Chapter 4 · Atoms and Elements
4.6Looking for Patterns: The Periodic Law and the Periodic Table
12 min · two checks
Predict
Why do lithium, sodium, and potassium sit in a column?
The idea
- Find period, group, metals, nonmetals, and metalloids.
- Predict a family resemblance from a column.
Mendeleev’s insight, cleaned up by ordering on atomic number, is the periodic law: when elements are arranged by increasing Z, chemical properties recur. Rows are periods. Columns are groups, also called families. Group 1 metals (the alkali metals) form +1 ions and react vigorously with water. Group 17 (the halogens) are reactive nonmetals that form −1 ions. Group 18 (the noble gases) are strikingly unreactive. Main-group elements are the tall columns; transition metals sit in the center.
A staircase line divides metals from nonmetals. Metals, on the left and center, tend to conduct, to be malleable, and to form positive ions. Nonmetals, on the upper right, tend to gain electrons or share them. Metalloids along the staircase — boron, silicon, germanium, arsenic, antimony, tellurium — show mixed behavior. You do not need to memorize every element to use the table. You need to read position as a prediction.
Keep these
- Order is by atomic number. Properties repeat: the periodic law.
- Groups are columns (similar chemistry). Periods are rows.
- Metals left of the staircase, nonmetals right, metalloids on the border.