Periodic trends: reading change across the table
A trend is a general direction, not a promise about every detail. Shell number and nuclear attraction help explain why size and character change.
Learning goals
- describe radius and metallic-character trends;
- separate across-period and down-group movement;
- compare group-1 and group-17 reactivity;
- characterize an element from its position and shell arrangement.
1. Why trends appear
Across a period, proton number increases while new electrons enter the same main shell. The stronger nuclear pull generally draws the electron cloud closer. Down a group, a new occupied shell is added, distance and shielding increase, and the atom becomes larger.
2. Core directions
| Property | Across a period, left → right | Down a group |
|---|---|---|
| Atomic radius | Generally decreases | Generally increases |
| Metallic character | Generally decreases | Generally increases |
| Nonmetallic character | Generally increases | Generally decreases |
Memory map: larger and more metallic toward the lower left; more nonmetallic toward the upper right.
3. Reactivity needs a family
Do not use one reactivity arrow for the entire table. In group 1, outer electrons are lost more easily down the group, so metal reactivity generally increases downward. In group 17, gaining an electron becomes less favorable down the group, so halogen reactivity generally decreases downward and increases upward.
4. Element-characterization checklist
- Identity: atomic number gives proton count and element.
- Period: for the simple model, it matches the number of occupied shells.