach a FULL d subshell: [Ar]4s1 3d10.

What are valence electrons?

The electrons in the outermost occupied shell. They determine chemical bonding and reactivity.

What is effective nuclear charge (Z eff)?

The net positive pull an electron actually feels, after inner electrons shield it from the full nuclear charge. Roughly Z minus the shielding electrons.

How does atomic radius change ACROSS a period, and why?

It DECREASES left to right. Protons are added without adding a new shell, so Z eff rises and pulls the electrons in tighter.

How does atomic radius change DOWN a group, and why?

It INCREASES. Each row adds a new principal energy level, so valence electrons sit further out and are better shielded.

What is first ionisation energy?

The energy required to remove the most loosely held electron from a gaseous atom. Always endothermic (positive).

How does ionisation energy trend across a period and down a group?

INCREASES across a period (higher Z eff, tighter hold). DECREASES down a group (valence electrons further out and more shielded). Opposite of the atomic radius trend.

Why is there a huge jump between successive ionisation energies at a certain point?

Because you have stripped the valence shell and begun removing electrons from the stable noble-gas core. The size of the jump reveals how many valence electrons the atom had.

What is electron affinity?

The energy change when a gaseous atom gains an electron. Generally becomes more negative (more favourable) across a period; halogens are the most negative.

What is electronegativity, and which element is highest?

The tendency of an atom to attract shared electrons within a bond. FLUORINE is highest at about 4.0. It increases across a period and decreases down a group.

How does a cation's size compare to its parent atom?

SMALLER. It has lost electrons, so the remaining electrons feel a greater effective nuclear charge and are pulled in, often losing an entire shell.

How does an anion's size compare to its parent atom?

LARGER. Added electrons increase repulsion while nuclear charge stays the same, so the electron cloud expands.

What does isoelectronic mean, and how does radius vary in an isoelectronic series?

Isoelectronic species have the SAME number of electrons. Within the series, radius DECREASES as nuclear charge (proton count) increases.