Grade 10

Periodic table and periodic trends

Why the periodic table has its shape, and how atomic size, ionization energy and electronegativity change across it.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: a table that predicts

In 1869 Mendeleev ordered the elements by atomic weight and left gaps for elements not yet found. Today we order them by atomic number $Z$ , and the rows and columns reflect how electrons fill shells: elements in one column have the same number of valence electrons and behave alike.

Click an element. Switch the coloring to see how electronegativity, atomic radius and ionization energy vary.

SchoolSchool level: three trends

Periodic trends (main-group elements)
Property	Across a period (left → right)	Down a group (top → bottom)
Atomic radius	decreases	increases
First ionization energy	increases (with small dips)	decreases
Electronegativity	increases	decreases

Across a period the nuclear charge grows while electrons enter the same shell, so the nucleus pulls harder and atoms shrink. Down a group new shells are added, so atoms grow and the outer electrons are held more loosely.

Switch the property and follow the ridges: electronegativity climbs toward fluorine at the upper right, while atomic radius peaks at the alkali metals on the left.

\mathrm{IE}_1:\qquad \ce{X(g) -> X+(g) + e-}

Example: Which is larger, Na or Cl?

Compare the atomic radii of sodium and chlorine.

Solution

Both are in period 3; Na is at the far left and Cl near the right, so Na is larger: 180 pm against 100 pm in the table above.