Atomic Structure & the Periodic Table
Periods: Horizontal rows that indicate the number of electron shells an atom possesses; moving across a period adds one proton and one electron to the same shell.
Groups: Vertical columns containing elements with the same number of valence electrons, resulting in similar chemical properties and reactivity patterns.
Atomic Number Order: Elements are arranged by increasing atomic number (), which corrected early mass-based tables by ensuring elements with similar properties aligned correctly.
Metal/Non-metal Divide: A 'staircase' line separates metals (left/center) from non-metals (right), with metalloids found along the boundary.
| Feature | Isotopes | Ions |
|---|---|---|
| Definition | Atoms of the same element with different neutron counts | Atoms that have gained or lost electrons |
| Effect | Changes the mass number () but not the charge | Changes the electrical charge but not the mass |
| Chemical Behavior | Identical chemical properties | Significantly different reactivity and properties |
The Proton Rule: Always identify an element by its proton count (); never use electron count alone, as ions may have different numbers of electrons.
Shell Filling: Remember the 2, 8, 8 rule for the first 20 elements; always fill the inner shells completely before moving to the next level.
Group Trends: In exams, if you know the reaction of one element in a group (e.g., Lithium), you can predict that others in the same group (e.g., Sodium) will react similarly.
Noble Gases: Group 0 elements are unreactive because they already possess a full outer shell, making them the 'standard' for stability.