Times are illustrative, for a piece the size of a grain of rice. Never try this at home: teachers do it behind a safety screen.
Across a period (left to right), atoms get smaller and hold their electrons more tightly: atomic radius falls (lithium 133 pm, fluorine 64 pm), while first ionization energy and electronegativity rise. Fluorine has the highest electronegativity, 3.98. Down a group the opposite happens, because each row adds a new electron shell. Metallic character increases down and to the left.
| Element | Radius (pm) | IE (kJ/mol) | Electronegativity |
|---|---|---|---|
| Na | 155 | 496 | 0.93 |
| Mg | 139 | 738 | 1.31 |
| Al | 126 | 578 | 1.61 |
| Si | 116 | 787 | 1.90 |
| P | 111 | 1,012 | 2.19 |
| S | 103 | 1,000 | 2.58 |
| Cl | 99 | 1,251 | 3.16 |
| Ar | 96 (estimate) | 1,521 | none |
Effective nuclear charge. The outer electrons do not feel all Z protons, because inner electrons shield part of the charge: Zeff = Z − σ, where σ is the shielding constant. The simplest estimate (“core charge”) counts each inner electron as a full shield: sodium has 11 protons and 10 inner electrons, so its outer electron feels about +1; chlorine feels about +7.
Slater’s rules give a better estimate for an outer s or p electron: each other electron in the same (ns, np) group shields 0.35 (0.30 in 1s), each electron in shell n−1 shields 0.85, and each deeper electron 1.00. Fluorine: σ = 2 × 0.85 + 6 × 0.35 = 3.80, so Zeff = 9 − 3.80 = 5.20. Sodium: σ = 2 × 1.00 + 8 × 0.85 = 8.80, so Zeff = 2.20. Across period 2 Zeff climbs by 0.65 per step (lithium 1.30 → neon 5.85), while potassium (2.20) is no higher than sodium, but one shell farther out.
Size and energy together. For a hydrogen-like orbital the size grows as n²/Zeff and the binding energy as Zeff²/n². Across a period n stays the same and Zeff rises, so the atom shrinks and the ionization energy climbs. Down a group n rises faster than Zeff, so the atom grows and the outer electron is easier to remove.
The dips. Ionization energy falls from Be (900) to B (801 kJ/mol): boron’s outer electron is in a 2p orbital, a little higher in energy and better shielded than beryllium’s 2s electrons. It falls from N (1,402) to O (1,314 kJ/mol): oxygen’s fourth 2p electron must pair up in an orbital that already holds one, and the repulsion makes it easier to remove. The same dips appear in period 3 (Mg → Al, P → S) and period 4 (Zn → Ga, As → Se); the small wiggles among the transition metals come from 3d/4s filling.
A size exception. Gallium (124 pm) is a little smaller than aluminum (126 pm), although it is one row lower. Between them sit the ten d-block elements: their 3d electrons shield the outer electrons poorly, so gallium’s outer shell feels a stronger pull (the “d-block contraction”).
Ions of the same size pattern. F−, Na+ and Mg2+ all have 10 electrons (isoelectronic), but 9, 11 and 12 protons. More protons pull the same electron cloud tighter: ionic radii are about 133, 102 and 72 pm (Shannon radii). Cations are smaller than their atoms (Na 155 → Na+ 102 pm; covalent vs ionic radius, so compare roughly) because they lose the whole outer shell; anions are larger.
Electron affinity and electronegativity. Electron affinity is the energy released when a lone gas atom gains an electron (chlorine 349 kJ/mol, fluorine 328). Electronegativity is a 0.7–4.0 scale for the pull on electrons in a bond. Mulliken defined it as the average of ionization energy and electron affinity, which is why it follows the same diagonal trend: highest at the upper right (F), lowest at the lower left (Cs).
About the data. Atomic radius is not one fixed number: it depends on how it is measured. This page uses one consistent set, the single-bond covalent radii of Pyykkö & Atsumi (2009); their noble-gas values are calculated, so they are drawn dashed. First ionization energies are NIST values in eV × 96.485 kJ/mol per eV.
Pick one answer for each question, then press Check my answers to see the explanations. New questions picks another set.
A 45-minute lesson. Start in the Reactivity lab on a projector (Full screen button): ask the class to predict which of lithium, sodium and potassium will react fastest, then drop them one by one. Use the surprise to ask why, and move to the Trend map to find the answer in the atom sizes and ionization energies. Pairs then work through the six missions; each asks for a prediction before the simulator shows the result, and a certificate prints when all six are done.
The atomic radii (Pyykkö & Atsumi 2009 covalent radii), first ionization energies (NIST) and Pauling electronegativities (RSC) are real data for elements 1–36. The reaction times in the lab are illustrative (a rice-grain-sized piece); the order and the looks (fizz, molten ball, lilac flame, cesium sinking) are real. Alkali metals in water are a teacher demonstration behind a safety screen, never a student practical; rubidium and cesium are never used in schools.
