Periodic Trends
Explain periodic comparisons with charge, distance, shielding and supplied exception data.
AP Chemistry Topic 1.7 · CED printed p. 37 / PDF p. 44. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Configurations
- Coulombic attraction
- PES positions
Trends come from charge, distance and shielding
The periodic table repeats chemical behavior because outer electron patterns repeat. Main-group elements in a column have analogous valence configurations; going down adds shells. Across a period, more protons are added while valence electrons enter the same principal shell. Core shielding changes relatively little, so effective nuclear attraction generally increases. The electron cloud contracts and first ionization energy generally increases. Down a group, larger distance and added core shielding generally outweigh added protons for valence-electron removal.
Atomic radius is an operational estimate from distances, not the edge of a hard ball. Compare like radius definitions. Losing outer electrons commonly makes a cation smaller than its neutral atom, often eliminating a shell; adding electrons commonly makes an anion larger through increased electron repulsion. For an isoelectronic set, the electron arrangement is shared, so greater proton number usually means smaller radius.
Similar trends, different physical meanings
First ionization energy removes an electron from a gaseous atom and is positive. Successive values refer to increasingly positive gaseous ions. A large jump after the third removal suggests three relatively accessible valence electrons before reaching core electrons. Electronegativity describes an atom’s pull on shared electrons in a bond; it is not the same measurement as ionization energy. It generally increases across representative-element periods and decreases down groups.
For the sign convention in an electron-addition table, an enthalpy change describes energy exchanged as heat at constant pressure. Exothermic means the process releases heat and has a negative enthalpy change; endothermic means it absorbs heat and has a positive change. These signs describe the process from its stated starting species to its final species. Electron affinity concerns adding an electron to a gaseous species. Sign conventions vary: an electron-gain enthalpy is negative for favorable exothermic addition, while some affinity tables give the released energy as positive. Say “more energetically favorable addition” if the sign convention is unstated. Affinity is less smoothly periodic than radius; filled shells, half-filled subshells and electron repulsion create exceptions. A compact valence shell can increase the repulsion experienced by an incoming electron; greater nuclear attraction alone does not settle every comparison. Do not predict exact values from one arrow.
Trend exceptions are good tests of whether you understand subshells. A p electron can be easier to remove than an s electron in the same shell, and a paired p electron can be easier to remove than a corresponding singly occupied p electron because of repulsion. The AP exclusion removes memorization of Aufbau-exception configurations, not reasoning from a supplied configuration or data. Use trends to estimate missing values, then check the data when available.
Let's work through it
Work it through
Rank O2−, F−, Na+ and Mg2+ by increasing radius, and interpret a large ionization-energy jump after the second electron of an unknown main-group atom is removed.
- Count electrons: all four ions have 10. Their occupied shells are therefore analogous.
- Compare proton counts: O 8, F 9, Na 11, Mg 12. Stronger nuclear attraction pulls the same electron population inward.
- Increasing radius is Mg2+ < Na+ < F− < O2−.
- A large jump from IE2 to IE3 indicates the third removal reaches a more tightly held core electron. For a main-group atom, this supports two valence electrons and a group-2 pattern.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Why does radius generally decrease across a period even though electron count increases?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Why does radius generally decrease across a period even though electron count increases?
Worked answer and reason
The increasing effective nuclear attraction in the same principal shell usually dominates the added electron repulsion.
Use charge and shielding rather than the claim that adding electrons always enlarges an atom.
Retrieval 2
IE values are 600, 1800, 14,800 and 21,000 kJ mol−1. What main-group valence count is suggested?
Worked answer and reason
Two; the large increase begins at the third removal.
The jump marks entry into a core shell.
Retrieval 3
Why should “electron affinity increases” be qualified?
Worked answer and reason
The trend has substantial subshell exceptions and different tables use different energy sign conventions.
Electron addition is a specific energy process, not a universal arrow.
