Atomic Structure

• Protons and neutrons reside together in the nucleus, which holds almost all the mass and all the positive charge.
• Electrons move in regions of space around the nucleus and are attracted electrostatically by the protons.
• An atom is neutral, so protons = electrons.

Nucleon number (mass number): total number of protons and neutrons in the nucleus of an atom.

Proton number (atomic number): number of protons in the nucleus. It determines the identity of the element.

Neutrons = nucleon number − proton number.

• Same electronic configuration → same chemical properties: chemical reactions involve electrons, not the nucleus.
• Different masses → different physical properties, for example density and rate of diffusion.

Name an isotope with its nucleon number, for example chlorine-35, ³⁵Cl, or ²H.

Why does the electron bend more?

• Between the plates, each particle feels a sideways force that depends on its charge. A proton and an electron feel forces of the same size.
• The same force changes the path of a light particle far more than a heavy one. A proton is 1836 times heavier.

AnalogyGuests (electrons) check in one at a time. Every guest wants to pay as little as possible.

• Rooms on lower floors are cheaper, and within a floor s < p < d.
• Guests take the cheapest room available.
• Nobody wants to share with a stranger, but if the next empty room costs much more, sharing is the better deal.
• Rooms of the same price: each guest takes an empty one before anyone shares.

These are the three filling rules in 2.3.3.

What is an orbital?

We cannot know exactly where an electron is. Instead, imagine taking a snapshot of its position again and again, and marking a dot each time.

The dots build up a cloud: dense near the nucleus, thinning out further away.

Relative energies of the orbitals

• Energy rises with n: 1s < 2s < 3s < 4s.
• Within a shell: s < p < d < f. Orbitals of the same subshell have the same energy: they are degenerate.
• The levels get closer together as n increases, until they overlap. The first overlap: 4s is lower in energy than 3d, so 4s fills first.

Two ways to represent it

Electrons-in-boxes

Each orbital is a box, each electron is a half arrow. Two electrons in one box point in opposite directions (opposite spins).

Argon. Label boxes with the subshell only (2p, not 2p⁶).

Rules for filling orbitals

Using the order

• Follow the arrows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p.
• The block of an element is the subshell filled last: s-block, d-block, p-block.
• The period number is the n of the valence shell. For example Na is Period 3 and the s block, so it ends 3s¹. P is the 3rd element of the p block in Period 3, so it ends 3p³.
• A noble gas shorthand shows the inner electrons as the previous noble gas: Br is [Ar] 3d¹⁰ 4s² 4p⁵.

Anomalies: Cr and Cu

Cr expected [Ar] 3d⁴ 4s², actual [Ar] 3d⁵ 4s¹

Cu expected [Ar] 3d⁹ 4s², actual [Ar] 3d¹⁰ 4s¹

3d and 4s are very close in energy, so moving one electron costs little, and the more stable d⁵ or d¹⁰ arrangement wins.

Configurations of ions

Fe fills 4s before 3d.

PredictSo does Fe²⁺ lose its last-in electrons, two of the 3d?

Anion: electrons are added to the empty or partially-filled valence orbitals, for example S²⁻: 1s² 2s² 2p⁶ 3s² 3p⁶.

• All IEs are endothermic: energy must be put in to pull an electron away from the attraction of the nucleus.
• Gaseous atoms are far apart, so the energy measured is only for removing the electron. (g) is the state symbol for gas.
• A mole is a fixed number of particles, 6.02 × 10²³. kJ mol⁻¹ means kilojoules for one mole of atoms.

Shielding and effective nuclear charge

The outer electron is pulled by every proton, but the inner electrons sit between it and the nucleus and repel it, cancelling part of that pull: shielding.

AnalogyStanding in the back row of a concert: the crowd in front (inner electrons) blocks most of the stage, however big the stage (nucleus) is.

Electrons in the same shell shield each other very little.

First IE of elements 1 to 20 / kJ mol⁻¹

Across periods 2 and 3, 1st IE generally ↑

1. Nuclear charge ↑, while there is a negligible ↑ in inner shell shielding effect, as e⁻ are added to the same shell
2. → effective nuclear charge ↑, atomic radius ↓
3. → valence e⁻ more strongly attracted by the nucleus → more energy required to remove them

Down a group, 1st IE ↓

1. Atomic radius ↑
2. No. of principal quantum shells ↑ → shielding effect ↑, which largely cancels out the ↑ in nuclear charge
3. → valence e⁻ less attracted by the nucleus → less energy required to remove them

Dips: Be → B (also Mg → Al)

PredictThe 2p electron is higher in energy. Does that make it harder or easier to remove?

AnalogyThink of a deep well. Removing an electron means lifting it to the top (where it is free). A ball resting on a higher ledge needs a shorter lift.

Dips: N → O (also P → S)

AnalogyBack to the hotel: O's 8th guest had to share a 2p room with a stranger. Two guests in one room push each other, so one of them is easier to move out.

• Successive ionisation energies always increase: the same protons hold fewer and fewer electrons.
• Gradual increase: electrons removed from the same subshell. A small jump: the next electron is from a lower energy subshell of the same shell.
• Big jump: the next electron comes from an inner shell, nearer the nucleus and less shielded. The number removed before the jump is the group's valence electrons.