One metal explodes in water. One sits in your wiring doing nothing.
Drop a pea-sized lump of sodium into water and it skitters, hisses, and bursts into flame. Drop in a copper penny and… nothing. Same periodic table, different addresses. By the end of this page, you can predict that difference from an element's position alone.
Core Claims
- Atomic Radius: Decreases left to right, because more protons pull the same shell tighter. Increases top to bottom, because each row adds a new electron shell.
- Ionization Energy (IE): Energy to remove an electron. Increases left-to-right (tighter nuclear grip); decreases top-to-bottom (outer electrons farther and shielded).
- Electronegativity (EN): Atom's power to pull shared electrons. Follows the same trends as IE (excluding noble gases).
- Reactivity: Metals react by losing electrons (highest bottom-left); nonmetals react by gaining electrons (highest top-right).
Trend Summary Map
Retrieval Checklist
- Recall trend direction for radius, IE, and EN.
- Explain radius shrink using nuclear pull (protons) in same shell.
- Explain shielding/distance effect on ionization energy down groups.
- Contrast metal and nonmetal reactivity trends based on electron loss/gain.
An element's address on the table tells you how it behaves.
Every periodic trend comes down to one tug-of-war over the outermost electrons. The positive nucleus pulls them in. Distance and the layers of inner electrons weaken that pull. Two things change the strength of the tug-of-war. They line up with the two directions you can move on the table.
Across a period
Each step right adds one proton, but the new electron joins the same shell. More pull at the same distance: the grip tightens and the atom shrinks.
Down a group
Each step down adds a whole new shell. The outer electrons sit farther out, and the inner electrons shield them. The grip loosens and the atom grows.
Tighter grip on the outer electrons → smaller atom, harder to remove an electron, stronger pull on shared electrons. Looser grip → all of that, reversed. Everything below is this idea from a different angle.
Thinking more protons and electrons make a bigger atom. Across a period the new electrons join the same shell, and the extra protons pull that shell in.
The same two levers, drawn as atoms.
Step through the elements and watch what changes. Switch between the two directions to see why one shrinks the atom and the other grows it.
Why does the atom shrink across a period and grow down a group?
The explanation appears after you predict.
- Inner electron
- Outer-shell electron
- Nucleus, labeled with its protons
- Shell (the outer ring is drawn to the atomic radius)
- Choose Across a period. Step to lithium and press Record, then step to neon and press Record. Look for: the shell count stays at 2 while the outer ring shrinks.
- Choose Down a group. Step from lithium to potassium and press Record at each element. Look for: every step adds a whole shell, and the outer ring grows.
Use your evidence
Pick a property. Watch the table light up.
Cool colors mark low values, and warm colors mark high values. The readout names the direction each property increases. Select any two elements to compare them head to head.
Which way does each trend point on the periodic table?
The numbers are supporting data. Read the pattern: which cells are higher or lower, not the exact value.
The explanation appears after you predict.
Select two elements to compare them.
- Choose Ionization energy. Read across period 2, then down Group 1. Look for: the values rise across a period and fall down a group.
- Choose Reactivity. Look for: the colors split. Metals are strongest toward the bottom left, and nonmetals toward the top right.
Reactivity isn't one arrow. It depends on whether the atom wants to lose or wants to gain electrons.
Atomic radius, ionization energy (IE), and electronegativity (EN) each run in one consistent direction. Reactivity does not, and that is what trips people up. Stop asking "is it reactive?" Start asking "reactive how?"
Metals lose electrons. Most reactive where that is easiest (low ionization energy), so down a group and to the left. The alkali metals (Li → Na → K) are the wild ones.
Nonmetals gain electrons. Most reactive where the pull is strongest (high electronegativity), so up a group and to the right. Fluorine is the champion.
Noble gases barely react. A full outer shell means no reason to lose or gain. The far-right column stays calm.
Reading reactivity as one trend. Metals get more reactive toward the bottom left. Nonmetals get more reactive toward the top right.
In the explorer, select Reactivity. The coloring splits (metals one way, nonmetals the other) instead of a single gradient.
