Periodic law: order that reveals patterns
The periodic table is not just a list of elements. It is a map arranged by increasing atomic number, and this order makes chemical properties repeat in recognizable patterns.
Learning goals
- state the modern periodic law in grade-appropriate words;
- distinguish a period from a group;
- use an element position to make cautious predictions;
- identify broad regions and important element families.
1. The big idea
Every element has an atomic number, equal to the number of protons in one atom of that element. The modern periodic law says that when elements are arranged by increasing atomic number, their physical and chemical properties change periodically. “Periodically” means that related patterns appear again after a sequence of elements. Atomic number is therefore the organizing key; relative atomic mass is useful, but it is not the rule used to place the elements.
2. How the table is built
Periods
Horizontal rows are called periods. There are seven periods. Moving one cell to the right normally increases atomic number by one. Properties change across a period rather than remaining the same.
Groups
Vertical columns are called groups. There are eighteen groups. Main-group elements in the same group often have related reactions because their outer electron arrangements follow a common pattern.