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1.4Atomic Structure and Properties

Atoms & Electrons — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 12 of 16 1 quiz

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1.4 Atoms & Electrons — AP Notes — AP CHEMISTRY notes: Represent electron configurations with the Aufbau principle and relate photoelectron spectra and ionisation energies to electron structure.The atom has a small, dense, positive nucleus of protons and neutrons, and almost all …

  • Number of neutrons
  • Rutherford's gold-foil experiment
  • Coulomb's law
  • Photon energy
  • Wave equation
  • Hund's rule

Section 1 of 10Atomic structure

Atomic structure
The atom has a small, dense, positive nucleus of protons and neutrons, and almost all the mass is in the nucleus.
Number of neutrons
A − Z, and electrons in an ion = Z − charge.
ParticleRelative chargeRelative mass (amu)Location
Proton+11.007Nucleus
Neutron01.009Nucleus
Electron−10.00055Around the nucleus
  • Rutherford's gold-foil experiment
    • most alpha particles passed through but a few were strongly deflected or bounced back, showing a tiny, dense, positive nucleus and mostly empty space.

Section 2 of 10Coulombic attraction and shielding

Coulomb's law
  • F ∝ q1q2/r2 and potential energy E ∝ q1q2/r.
Attraction increases with greater nuclear charge and decreases with greater distance.
Electrons in higher shells are on average farther from the nucleus and held less tightly.
Inner electrons shield outer electrons from the full nuclear charge.

Section 3 of 10Light and photons

Photon energy
  • E = hν, with h = 6.626 × 10−34 J s.
Wave equation
  • Wave equation c = λν, with c = 2.998 × 108 m/s.

Section 4 of 10Orbitals and subshells

Orbitals and subshells
Electrons occupy orbitals, regions of probability, and each orbital holds up to two electrons of opposite spin.
An s subshell has 1 spherical orbital, p has 3 dumbbell orbitals (px, py, pz), d has 5 and f has 7.
Shell n has n2 orbitals and holds up to 2n2 electrons; subshells hold 2, 6, 10 and 14.

Section 5 of 10Writing electron configurations

Fill subshells in the order 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p.
Noble-gas shorthand is acceptable, for example P is [Ne] 3s2 3p3.
Hund's rule
  • Hund's rule degenerate orbitals fill singly before pairing.
Transition-metal cations lose 4s electrons first: Fe is [Ar] 4s2 3d6, Fe2+ is [Ar] 3d6, Fe3+ is [Ar] 3d5.
Exceptions
  • Cr is [Ar] 4s1 3d5 and Cu is [Ar] 4s1 3d10.
Isoelectronic species such as O2−, F−, Na+ and Mg2+ have the same configuration; the one with more protons holds electrons more tightly.

Section 6 of 10Ionisation energy

Ionisation energy
Successive ionisation energies always increase because electrons are removed from an increasingly positive ion.
A large jump occurs when a core electron is removed, for example magnesium between IE2 and IE3.
Jumps reveal the number of valence electrons.

Section 7 of 10Photoelectron spectroscopy

Binding energy
Binding energy photon energy − kinetic energy of the ejected electron.
Each peak is a subshell, and the peak height is proportional to the number of electrons.
1s has the highest binding energy because it is closest to the nucleus with no shielding.
For the same subshell, binding energy increases with atomic number.
Within a shell, s electrons have slightly higher binding energy than p, which is evidence for subshells.
Binding energy often increases to the left on AP spectra.
1
Identify each peak starting from the highest binding energy (1s).
2
Assign peaks in order 1s, 2s, 2p, 3s, 3p, 4s, 3d.
3
Use peak heights to count electrons in each subshell.
4
Add the electrons to find Z and identify the element.

Section 8 of 10More from the specification

1Periodic
  • Describing properties that repeat at regular intervals along the table
2Photoelectron spectroscopy
  • Technique measuring the energies of electrons ejected by radiation.

Section 9 of 10Key terms

TermMeaning
Electron configurationArrangement of electrons in shells and subshells.
Aufbau principleElectrons occupy the lowest-energy available orbitals first.
Pauli exclusion principleAn orbital holds at most two electrons with opposite spins.
Hund's ruleDegenerate orbitals are filled singly before pairing.
Binding energyEnergy needed to remove an electron from a subshell.
Valence electronsElectrons in the outermost occupied shell.

Section 10 of 10Exam technique

1
Reading peak height as energy — height shows the number of electrons
2
Removing 3d electrons before 4s when forming transition-metal cations
3
Forgetting that electrons in an ion equal Z minus the charge
4
Putting more than two electrons in one orbital

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