The Periodic Table and s, p, d Blocks

Learning Objectives (OCR Spec 3.1.1)

By the end of this lesson you should be able to:

• Describe how the modern Periodic Table is arranged in order of increasing atomic (proton) number
• Define the terms period, group, block and periodicity
• Classify elements into s, p, d (and f) blocks based on their outer electron configuration
• Recognise that elements in the same group have the same number of outer-shell electrons and therefore similar chemical properties
• Appreciate the historical and predictive power of Mendeleev's table

Arrangement by Atomic Number

In the modern Periodic Table, elements are arranged in order of increasing atomic (proton) number. Each horizontal row is called a period and each vertical column is called a group.

• Periods are numbered 1 to 7 from the top
• Groups are numbered 1 to 18 (modern IUPAC) or using the older Roman numeral + letter system (I-VIII, A/B) that OCR still refers to for s- and p-block groups (Group 2, Group 7 etc.)

The key physical idea: because the elements are arranged in order of atomic number, elements within a group have the same pattern of outer-shell electrons. It is the outer-shell electron configuration that controls the chemistry, which is why group members behave similarly.

Periodicity

Periodicity is the repeating trend in physical and chemical properties across periods of the Periodic Table. When we move across Period 3 (Na → Ar) and then into Period 4 (K → Ca and beyond), we see the pattern of properties repeating. Examples of periodic properties include:

• Atomic radius
• First ionisation energy
• Electronegativity
• Melting point
• Type of structure and bonding (giant metallic → giant covalent → simple molecular)

We will meet each of these in later lessons.

Classification by Electron Configuration: s, p, d, f Blocks

The Periodic Table can be split into blocks according to which sub-shell the highest-energy electron occupies:

graph LR
  A[Periodic Table] --> B[s-block: Groups 1 and 2]
  A --> C[p-block: Groups 13-18 Groups 3-0]
  A --> D[d-block: transition metals]
  A --> E[f-block: lanthanides and actinides]
  B --> B1[highest e- in s sub-shell]
  C --> C1[highest e- in p sub-shell]
  D --> D1[highest e- in d sub-shell]
  E --> E1[highest e- in f sub-shell]

Block	Groups	Outer configuration	Examples
s	1, 2 (and He)	ns1 or ns2	Na [Ne] 3s1, Mg [Ne] 3s2
p	13 - 18	ns2 npx	Cl [Ne] 3s2 3p5, Ar [Ne] 3s2 3p6
d	Transition metals	(n-1)dx ns2	Fe [Ar] 3d6 4s2, Cu [Ar] 3d10 4s1
f	Lanthanides + actinides	(n-2)fx	U [Rn] 5f3 6d1 7s2

Helium is a special case: its electron configuration is 1s2 (s-block), but chemically it is grouped with the noble gases (Group 0/18) because of its full outer shell. OCR expects you to know this anomaly.

Why Group Members Behave Similarly

Consider Group 2:

Element	Electron configuration	Outer electrons
Be	[He] 2s2	2 (in s)
Mg	[Ne] 3s2	2 (in s)
Ca	[Ar] 4s2	2 (in s)
Sr	[Kr] 5s2	2 (in s)
Ba	[Xe] 6s2	2 (in s)

All Group 2 elements have two electrons in their outer s sub-shell. They therefore all tend to lose these two electrons to form 2+ ions, and they show similar chemistry (reaction with water, formation of oxides and hydroxides). The same principle applies across every group.

Mendeleev's Predictive Table

Dmitri Mendeleev (1869) arranged the then-known 63 elements by atomic mass, but crucially left gaps where he believed undiscovered elements should go and predicted their properties. His prediction of "eka-silicon" (later discovered as germanium, 1886) was accurate to within a few percent for atomic mass, density and oxide formula. This success entrenched the Periodic Table as a predictive tool.

The modern table differs from Mendeleev's in two major respects:

1. Elements are ordered by atomic number (not atomic mass), removing anomalies such as Ar/K and Te/I that puzzled Mendeleev.
2. A separate d-block (transition metals) was added later, plus the f-block.

Worked Example

Q: Classify each of the following elements by block and state the outer electron configuration: (a) Sulfur (Z = 16) (b) Vanadium (Z = 23) (c) Strontium (Z = 38)

Answer:

(a) S: 1s2 2s2 2p6 3s2 3p4 → outer sub-shell is 3p → p-block; outer config 3s2 3p4 (b) V: [Ar] 3d3 4s2 → highest occupied sub-shell is 3d → d-block (transition metal); outer config 3d3 4s2 (c) Sr: [Kr] 5s2 → outer sub-shell is 5s → s-block (Group 2); outer config 5s2

Exam Tips

• Learn the outer electron configurations for the first 20 elements (H → Ca) inside out; examiners frequently probe anomalies such as Cr [Ar] 3d5 4s1 and Cu [Ar] 3d10 4s1
• When identifying the block, look at the highest-energy sub-shell occupied (3d for transition metals, not 4s)
• Remember that He, despite its 1s2 configuration, is placed with Group 0 noble gases because of its full outer shell
• Elements in the same group have the same number of outer-shell electrons - this is the single most important point

Common Exam Mistakes

• Claiming that elements are ordered by atomic mass in the modern table (they are ordered by atomic number)
• Classifying Ca as d-block because it is in Period 4 (it is s-block: [Ar] 4s2)
• Writing Fe as [Ar] 4s2 3d6 in exam answers - OCR prefers the order [Ar] 3d6 4s2, with the 3d written before 4s for transition metals because the 4s electrons are lost first on ionisation
• Forgetting that H is often placed above Group 1 but is not a Group 1 metal
• Confusing "period" (row) with "group" (column)

Summary

The modern Periodic Table arranges elements in order of increasing atomic number into periods (rows) and groups (columns). Elements in the same group share outer-shell electron arrangements and therefore show similar chemistry. Elements can be further classified into s-, p-, d- and f-blocks based on the sub-shell of their highest-energy electron. This framework underpins all of OCR spec 3.1, from periodic trends and Group 2 chemistry to the halogens and beyond.