How do atomic radius, ionization energy and electronegativity change across a period and down a group?
Periodic trends: describe and explain the trends in atomic radius, ionization energy, electronegativity and metallic character across a period and down a group, using Table S where appropriate.
A focused Regents Chemistry answer on periodic trends: atomic radius, ionization energy, electronegativity and metallic character, why each trend runs the way it does, and how to read the numbers from Table S of the Reference Tables.
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What this topic is asking
The Core Curriculum asks you to describe and explain how four properties change across the periodic table: atomic radius, first ionization energy, electronegativity and metallic character. The Regents exam tests both the direction of each trend and a short explanation in terms of atomic structure, and Table S of the Reference Tables lists the actual values for atomic radius, electronegativity and first ionization energy so you can read them off directly.
Atomic radius
So the largest atoms sit at the bottom left of the table (such as francium and caesium) and the smallest sit at the top right (excluding the noble gases). Table S lists atomic radii in picometres, which lets a question ask you to compare two specific elements directly.
First ionization energy
Ionization energy increases across a period (the rising nuclear charge holds electrons more tightly in a smaller atom) and decreases down a group (the outer electron is farther away and shielded by inner electrons, so it is easier to remove). This is why metals at the bottom left lose electrons readily, while nonmetals at the top right resist losing them. Table S gives first ionization energies in kilojoules per mole.
Electronegativity
Electronegativity drives bond polarity, so this trend feeds directly into the bonding module: a large electronegativity difference between two atoms gives an ionic bond, a moderate difference a polar covalent bond, and a small difference a nonpolar covalent bond. Table S lists electronegativity values you can subtract to judge a bond.
Metallic character
Metallic character (the tendency to behave as a metal, losing electrons easily) follows the opposite pattern to ionization energy: it decreases across a period and increases down a group. The most metallic elements are at the bottom left; the most nonmetallic are at the top right. This is consistent with the trends above, because metals are the elements that lose electrons readily and so have low ionization energies.
Try this
Q1. State the trend in atomic radius down Group 17 from fluorine to iodine. [1 point]
- Cue. It increases, because each element down the group has an additional energy level.
Q2. Which element has a higher first ionization energy, lithium or fluorine? Explain briefly. [1 point]
- Cue. Fluorine; it is further right with a greater nuclear charge and smaller radius, so it holds its electrons more tightly.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part A style)1 marksAs the elements in Period 3 are considered in order from sodium to chlorine, the atomic radius generally (1) decreases (2) increases (3) remains the same (4) decreases then increasesShow worked answer →
A 1-point Part A trend item. The answer is (1) decreases.
Across a period from left to right, the nuclear charge (number of protons) increases while electrons are added to the same outermost energy level. The greater pull of the larger nuclear charge on the same shell draws the electrons in, so the atomic radius generally decreases. Table S in the Reference Tables lists atomic radii that confirm this trend.
The trap is assuming more particles means a bigger atom; the increasing nuclear charge dominates across a period.
Regents (Part B-2 style)3 marksRefer to the elements lithium, sodium and potassium in Group 1. (a) State the trend in atomic radius from lithium to potassium. (b) State the trend in first ionization energy from lithium to potassium. (c) Explain the trend in first ionization energy in terms of atomic structure.Show worked answer →
A 3-point constructed-response item on group trends, using Table S values.
(a) Atomic radius (1 point): the atomic radius increases from lithium to potassium (down the group).
(b) Ionization energy (1 point): the first ionization energy decreases from lithium to potassium.
(c) Explanation (1 point): going down the group, atoms gain additional energy levels, so the outermost electron is farther from the nucleus and more shielded by inner electrons. It is held less tightly and is easier to remove, so less energy is required.
Markers reward the correct directions of both trends and an explanation that links larger size and greater shielding to weaker attraction on the outer electron.
Related dot points
- The periodic table and its organization: explain periods, groups and the periodic law, and classify elements as metals, nonmetals or metalloids using position and physical properties.
A focused Regents Chemistry answer on how the periodic table is arranged: periods and groups, the periodic law, the families (alkali metals, alkaline earth metals, halogens, noble gases), and how to classify metals, nonmetals and metalloids from position and properties.
- Electron configuration and energy levels: write Regents electron configurations, distinguish ground state from excited state, and explain how electrons absorb and emit specific amounts of energy as photons.
A focused Regents Chemistry answer on electron configuration the New York way (shell notation such as 2-8-1), the ground state versus excited state distinction, valence electrons, and how absorbed and emitted energy produces bright-line spectra.
- Ions and nuclide notation: explain how positive and negative ions form by losing or gaining electrons, and interpret nuclide symbols to count protons, neutrons and electrons.
A focused Regents Chemistry answer on ion formation and nuclide notation: how losing or gaining electrons makes cations and anions, why protons and neutrons stay fixed, and how to read mass number, atomic number and charge from a nuclide symbol.
- Electronegativity and bond polarity: use electronegativity differences from Table S to classify bonds as ionic, polar covalent or nonpolar covalent.
A focused Regents Chemistry answer on electronegativity difference and bond polarity: how subtracting Table S electronegativities classifies a bond as nonpolar covalent, polar covalent or ionic, and how that difference shapes the unequal sharing of electrons.
- Types of chemical bonds: distinguish ionic, covalent and metallic bonding in terms of electron transfer or sharing, and relate bond type to the elements involved.
A focused Regents Chemistry answer on ionic, covalent and metallic bonding: how electrons are transferred or shared, why bonds form to reach stability, the role of energy, and how to predict bond type from the elements involved.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)