Build the 5-step explanation chain.
To explain why one element is more reactive than another, never jump straight from position to behavior. Build a connected chain of five steps. It links atomic structure to ionization energy and to electron loss or gain.
Why does one metal fizz gently and another explode in water?
Select an element to trace its 5-step causal chain. Notice how Group 1 metals (electron loss) contrast with Group 17 nonmetals (electron gain).
Case study: why Mg fizzes in acid but Cu does nothing
In Block 1.4, you drop strips of magnesium (Mg) and copper (Cu) into dilute hydrochloric acid (HCl).
| Magnesium (Mg + 2 HCl) | Copper (Cu + HCl) | |
|---|---|---|
| Observation | Vigorous fizzing and gas evolution (H2 gas). | No reaction. The strip stays shiny. |
| Reasoning | Low ionization energy. Mg easily sheds 2 valence electrons to H+ ions: Mg(s) + 2 H+(aq) → Mg2+(aq) + H2(g). | High ionization energy. Copper holds its valence electrons tightly, and H+ ions cannot pull electrons away from Cu. |
This is why you can read a hazard off the table, before you ever see the bottle.
Back to where we started, and now the address explains it. At the particulate level, sodium loosely holds its single outer electron. Its low ionization energy allows that electron to leave with minimal energy input. That eagerness to lose an electron is what makes sodium tear into water. Copper is a transition metal, outside our main-group scope. It holds its electrons far more tightly, so it just sits there.
Bottom-left metals → flagged reactive or water-reactive. Top-right nonmetals (fluorine, chlorine) → corrosive, aggressive oxidizers. Far-right noble gases → inert and safe. Position first, label second.
For example, you need to know the exact hazard profile for elemental sodium (Na). Sodium reacts violently with water and carries two GHS pictograms. The Flame marks extreme water reactivity and flammable hydrogen release. The Corrosion symbol marks generated sodium hydroxide, a caustic base causing severe tissue damage.
Position predicts intrinsic reactivity: what an element tends to do. It does not tell you concentration, quantity, or context. A trace behind glass can be safe; a drum of a "mild" element may not be. The table predicts the tendency, not the situation.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
Moving across a period, each atom gains a proton in the same shell, so atomic radius and ionization energy . Moving down a group, each atom adds a new shell, so the outer electrons are held more . Metals are most reactive at the of the table, because they lose electrons most easily. The most reactive nonmetal is .
Ionization energy increases across a period but decreases down a group. In your own words, why do those two directions go opposite ways?
No one is grading this. Writing the reasoning out moves it from "I recognize it" to "I can produce it." Then compare with a model answer.
Ionization energy is the energy needed to remove an outer electron, so it tracks how tightly that electron is held. Across a period, protons pile up in the same shell. The pull tightens, the electron is harder to remove, and ionization energy rises. Down a group, the outer electron sits in a farther, shielded shell. The nuclear pull loosens, the electron leaves more easily, and ionization energy drops. Same tug-of-war, two different things changing.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Rubidium (Rb) sits directly below potassium (K) in Group 1. Predict whether Rb or K reacts more vigorously with water, and explain your reasoning using periodic trends. Then state one thing this prediction does not tell you about handling the actual substance.
- Prediction (1 mark): You say Rb reacts more vigorously than K.
- Position (1 mark): You say Rb sits lower, so its valence electron is farther out and more shielded.
- Link (1 mark): You say Rb has a lower ionization energy, so it loses the electron more easily.
- Limit (1 mark): You say the trend shows what an element tends to do, not its quantity or storage.
Lithium battery technology relies on periodic trends. Lithium's extremely low ionization energy and small atomic radius make it incredibly eager to shed its single valence electron. This high electron-releasing drive is what makes lithium the optimal anode material for the lightweight, high-energy rechargeable batteries in our smartphones.
Adaptive Competency Practice: HS-PS1-1
Test your mastery of atomic radius, ionization energy, electronegativity, and periodic trends one question at a time. The adaptive engine identifies individual skill gaps, provides tutoring hints on missed questions, and awards an official authenticated completion receipt.