Grades 6–8 can use the patterns (MS-PS1-1 atomic models; MS-PS1-2 signs of a reaction: gas, color change). Go deeper adds effective nuclear charge, shielding, Slater’s rules, the ionization-energy dips and isoelectronic ions for HS-PS1-1 (periodic table as a model of outer electrons) and HS-PS1-2 (explaining reactions with trends). For the full table of all 118 elements use the periodic table; for protons, neutrons and shells use atomic structure.
Standards: NGSS MS-PS1-1, MS-PS1-2 · HS-PS1-1, HS-PS1-2 (Go deeper: effective nuclear charge, shielding, Slater’s rules)
| Time | Step | What students do | Go to |
|---|---|---|---|
| 5 min | Hook | Predict, then drop Li, Na and K in the Reactivity lab | Simulator |
| 10 min | Explore | Missions 1–3: shrinking atoms, the group 1 race, ionization energy | Simulator |
| 10 min | Apply | Missions 4–6: halogen swap, the N → O dip, the top electron grabber | Simulator |
| 5 min | Order | Order atoms by size and by ionization energy | Order game |
| 10 min | Check | Quiz (10 questions) or the worksheet | Quiz |
| 5 min | Exit ticket | Explain in two sentences why K reacts faster than Li | Key facts |
Worksheet: a print-out for students and a separate answer key page for the teacher (opens the print dialog; choose "Save as PDF" to keep a file).
Periodic trends are patterns in how the properties of elements change across the periodic table. Across a period (left to right) atoms get smaller, hold their electrons more tightly (higher ionization energy) and pull harder on shared electrons (higher electronegativity). Down a group they get bigger and give up electrons more easily.
Key takeaway: smaller and grabbier to the right, bigger and looser going down.Each element across a period has one more proton. Its new electron goes into the same outer shell. The inner electrons stay the same, so the outer shell feels a stronger pull and is drawn closer. From sodium (155 pm) to chlorine (99 pm) the atom shrinks by about a third.
Key takeaway: more protons, same shell: the shell is pulled in.Each row down adds a new electron shell farther from the nucleus. The extra protons are mostly balanced by the extra inner electrons that shield them. So the outer electrons do not feel a much bigger pull. Lithium 133 pm, sodium 155 pm, potassium 196 pm.
Key takeaway: a new shell each row.First ionization energy rises across a period and falls down a group, the opposite of atomic radius. A small atom holds its outer electron close, so it takes more energy to remove it. Helium has the highest value (2,372 kJ/mol) and potassium the lowest in periods 1–4 (419 kJ/mol).
Key takeaway: small atom, tight grip, high ionization energy.Fluorine, 3.98 on the Pauling scale. It is small, and its outer electrons feel a strong pull from the nucleus with little shielding, so it pulls hard on electrons in a bond. Electronegativity rises toward the upper right of the table (most tables give the noble gases no value) and is lowest at the lower left (cesium 0.79).
Key takeaway: fluorine: small atom, strong pull.Alkali metals react by losing their single outer electron. Down the group that electron is farther from the nucleus and better shielded. So it is easier to remove (lithium 520, sodium 496, potassium 419 kJ/mol). That is why lithium fizzes, sodium melts and skates, and potassium catches fire in water.
Key takeaway: easier to lose the electron, faster reaction.Halogens react by gaining one electron. A smaller atom pulls an extra electron in harder, so fluorine is the most reactive and iodine the least. That is why chlorine water turns potassium bromide orange (bromine is made), but bromine water does nothing to potassium chloride.
Key takeaway: harder to gain an electron, slower reaction.Metallic character is how much an element behaves like a metal, mainly how easily its atoms give up electrons. It increases down a group and toward the left, so the most metallic elements are at the bottom left (cesium) and the least metallic at the upper right (fluorine). Metalloids such as silicon sit on the border.
Key takeaway: metals lose electrons easily: bottom left.Boron’s outer electron is in a 2p orbital, which is a little higher in energy and better shielded than beryllium’s 2s electrons, so it is easier to remove (900 → 801 kJ/mol). In oxygen, the fourth 2p electron pairs up with another one in the same orbital; their repulsion makes it easier to remove (1,402 → 1,314 kJ/mol).
Key takeaway: a new 2p electron (B) and the first paired 2p electron (O) are easier to remove.Sodium has one outer electron, and it takes only 496 kJ/mol to remove it. When it loses that electron, sodium is left with the same full shells as neon. Neon already has a full outer shell, and removing one of its electrons takes 2,081 kJ/mol, about four times more. So sodium gives up its electron easily and reacts, while neon hardly reacts at all.
Key takeaway: one easy electron to lose vs a full shell.