1.5 Atomic structure and electron configuration

Graded Do Now: 1.1 - 1.4

Review: Subatomic Particles

Ex. Lithium

Atoms

Atomic Structure

• Atoms = protons (+), neutrons (neutral), electrons (–)
• Nucleus holds protons + neutrons; electrons orbit around it
• Mass comes from the nucleus (protons/neutrons)
• Volume comes from the electrons

Focus: Electrons

The inner electrons are called core electrons.

The outer electrons are called valence electrons.

Energy levels (shells) around the nucleus correspond to a specific, fixed amount of energy. Higher energy levels are farther from the nucleus.

Orbitals are different cloud-shapes electrons are likely to occupy

Orbital Shapes you need to know

s = sphere

p = dumbbell

d = four leaf clover

Review: RULES FOR ELECTRON CONFIGURATIONS:

1. Aufbau principle: electrons are added to the lowest subshells first and build up.

2. Hund’s Rule: each subshell should have one electron before any are doubled up.

3. Pauli Exclusion Principle: no two electrons can occupy the same specific space.

Writing Electron Configuration (at least 1 pt on aP Test)

Fluorine Example: 1s² 2s² 2p⁵

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Writing Electron Configurations is more convenient than filling diagrams.

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Write ground state electron configuration of Scandium (Sc):

Noble Gas Configuration

Example: Scandium

Instead of Sc:

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We can abbreviate:

You Try:

1. Write the ground state configuration for Gallium (Ga):

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• Write the ground state, noble gas configuration Rubidium (Rb):

★ Coulomb’s Law ★

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Coulomb's Law (qualitative)

• Force ∝ (charge × charge) ÷ distance²
• Bigger charges → stronger force.
• Bigger distance → weaker force.
• Closer + more charge = harder to pull apart

Why it matters: electrons closest to the nucleus feel the strongest pull — that's why they take the most energy to remove.

Shielding

Core electrons block valence electrons from feeling the full pull of the nucleus

Result: valence electrons experience less effective nuclear charge — even before distance is factored in

Two forces working together: farther away and shielded = valence electrons are the easiest to remove

Photoelectron Spectroscopy (PES)

• Measures the relative energies of electrons in atoms or molecules
• Method: blast the sample with high-energy radiation (UV or X-ray) → electrons get ejected
• Measure the kinetic energy of those ejected electrons
• This ejection process = photoionization

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PES Graphs

• X-axis: binding energy — energy needed to pull that electron away from the atom
• Y-axis: relative number of electrons at that energy (peak height = how many electrons share it)
• Highest binding energy = closest to the nucleus → strongest Coulombic attraction → hardest to remove

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Reading a PES Graph → Electron Configuration

Reading a PES Graph → Electron Configuration

• A PES graph is a direct map to an atom's electron configuration
• Rule #1: always check both axes before analyzing — orientation isn't guaranteed
• Higher binding energy = closer to the nucleus — locate that end of the x-axis first

Reading a PES Graph → Electron Configuration

Reading peak heights:

• Peaks often aren't labeled with electron counts
• Use the 1s peak as your ruler — it's always 2 electrons
• Compare every other peak's height to that 1s peak to estimate its electron count

Unlabeled PES Graph

• Where is the highest binding energy, left or right?
• Which peak represents the 1s orbital?
• How many electrons are in the 1s orbital?
• What is the electron configuration? What element?

Together

Which element is represented by the PES below?

You Do, We Review

Which element is represented by the PES absorption spectra shown?

Homework For Monday

• Practice Questions 1.5 and 1.6
• Graded Do Now 1.5 and 1.6 Monday