Fourth-Period Ordering and the d Block

Introduction

Period four has eighteen elements, from potassium to krypton. Its extra width compared with periods two and three comes from a central ten-element d-block region. The familiar filling story begins with 4s, then includes 3d, then 4p, but real configurations and ion formation need more nuance than a rigid energy ladder.

Core explanation

After argon, neutral potassium is [Ar]4s¹ and calcium is [Ar]4s² in the introductory ground-state sequence. These two elements occupy the left s-block positions of period four. The 3d subshell has five orbitals and room for ten electrons, corresponding broadly to the central d-block run from scandium to zinc. Then the 4p subshell has six-electron capacity across gallium to krypton. Counting positions gives 2 + 10 + 6 = 18, matching the row length.

The period number is four even though the d-block's characteristic electrons are called 3d. Neutral period-four atoms include occupied n = 4 states, while the developing 3d subshell has n = 3. A rule that “all added electrons in period four are fourth-shell electrons” would be wrong. The table's row structure follows the sequence of ground-state elements ordered by Z, with orbital occupancy providing an explanation for its width.

The simple shorthand “4s fills before 3d” works for neutral potassium and calcium and helps introduce the row. It becomes less reliable if treated as an absolute energy hierarchy for every atom and ion. Subshell energies are close and depend on nuclear charge, occupancy and electron repulsion. Chromium and copper are familiar neutral-configuration exceptions to the most naive filling pattern. In many transition-metal cations, electrons are removed from 4s before 3d even when a neutral configuration was written with 4s filled first.

For example, a common ground-state notation for iron is [Ar]3d⁶4s², totalling 26 electrons. Fe²⁺ is commonly written [Ar]3d⁶, not [Ar]3d⁴4s². The two 4s electrons are removed in forming this ion. This does not mean the neutral filling sequence was useless; it means orbital energies and electron removal depend on the state being considered. A period-four overview should not pretend to solve every transition-metal configuration with one memorised arrow diagram.

The d-block width itself follows Pauli exclusion: each of five d orbitals can hold two electrons. That explains ten positions in a complete d-filling progression. Yet “one position equals exactly one new 3d electron with every other occupancy unchanged” is too strong because observed configurations can redistribute electrons between near-energy subshells. Count the region's width as a pattern and check individual configurations against reliable data when precision matters.

The fourth period ends when the 4p sequence reaches krypton, [Ar]3d¹⁰4s²4p⁶ in a conventional notation. Rubidium, the next element, begins the fifth period with an outer 5s electron. The recurrence of group-one ns¹ after a filled p-block end shows periodicity across rows of unequal lengths. The table is not a simple repetition of an eight-element strip.

This row also introduces chemistry not captured by a simple main-group group-number rule. Transition metals often form more than one common oxidation state, and d-electron occupancy can matter in colour, magnetism and coordination compounds. The present lesson concerns classification and configuration; explaining specific complexes needs later bonding models.

Step-by-step reasoning

1. Start at [Ar] and place K and Ca in the 4s-associated s-block positions. 2. Count the ten d-block positions associated broadly with 3d filling. 3. Count the six 4p positions from Ga to Kr. 4. Add 2 + 10 + 6 and qualify individual configuration and ion exceptions.

Visual explanation

Draw a period-four strip divided into three coloured regions: K–Ca labelled 4s, Sc–Zn labelled 3d and Ga–Kr labelled 4p. Put widths 2, 10 and 6 above them. Below, draw Fe [Ar]3d⁶4s² changing to Fe²⁺ [Ar]3d⁶ to warn that electron removal is not the reverse of a simplistic filling list.

Real-world analogy

A road with two lanes, then ten, then six has eighteen lane positions across its sections. The count resembles block widths, but traffic can rearrange between lanes, just as electrons can redistribute between near-energy subshells. The analogy is for counting, not quantum dynamics.

Real-world example

Iron's position in the period-four d block helps signal that +2 and +3 compounds are both important. A group-one style “lose exactly one outer electron” shortcut would not describe iron's observed ion chemistry.

Why?

Why is period four longer than period three? Besides outer 4s and 4p positions, the sequence includes a central ten-position 3d region that period three did not fill.

Common misconception

“4s is always lower in energy than 3d, so a transition-metal ion must lose 3d electrons first.” Relative energies change with the species; common transition-metal cations lose 4s electrons first.

Worked example

Count period-four elements by region: K and Ca give two s-block elements; Sc through Zn give ten d-block elements; Ga through Kr give six p-block elements. Total 2 + 10 + 6 = 18. Check by atomic numbers: K is 19 and Kr is 36, so 36 − 19 + 1 = 18. The two methods agree.

Quick check

1. Which subshell's ten-electron capacity accounts for the central width of period four? Answer: The 3d subshell has five orbitals and a total capacity of ten electrons.

Exam focus

Use the 2 + 10 + 6 structure and name the K–Kr boundaries. Separate period number from a differentiating subshell's n label. When writing individual transition-metal ions, check 4s removal and known exceptions.

Advanced insight

The energy difference between 4s and 3d states varies with occupancy, charge and nuclear attraction. Quantum calculations and observed spectra, rather than a single fixed ordering diagram, determine detailed configurations. The block-count model remains useful for table shape.

Summary

Period four runs from K to Kr and contains eighteen elements: two associated with 4s, ten with the 3d region and six with 4p. Neutral filling mnemonics explain the row broadly, while transition-metal exceptions and ionisation require more careful treatment.

Practice questions

1. How many elements are in period four? Answer: Eighteen, from atomic numbers 19 through 36 inclusive. 2. Why does the 3d region span ten positions? Answer: Five d orbitals can hold two electrons each. 3. Is Fe²⁺ commonly written by removing 3d electrons before 4s? Answer: No; the common configuration loses the two 4s electrons, giving [Ar]3d⁶. 4. Which subshell ends the period-four main-group sequence at krypton? Answer: The 4p subshell reaches 4p⁶.