Atomic Structure and Properties
Atoms & Electrons — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 10 parts Lesson 3 of 16 Unit quiz
Lesson summary
Atoms & Electrons — AP CHEMISTRY notes: Describe atomic structure, Coulomb's law, electron configuration, ionisation energy and photoelectron spectroscopy.Protons are positive particles in the nucleus, neutrons are neutral particles in the nucleus, and electrons are negative part…
- Atomic number (Z)
- Mass number (A)
- Conclusion
- Photon energy
- Aufbau principle
- Binding energy
Section 1 of 10Atomic structure
Atomic structure
Protons are positive particles in the nucleus, neutrons are neutral particles in the nucleus,
- and electrons are negative particles in shells around the nucleus.
Atomic number (Z)
Atomic number (Z) the number of protons, which defines the element.
Mass number (A)
Mass number (A) protons + neutrons; electrons contribute almost no mass.
A neutral atom has equal numbers of protons and electrons; in an ion, electrons = Z − charge.
Isotopes
Isotopes atoms of the same element with the same Z but different mass numbers, so different neutron numbers.
Notation
- Notation the mass number is written above and the atomic number below the atomic symbol.
Section 2 of 10Rutherford's experiment
Alpha particles were fired at gold foil.
- Most passed straight through, some were deflected strongly and a few bounced back.
Conclusion
- Conclusion the atom has a small, dense, positively charged nucleus containing the protons and neutrons.
Section 3 of 10Coulomb's law
Coulomb's law
Coulomb's law the electrostatic force between charges is proportional to the product of the charges and inversely proportional to the square of the distance.
- F = k q1q2/r2, with k ≈ 8.99 × 109 N m2 C−2.What the symbols mean: F = electric force (N) · Q1, Q2 = charges (C) · r = separation (m) · k = 1/4πε0 = 8.99 × 109 N m2 C−2
- The force is attractive for opposite charges and repulsive for like charges.
Larger charges and smaller separation give stronger attraction.
Units must be consistent (coulombs and metres) before substituting.
Section 4 of 10Electrons, light and shells
Light behaves as a wave and as a particle, the photon; c = λν.
Photon energy
- E = hν, with h = 6.626 × 10−34 J s.
Absorbing a photon moves an electron to an outer shell, and emitting a photon moves it to an inner shell.
Shells farther from the nucleus have higher energy.
Subshells s, p, d and f contain 1, 3, 5 and 7 orbitals, and each orbital holds at most two electrons.
| Shell | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Subshells | s | s, p | s, p, d | s, p, d, f |
| Maximum electrons | 2 | 8 | 18 | 32 |
Section 5 of 10Electron configuration
Aufbau principle
- Aufbau principle electrons fill the lowest-energy orbitals first, following the Aufbau diagram.
Electron configuration is written with subshell labels and electron numbers as exponents, for example fluorine 1s2 2s2 2p5.
Condensed configuration uses the previous noble gas, for example fluorine [He] 2s2 2p5.
Valence electrons are in the outer shell, and core electrons are in inner shells.
Elements in the same group have similar valence configurations and so similar properties.
Orbital diagrams use boxes and arrows; electrons fill orbitals of equal energy singly before pairing, and paired electrons have opposite spins.
Section 6 of 10Ionisation energy
Ionisation energy
Ionisation energy the energy needed to remove an electron from a gaseous atom or ion; it is always positive.
Factors, all explained by Coulomb's law.
- Greater nuclear charge increases ionisation energy.
- Greater distance from the nucleus decreases it.
- More shielding by core electrons decreases it by reducing effective nuclear charge.
Distance and shielding usually outweigh nuclear charge when comparing atoms, so fluorine has a higher first ionisation energy than chlorine.
Section 7 of 10Photoelectron spectroscopy
PES measures the energies of electrons ejected by radiation.
Binding energy
Binding energy the energy needed to overcome attraction to the nucleus.
A spectrum plots binding energy (x-axis) against relative number of electrons (y-axis).
- Each peak is a subshell, and the peak position gives the binding energy.
- Higher binding energy means electrons closer to the nucleus.
- Peak height is proportional to the number of electrons in the subshell.
The peaks give the electron configuration, and so the element.
Section 8 of 10More from the specification
- Periodic
- Describing properties that repeat at regular intervals along the table
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Atomic number | Number of protons in the nucleus. |
| Isotope | Atom of the same element with a different number of neutrons. |
| Coulomb's law | Force between charges is proportional to q1q2 and inversely proportional to r squared. |
| Aufbau principle | Electrons fill the lowest-energy orbitals first. |
| Ionisation energy | Energy needed to remove an electron from a gaseous atom or ion. |
| Binding energy | Energy needed to remove an electron against the nuclear attraction. |
Section 10 of 10Exam technique
1
Reading peak height in PES as energy — height shows the number of electrons and position shows binding energy
2
Forgetting that electrons in an ion equal Z minus the charge
3
Confusing isotopes (different neutrons) with ions (different electrons)
4
Comparing ionisation energies by nuclear charge alone — distance and shielding must also be considered
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