Modern Periodic Table
Easy Overview
The periodic table isn't just a chart on a wall — it's the ultimate cheat sheet for chemistry. Once you understand how it's organized, you can predict almost anything about an element: its size, its reactivity, what kind of bonds it forms. This chapter shows you the patterns (periodic trends) that make the table so powerful.
Mendeleev vs Modern Periodic Law
Mendeleev arranged elements by atomic mass and left gaps for undiscovered elements — bold move. Modern periodic law says properties repeat when elements are arranged by atomic number, not mass. That's why the modern table fixes the few places Mendeleev got wrong (like tellurium and iodine). Atomic number is the real deal.
Blocks of the Periodic Table
The table is divided into blocks based on which orbital the last electron enters. s-block (groups 1-2): last electron in s orbital. p-block (groups 13-18): last in p. d-block (groups 3-12): last in d. f-block (lanthanides and actinides): last in f. The block tells you the electron configuration of the element.
Atomic Radius — How Big Is an Atom?
Atomic radius decreases as you go left to right across a period (more protons pull electrons tighter). It increases as you go down a group (new shells added). So the biggest atoms are at the bottom-left (francium), and the smallest are at the top-right (helium). Size matters for practically everything an atom does.
Ionization Energy and Electronegativity
Ionization energy is how much energy you need to rip an electron off an atom. It increases across a period (tighter hold) and decreases down a group (further from nucleus). Electronegativity is how badly an atom wants to steal electrons from others. Same trend. Fluorine is the most electronegative — it's the ultimate electron thief.
Periodic Trends in Chemical Behavior
Metallic character decreases across a period (left elements are metals, right are non-metals). Reactivity of metals increases down a group (Cs explodes in water worse than Na). Reactivity of non-metals decreases down a group (F₂ is terrifying, I₂ is mild). Everything follows the same pattern — just read the table.
Key Points
- •Modern periodic law: properties are a periodic function of atomic number
- •s-block (1-2), p-block (13-18), d-block (3-12), f-block (lanthanides + actinides)
- •Atomic radius decreases across a period, increases down a group
- •Ionization energy increases across a period, decreases down a group
- •Electronegativity increases across a period, decreases down a group (F is highest)
- •Metallic character decreases left to right, increases top to bottom
- •Noble gases (Group 18) have full valence shells — they're the chill ones that don't react
Practice Questions
- Explain the trend in atomic radius across a period and down a group.
- Why is ionization energy of nitrogen higher than oxygen?
- Define electronegativity. Which element has the highest electronegativity?
- Compare the metallic character of Na, Mg, and Al across period 3.
- What are s-block, p-block, d-block, and f-block elements? Give one example of each.