Period 4 element

A chemical element in the fourth row of the periodic table, from potassium through krypton. These 18 elements include s-, d-, and p-block elements.

Electron shell: A group of atomic orbitals with the same principal quantum number, occupied by electrons around an atomic nucleus. The fourth-period label corresponds to filling the fourth principal electron shell.
Atomic radius: A measure of the size of an atom, defined through its boundary or distance to a bonded neighbor. Atomic radii generally contract across Period 4 as nuclear charge increases.
Potassium: A soft, highly reactive alkali metal with atomic number 19 and symbol K. Its ions are essential for nerve signaling and fluid balance in organisms.
Period 3 element: A chemical element in the third row of the periodic table, from sodium through argon. The preceding period lacks the 3d transition-metal series found in Period 4.
Calcium: An alkaline-earth metal with atomic number 20 and symbol Ca. It follows potassium before the 3d transition series begins.
Aufbau principle: A rule of thumb for arranging electrons in atomic orbitals from lower to higher energy. Its filling sequence explains why the 3d subshell fills after 4s in this period.
First ionization energy: The energy required to remove the most weakly bound electron from a neutral gaseous atom. Its overall rise across the period has exceptions tied to subshell structure.
Titanium: A strong, low-density transition metal with atomic number 22 and symbol Ti. Its corrosion resistance and strength suit implants, aircraft, and chemical equipment.
Period 5 element: A chemical element in the fifth row of the periodic table, from rubidium through xenon. The next row follows a similar pattern but fills 4d rather than 3d orbitals.
Scandium: A transition metal with atomic number 21 and symbol Sc. It is the first Period 4 element in which the 3d subshell begins to fill.

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