Blocks, Periods and Groups from Electron Configurations – Trends in Physical Properties of Elements in the Periodic Table (Senior 4 Chemistry) | Ganzaa

Trends in Physical Properties of Elements in the Periodic Table

Lessons

Senior 4 · Chemistry

Trends in Physical Properties of Elements in the Periodic Table

3 lessons

Lesson 2 of 3

Blocks, Periods and Groups from Electron Configurations

25 min

A map of electron configurations

Mendeleev built his table from properties alone. Fifty years later, Unit 2's configurations explained it. The table is really a map of how electrons fill subshells: each row is a new shell, and each region of the table is the place where one type of subshell (s, p, d or f) is being filled. That means you can find any element's position from its configuration, and write the configuration of any element from its position, without memorising either. This lesson shows how.

Definition

Period: a horizontal row. Its number is the principal quantum number $n$ of the outermost occupied shell.

Group: a vertical column of elements with the same outer electron arrangement and similar chemical properties. The modern table numbers them 1 to 18.

Block: the region of the table in which the last electron (in the Aufbau order) enters an s, p, d or f subshell.

The four blocks

Table 2.1: The blocks and their elements

Block	Groups	Outer configuration	Typical elements and behaviour
s	1 and 2 (and He)	$n s^{1}$ or $n s^{2}$	reactive metals (Na, K, Mg, Ca); form ions $\mathrm{M^{+}}$ or $\mathrm{M^{2+}}$ ; mostly ionic compounds
p	13 to 18	$ns^2\,np^1$ to $ns^2\,np^6$	metals, metalloids and non-metals (Al, Si, N, O, Cl, the noble gases); many covalent compounds
d	3 to 12	$(n-1)d^{1 \text{ to } 10}\,ns^{1 \text{ or } 2}$	transition metals (Fe, Cu, Zn); variable oxidation states, coloured compounds, catalysts
f	lanthanoids and actinoids	filling 4f or 5f	lanthanoids, very similar to each other; actinoids, all radioactive (uranium)

The width of each block matches the number of electrons its subshell holds: s is 2 columns wide, p 6, d 10 and f 14, so the shape of the table follows from the quantum numbers of Unit 2.

Finding the position from the configuration

Period = the highest value of $n$ in the configuration.
Block = the type of the last subshell filled (in Aufbau order).
Group:
- s-block: the number of s electrons in the outer shell (1 or 2).
- p-block: $10 +$ the number of outer s and p electrons (in the old numbering, used in many books: Group III to VIII = the number of outer s and p electrons).
- d-block: the number of $(n - 1) d$ electrons plus the outer s electrons.

Worked example 1: locating four elements

Find the period, group and block of the elements with Z = 15, 35, 25 and 56.

Solution.

Z = 15: $1s^2\,2s^2\,2p^6\,3s^2\,3p^3$ . Highest n = 3: Period 3. Last subshell p. Outer s + p = 5, so Group $10 + 5 = 15$ . Phosphorus, Period 3, Group 15, p-block.
Z = 35: [Ar] $3d^{10}\,4s^2\,4p^5$ . Period 4; p-block; $10 + 7 = 17$ . Bromine, Group 17. (The full 3d subshell is not counted.)
Z = 25: [Ar] $3d^5\,4s^2$ . Period 4; d-block; $5 + 2 = 7$ . Manganese, Group 7.
Z = 56: [Xe] $6 s^{2}$ . Period 6; s-block; barium, Group 2.

Worked example 2: from position to configuration

An element is in Period 4, Group 16. Write its configuration and atomic number, and predict the charge on its simple ion.

Solution. Period 4, so the outer shell is $n = 4$ . Group 16 is in the p-block with $16 - 10 = 6$ outer s and p electrons: $4s^2\,4p^4$ . Before 4p fills, the 3d subshell must be full: [Ar] $3d^{10}\,4s^2\,4p^4$ . Electrons: $18 + 10 + 2 + 4 = 34$ .

Answer: Z = 34, selenium. It needs 2 electrons to complete its octet, so it forms $\mathrm{Se^{2-}}$ , like oxide and sulfide.

Warning

Common mistakes. (1) Counting the full $(n-1)d^{10}$ electrons when finding the group of a p-block element: bromine is Group 17, not Group 27. (2) Taking the period from the last subshell written: in [Ar] $3d^5\,4s^2$ the period is 4 (from 4s), not 3. (3) Forgetting that helium ( $1 s^{2}$ ) is an s-block element placed in Group 18 because, like the other noble gases, its outer shell is full.

Tip

Read the table like a book, left to right, top to bottom, and you read the Aufbau order: 1s; 2s 2p; 3s 3p; 4s 3d 4p; 5s 4d 5p; 6s 4f 5d 6p. The d-block always sits one shell behind its period number, and the f-block two behind.

Wayne Breslyn: Writing Electron Configurations Using Only the Periodic Table (5 min). Watch how he walks across each block to build a configuration, and use the same method in reverse to place an element.

✦

Practice

State the period, group and block of the elements with configurations: (a) $1s^2\,2s^2\,2p^5$ ; (b) [Ar] $4 s^{2}$ ; (c) [Ar] $3d^{8}\,4s^2$ ; (d) [Kr] $4d^{10}\,5s^2\,5p^2$ .
Identify each element in question 1.
Write the configuration of the element in Period 5, Group 1, and of the element in Period 3, Group 13.
Why is the d-block ten columns wide and the p-block six?
Explain why the transition elements in Period 4 have their d electrons in the 3d subshell, not 4d.
Element X forms an ion $\mathrm{X^{3-}}$ with the configuration of argon. Give the period, group and block of X, and name it.
Uranium (Z = 92) is an actinoid. Which subshell is being filled across the actinoids, and in which period are they?

Show answersHide answers

(a) Period 2, Group 17, p-block; (b) Period 4, Group 2, s-block; (c) Period 4, Group 10, d-block; (d) Period 5, Group 14, p-block.
(a) fluorine; (b) calcium; (c) nickel; (d) tin.
Period 5, Group 1: [Kr] $5 s^{1}$ (rubidium). Period 3, Group 13: [Ne] $3s^2\,3p^1$ (aluminium).
A d subshell has 5 orbitals and holds 10 electrons; a p subshell has 3 orbitals and holds 6. Each column adds one electron.
In the Aufbau order, 3d is filled after 4s (it has a higher $n + l$ ), so in Period 4 it is the 3d subshell that fills; 4d fills in Period 5.
X has $18 - 3 = 15$ electrons: phosphorus, Period 3, Group 15, p-block.
The 5f subshell, in Period 7.

Quick check

An element has the configuration [Ne] 3s² 3p⁴. Where is it in the periodic table?

Key points

Period = highest principal quantum number; block = subshell of the last electron filled.
s-block: Groups 1–2 (and He); p-block: Groups 13–18; d-block: Groups 3–12; f-block: lanthanoids and actinoids.
Group: s-block = outer s electrons; p-block = 10 + outer s and p electrons; d-block = $(n - 1) d$ + outer s electrons.
Block widths (2, 6, 10, 14) equal the electrons that s, p, d and f subshells hold.

Sources and credits

Written by Ganzaa. Group numbering follows the IUPAC 1 to 18 system.
Video: Writing Electron Configurations Using Only the Periodic Table, Wayne Breslyn (Dr. B.), embedded from YouTube.
The periodic table drawing, table, worked examples and Practice items were written and drawn by Ganzaa.

1A map of electron configurations

2The four blocks

3Finding the position from the configuration

A map of electron configurations

The four blocks

Finding the position from the configuration