Periodic Trends Notes
Periodic Trends Notes
This lesson develops Periodic Trends Notes as part of Electron Configuration and Spectra. It uses the source curriculum to organize the important facts, relationships, examples, and vocabulary into a focused Stemosphere teaching sequence.
Keep the central model in view: Periodic Trends Notes explains observable patterns by connecting structure, process, evidence, and quantitative relationships.
Broad/General Trends:
In Stemosphere terms, atomic Radius:generally,decreases across a period and increases down a group
In Stemosphere terms, ionic Radius:shows that cations are smaller than their parent atoms, while anions are larger
In Stemosphere terms, ionization Energy:typically increases across a period and decreases down a group
In Stemosphere terms, electron Affinity:generally becomes more negative (more exothermic) across a period and less negative down a group
In Stemosphere terms, electronegativity:increases across a period and decreases down a group Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, these trends are fundamental for understanding the reactivity and properties of elements throughout the periodic table
Atomic Radius:
In Stemosphere terms, the atomic radius is defined as half the distance between the nuclei of two identical atoms bonded together.
Across a Period (left to right)
Atomic radius generallydecreases. This is because the nuclear charge increases, pulling the valence electrons closer to the nucleus, while the number of electron shells remains the same, leading to poor shielding by inner electrons(Chemistry Libre Texts, “Periodic Trends”).
Down a Group (top to bottom)
Atomic radiusincreases. This is due to the addition of a new electron shell for each successive element, placing the valence electrons further from the nucleus, despite the increased nuclear charge(Chemistry Libre Texts, “Periodic Trends”).
Ionic Radius:
In Stemosphere terms, cations (positive ions):are alwayssmallerthan their parent atoms because they have lost outer shell electrons, and the remaining electrons are pulled closer by the same nuclear charge.(Chemistry Talk, “Periodic Trends Made Easy”).
In Stemosphere terms, anions (negative ions):are alwayslargerthan their parent atoms because they have gained electrons. The increased electron-electron repulsion and reduced effective nuclear charge on the valence electrons cause the electron cloud to expand(Chemistry Talk, “Periodic Trends Made Easy”).
Isoelectronic Series
For a series of ions with the same number of electrons (e.g., N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺, Al³⁺), the ionic radius decreases with increasing nuclear charge.(Chemistry Talk, “Periodic Trends Made Easy”).
In Stemosphere terms, ionization energy is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms in their ground state.
Across a Period
Ionization energy generallyincreases. This is due to increasing nuclear charge and decreasing atomic radius, which makes it harder to remove an electron(Chemistry Libre Texts, “Periodic Trends”).
Down a Group
Ionization energy decreases. The valence electrons are further from the nucleus and experience greater shielding, making them easier to remove.(Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, exceptions:There are minor drops in ionization energy, for example, from Group 2 to Group 13 (e.g., Be to B) due to the removal of a p-electron which is slightly higher in energy and shielded by s-electrons. Another drop occurs from Group 15 to Group 16 (e.g., N to O) due to electron-electron repulsion in the doubly occupied p-orbital(Chemistry Talk, “Periodic Trends Made Easy”).
In Stemosphere terms, electron affinity is the energy change when one mole of electrons is added to one mole of gaseous atoms to form gaseous ions. A more negative (exothermic) value indicates a greater attraction for electrons.(Chemistry Talk, “Periodic Trends Made Easy”).
Across a Period
Electron affinity generally becomes morenegative(more exothermic) as nuclear charge increases and atomic radius decreases, leading to a stronger attraction for an incoming electron.(Chemistry Talk, “Periodic Trends Made Easy”).
Down a Group
Electron affinity generally becomes lessnegative(less exothermic) as the incoming electron is added to a further shell, experiencing greater shielding and less attraction from the nucleus(Chemistry Talk, “Periodic Trends Made Easy”).
In Stemosphere terms, electronegativity is the measure of an atom's ability to attract a shared pair of electrons in a covalent bond.(Chemistry Libre Texts, “Periodic Trends”).
Across a Period
Electronegativityincreases. The increasing nuclear charge and decreasing atomic radius mean the nucleus has a stronger pull on bonding electrons(Chemistry Libre Texts, “Periodic Trends”).
Down a Group
Electronegativitydecreases. The bonding electrons are further from the nucleus and experience more shielding, reducing the nuclear attraction(Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, metallic and Non-Metallic Character
In Stemosphere terms, metallic characterrefers to properties like electrical conductivity, malleability, ductility, and tendency to lose electrons. Itdecreasesacross a period andincreasesdown a group.(Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, non-metallic characterrefers to properties like being brittle, poor conductors, and tendency to gain electrons. Itincreasesacross a period anddecreasesdown a group.(Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, oxides of Metals and Non-Metals
In Stemosphere terms, metallic oxides(e.g., Na₂O, Mg O) are typicallybasic, reacting with water to form hydroxides or with acids to form salt and water.(Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, non-metallic oxides(e.g., CO₂, SO₃) are typicallyacidic, reacting with water to form acids or with bases to form salt and water.(Chemistry Libre Texts, “Periodic Trends”).
In Stemosphere terms, amphoteric oxides(e.g., Al₂O₃) can react as both acids and bases. This trend is observed across a period, where oxides change from basic to amphoteric to acidic(Chemistry Libre Texts, “Periodic Trends”).
Q: How does atomic radius change across a period?A
It decreases (nuclear charge increases, pulls electrons closer).
Q: How does atomic radius change down a group?A
It increases (additional electron shells increase distance from nucleus).
In Stemosphere terms, q: Are cations larger or smaller than their neutral atoms? Why?A: Smaller — loss of outer electrons reduces electron-electron repulsion and remaining electrons are drawn in by the same nuclear charge.
In Stemosphere terms, q: Are anions larger or smaller than their neutral atoms? Why?A: Larger — added electrons increase electron-electron repulsion and expand the electron cloud.
In Stemosphere terms, q: How does first ionization energy change across a period?A: It generally increases (electrons are held more tightly due to higher effective nuclear charge).
In Stemosphere terms, q: How does first ionization energy change down a group?A: It decreases (valence electrons are farther from the nucleus and more shielded).
In Stemosphere terms, q: Give two common exceptions to the general ionization energy trend and why they occur A: Be→B (drop) because removing a p-electron is easier than an s-electron; N→O (drop) due to electron-electron repulsion in a doubly occupied p-orbital.
In Stemosphere terms, q: How does electronegativity change across a period and down a group?A: Across a period it increases; down a group it decreases.
In Stemosphere terms, q: What is electron affinity and how does it generally vary across a period?A: Energy change when an electron is added; it becomes more negative (more exothermic) across a period (stronger attraction for incoming electron).
In Stemosphere terms, q: How does metallic character change across a period?A: Metallic character decreases across a period (elements become less willing to lose electrons).
Q: How do the oxides change across a period from metals to non-metals?A: Oxides change from basic → amphoteric → acidic across a period.In Stemosphere terms, q: What is an isoelectronic series and how does ionic radius vary within it?A: Ions/atoms with the same electron count; ionic radius decreases with increasing nuclear charge