Why “periodic”?
On a calendar, Monday follows Sunday every 7 days: the pattern repeats. Elements do something similar. Line them up by atomic number and elements with similar properties, like lithium, sodium, and potassium, keep showing up at regular intervals. Where does that repeating pattern come from? The answer is in the electron arrangements from lesson 1.4.4.
Structure and position
Arrange the Bohr diagrams of the first 20 elements in their places on the periodic table, and two patterns jump out:
- Atoms in the same period have the same number of shells. From left to right, the valence electrons go from 1 up to 8 (in period 1, from 1 to 2).
- Atoms in the same group have the same number of valence electrons. Going down, each atom has one more shell.
So an element’s position tells you its structure. The period number is the number of shells:
For the main groups, the group number gives the valence electrons: group 1 has 1, group 2 has 2, and groups 13–17 have the group number minus 10 (group 13 has 3, group 17 has 7). The noble gases in group 18 have 8, except helium, which has 2. On older American tables the main groups are labeled 1A–8A, and that number is simply the number of valence electrons.
Since valence electrons decide how an atom reacts (lesson 1.4.4), elements in the same group have similar chemical properties. Lithium, sodium, and potassium each have 1 valence electron: all three are very reactive metals, and all react with water to release hydrogen. Fluorine and chlorine each have 7, and both readily gain 1 electron.
🔧 Math tool card · Reading Tables & PatternsSkip it if you already know this
The periodic table is one big data table. To find patterns in any table, work through these steps:
- Read the headings first. Know what each row and each column stands for. Here, rows are periods and columns are groups.
- Compare across, then down. Moving along a row, what changes and what stays the same? Then do the same moving down a column.
- Test the pattern, and look for exceptions. Once you spot a pattern, check it against more boxes. Treat special cases, like hydrogen and helium, separately.
- Use the pattern to predict. That’s exactly what Mendeleev did when he predicted elements that hadn’t been discovered yet.
In the table of the first 20 elements, what stays the same as you move down a column (a main group)?
Atoms in the same main group have the same number of valence electrons. The number of shells, the nuclear charge, and the size all increase going down.
Change what the boxes show. Start with “Shells,” then try “Valence electrons,” and click the period numbers at the start of the rows and the group numbers at the tops of the columns to read off each pattern:
The number in each box is how many electron shells the atom has.The number in each box is how many valence electrons the atom has.Each circle is drawn to scale by atomic radius. Noble gases (dashed) are not compared.Blue: metals. Yellow: metalloids. Green: nonmetals. Purple: noble gases.
Click an element, a period number at the start of a row, or a group number at the top of a column.
A preview of trends: size and metallic character
The table hides more patterns, which Stage 2 covers in detail. Here’s a qualitative first look at two (switch the table above to “Atom size” and “Type of element”):
- Atomic size: across a period from left to right, atoms get smaller; down a group, atoms get bigger.
- Metallic character: across a period, elements change from metals to nonmetals. Period 3 runs from sodium, magnesium, and aluminum (metals) through silicon (a metalloid) to phosphorus, sulfur, and chlorine (nonmetals), ending with argon (a noble gas). Down a group, elements get more metallic: potassium reacts with water even more violently than sodium does.
Why do atoms get bigger going down a group? Each one has an extra shell. Why do they get smaller going across a period, even though they gain electrons? The number of shells stays the same while the nuclear charge keeps growing, so the nucleus pulls the electrons in more tightly.
Example 1: From position to structure
An element is in period 3, group 16. How are its electrons arranged, and which element is it?
- Period number = number of shells. Period 3, so 3 shells.
- Group number → valence electrons. Group 16 has 16 − 10 = 6 valence electrons.
- Fill in the shells. The inner two are full, 2 and 8, plus 6 in the third: 2, 8, 6, for 16 electrons in all.
- Answer. Nuclear charge 16: it’s sulfur (S).
Example 2: From structure to position, then a prediction
An atom has a nuclear charge of +19 and the electron arrangement 2, 8, 8, 1. Where is it on the table, and how does its chemistry compare with sodium’s?
- 4 shells. It’s in period 4.
- 1 valence electron. It’s in group 1. The element is potassium (K).
- Same group as sodium. Sodium is 2, 8, 1, also with 1 valence electron, so potassium behaves like sodium: both readily lose 1 electron to form ions with a 1+ charge (, ).
- More reactive than sodium. Potassium has an extra shell, so its atom is bigger and its valence electron is farther from the nucleus and easier to lose. That’s why potassium reacts with water even more violently than sodium.
- Sodium or magnesium
Atoms get smaller across a period, and sodium is to the left of magnesium.
- Sodium or potassium
Atoms get bigger down a group, and potassium is below sodium.
- Chlorine or fluorine
Both are in group 17, and chlorine has one more shell.
- Oxygen or sulfur
Sulfur is below oxygen, with one more shell.
- Lithium or fluorine
Both are in period 2. Lithium is on the far left; fluorine's larger nuclear charge pulls its electrons in tighter.
- Magnesium or calcium
Calcium is below magnesium, with one more shell.
- Aluminum or silicon
Both are in period 3, and aluminum is to the left of silicon.
- Carbon or nitrogen
Both are in period 2, and carbon is to the left of nitrogen.
- Nitrogen or phosphorus
Phosphorus is below nitrogen, with one more shell.
Common mistakes
Mistake 1: “More electrons means a bigger atom.” Across a period, atoms gain electrons but get smaller: a sodium atom is bigger than a chlorine atom. Size depends mainly on the number of shells; with the same number of shells, the larger nuclear charge wins and the atom is smaller.
Mistake 2: “The group number always equals the number of valence electrons.” It works directly only for groups 1 and 2. Groups 13–17 have the group number minus 10, group 18 has 8 (helium has 2), and the rule doesn’t apply to the transition metals.
Mistake 3: “Elements in the same period have similar properties.” It’s elements in the same group that are alike. Across a period, elements change from metals to nonmetals, so their properties are very different.
Quick check
An atom has 3 shells and 7 valence electrons. Where is it on the periodic table?
3 shells means period 3, and 7 valence electrons means group 17 (7 + 10). This is chlorine.
Going down a main group, which statement is correct?
Going down a group, the valence electrons stay the same, the number of shells increases, atoms get bigger, and elements get more metallic.
Summary
- Period number = number of shells. Main-group elements: groups 1 and 2 have 1 and 2 valence electrons, groups 13–17 have the group number minus 10, and group 18 has 8 (helium: 2).
- Elements in the same group have the same number of valence electrons and similar chemistry; across a period, elements change from metals to nonmetals.
- Atomic size decreases across a period and increases down a group. Metallic character decreases across a period and increases down a group.
On to AP (for later)
Periodic trends (topic 1.7) are a core part of AP Unit 1: atomic radius, ionization energy (the energy needed to remove an electron), and electronegativity. Today’s metallic-character pattern can be restated: going down a group, ionization energy decreases, so electrons are easier to remove. AP explains these trends with Coulomb’s law and effective nuclear charge, which Stage 2 covers in lessons 2.3.2–2.3.5.