Catch the tempting mistake
- Bigger nuclear charge always means smaller atoms down a group.
- New shells and shielding increase valence distance; compare the full structure.
- A negative electron-gain enthalpy means addition is unfavorable.
- Negative enthalpy denotes release of energy under that convention.
- Electronegativity alone ranks all bond energies.
- It predicts attraction to shared electrons and bond polarity, not every bond dissociation energy.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 1 problem 14 · ap application · symbolic
- Unit 1 problem 15 · ap synthesis · data
- Unit 1 problem 16 · ap synthesis · verbal
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Scientific calculator
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For the teacher
- Require a cause→attraction→property chain in trend explanations.
- Use exception data and ask students to locate the removed electron rather than memorize exceptions.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 1.7.A
- Explain the relationship between trends in atomic properties of elements and electronic structure and periodicity.
- 1.7.A.1
- The organization of the periodic table is based on patterns of recurring properties of the elements, which are explained by patterns of ground-state electron configurations and the presence of completely or partially filled shells (and subshells) of electrons in atoms.
- 1.7.A.2
- Trends in atomic properties within the periodic table (periodicity) can be predicted by the position of the element on the periodic table and qualitatively understood using Coulomb’s law, the shell model, and the concepts of shielding and effective nuclear charge. These properties include: i. Ionization energy ii. Atomic and ionic radii iii. Electron affinity iv. Electronegativity.
- 1.7.A.3
- The periodicity (in 1.7.A.2) is useful to predict/estimate values of properties in the absence of data.
- Writing the electron configuration of elements that are exceptions to the aufbau principle will not be assessed on the AP Exam.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §6.5, Periodic Variations — Effective nuclear charge, shielding, ionization-energy exceptions and electron-affinity limits.
Glossary and deeper reading · 10 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Photoelectron spectroscopy
Photoelectron spectroscopy uses photons to remove electrons and measure their binding energies. Peak position indicates how tightly electrons are bound, while peak intensity can indicate the relative population of a subshell in the simplified model. For example, an inner-shell electron normally produces a peak at higher binding energy than a valence electron.
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Valence electron
Valence electrons are the electrons most directly involved in an atom's bonding and chemical behavior. For main-group atoms, they occupy the highest occupied principal shell. For example, neutral chlorine has seven valence electrons, helping explain its tendency to form a chloride ion.
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Electron shielding
Electron shielding is the reduction in nuclear attraction experienced by an electron because other electrons also repel it. Inner electrons generally shield an outer electron more effectively than electrons in the same shell. For example, adding an occupied inner shell helps explain why atomic size usually increases down a periodic-table group.
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Cation
A cation is a positively charged ion. Its total positive nuclear charge exceeds its total electron charge. For example, calcium commonly forms a cation with charge +2 by losing two electrons.
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Anion
An anion is a negatively charged ion. It has more electrons than needed to balance its positive nuclear charge. For example, a chlorine atom gaining one electron forms a chloride anion.
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Ionization energy
Ionization energy is the energy required to remove an electron from a gaseous atom or ion. Removing an electron requires energy, and successive removals can reveal a transition from valence to core electrons. For example, sodium's second ionization requires much more energy than its first because it removes a core electron.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Enthalpy
Enthalpy is a state function defined from internal energy, pressure, and volume. At constant pressure with only pressure–volume work, an enthalpy change equals the heat transferred to the system. For example, a negative reaction enthalpy indicates heat release for the written reaction under those conditions.
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Exothermic process
An exothermic process transfers heat from the chosen system to its surroundings. Under constant-pressure conditions with only pressure–volume work, its enthalpy change is negative. For example, freezing water releases heat to the surroundings.
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Electron subshell
A subshell is a set of atomic orbitals with the same principal and angular-momentum quantum numbers. The s, p, d, and f subshells contain one, three, five, and seven orbitals, respectively. For example, a p subshell can accommodate six electrons because each of its three orbitals can hold two.