Definition
Elements are arranged by increasing atomic number (not atomic mass) because atomic number — the number of protons — uniquely identifies an element and determines its chemical behaviour.
A period (row) groups elements by number of electron shells; a group (column) groups elements with the same number of outermost (valence) electrons, giving them similar chemical properties.
Remember
Periods get longer moving down the table because each new period adds another electron shell, and shells further out can hold more sub-shells/electrons — periods – are short, later periods include the transition metals and are much longer.
Rule
Atomic radius: decreases across a period (more protons pull electrons in tighter), increases down a group (more shells).
Ionization energy: increases across a period, decreases down a group (outer electrons are farther from the nucleus, easier to remove).
Electronegativity: increases across a period, decreases down a group — most electronegative elements sit in the upper-right (excluding noble gases), i.e. groups –.
Example 1 — comparing atomic radii
Boron's atomic radius is pm, aluminium's is pm. Which is expected to be larger, and why?
- Both are in the same group; aluminium is below boron, meaning it has an extra electron shell.
- More shells means a larger atom.
Answer: Aluminium (matches the given pm pm)
Remember
Magnesium's first ionization energy is lower than its second because the first electron is removed from a neutral atom, while the second has to be pulled away from a positively charged ion () that holds its remaining electrons more tightly — so the second electron takes much more energy.
Definition
The table has blocks (s, p, d, f), based on which sub-shell holds the outermost electron — useful because elements in the same block tend to share broad chemical behaviour, especially the reactive s-block and the less predictable d-block (transition metals, with variable outer configurations).
Quick Recap
- ●Arranged by atomic number; periods = shell count, groups = valence electron count
- ●Atomic radius: down , across . Ionization energy & electronegativity: down , across
- ●Most electronegative elements: upper-right, groups 6–7 (excluding noble gases)
- ●4 blocks (s, p, d, f) by which sub-shell holds the outermost electron
