General Properties of s-Block and p-Block Elements
1. Occurrence
s-Block Elements
- s-Block elements are generally highly reactive and therefore occur predominantly in the combined state in minerals and natural salts.
- Group 1 elements occur mainly as salts, for example NaCl in rock salt and KCl in sylvite.
- Lithium occurs in minerals such as spodumene, while magnesium occurs in minerals such as magnesite and dolomite and is also abundant in seawater.
- Calcium occurs widely in compounds such as CaCO3 in limestone and CaSO4 in gypsum.
- Beryllium occurs mainly in minerals such as beryl.
p-Block Elements
- p-Block elements occur in both the free and combined states, depending on their chemical reactivity.
- Nitrogen and oxygen occur freely in the atmosphere, while sulfur can occur in the native state.
- Carbon occurs in different forms, including diamond, graphite, and amorphous forms such as coal.
- Aluminium occurs mainly in bauxite, tin in cassiterite, and lead in galena.
- Metalloids such as silicon occur abundantly in minerals such as quartz and silicates.
Trend: s-Block elements are predominantly found in the combined state, whereas p-block elements occur in both free and combined forms.
2. Electronic Configuration
| Property | s-Block | p-Block |
|---|---|---|
| General configuration | ns1 (Group 1) or ns2 (Group 2) | ns2np1–6 (Groups 13–18) |
| Valence electrons | 1 or 2 | 3 to 8 |
| Examples |
Li: 1s22s1 Na: [Ne]3s1 |
B: 1s22s22p1 Cl: [Ne]3s23p5 |
Trend: The differentiating electron in s-block elements enters an s-orbital, whereas in p-block elements it enters a p-orbital.
3. Atomic and Ionic Radii
s-Block
- s-Block elements generally have relatively large atomic radii within a given period because their valence electrons experience comparatively lower effective nuclear attraction.
- Atomic radius generally increases down a group because new electron shells are added.
- In a given period, Group 1 atoms are generally larger than Group 2 atoms.
- For comparable ions in the same period, M+ is generally larger than M2+.
p-Block
- Atomic radius generally decreases from left to right across a period because effective nuclear charge increases.
- Atomic radius generally increases down a group because additional electron shells are added.
- The atomic radius of gallium is comparable to, and in some definitions slightly smaller than, that of aluminium because of the poor shielding effect of the filled 3d electrons.
General trend: Atomic radius decreases across a period and increases down a group.
4. Density
s-Block
- Many s-block metals have relatively low densities compared with many transition metals.
- The density generally tends to increase down a group, but the trend is not perfectly regular because density depends on both atomic mass and atomic volume.
- An important irregularity occurs in Group 1: potassium has a lower density than sodium despite its greater atomic mass.
- Group 2 elements generally have higher densities than the corresponding Group 1 elements, although exceptions to simple periodic trends can occur.
p-Block
- The p-block shows a very wide range of densities because it contains gases, non-metallic solids, metalloids, and metals.
- Examples range from low-density elements such as carbon in some allotropes to relatively dense metals such as lead.
Trend: Density does not show a simple uniform trend across the p-block; both atomic mass and atomic volume influence it.
5. Ionization Energy
s-Block
- Group 1 elements generally have low first ionization energies because their single valence electron is relatively easy to remove.
- First ionization energy generally decreases down Group 1: Li > Na > K > Rb > Cs.
- Group 2 elements generally have higher first ionization energies than Group 1 elements in the same period.
p-Block
- First ionization energy generally increases across a period because effective nuclear charge increases.
- It generally decreases down a group because atomic size and shielding increase.
- Small deviations from the general trend occur because of subshell energies and electron pairing.
- Group 15 elements generally have higher first ionization energies than the corresponding Group 16 elements because the half-filled p3 configuration has additional stability.
General trend: First ionization energy generally increases across a period and decreases down a group.
Read more: Ionization Energy
6. Metallic Character
s-Block
- Groups 1 and 2 consist of metallic elements.
- s-Block metals generally show good electrical and thermal conductivity and are malleable and ductile.
- Metallic character generally increases down a group.
p-Block
- The p-block contains metals, metalloids, and non-metals.
- Metallic character generally decreases from left to right across a period.
- Metallic character generally increases down a group.
- For example, Group 13 changes from the metalloid boron to metallic elements such as aluminium, gallium, indium, and thallium.
Trend: s-Block elements are metallic, whereas the p-block shows a gradual transition from metals to metalloids and non-metals.
7. Electropositive Character
s-Block
- s-Block elements are strongly electropositive and readily lose their valence electrons to form cations.
- Electropositive character generally increases down a group because atomic size increases and ionization energy decreases.
- Group 1 elements are generally more electropositive than Group 2 elements in the same period.
p-Block
- Electropositive character generally decreases from left to right across a period.
- Electropositive character generally increases down a group.
- The left side of the p-block contains more metallic and electropositive elements than the right side.
General trend: Electropositive character decreases across a period and increases down a group.
8. Electronegativity
s-Block
- s-Block elements generally have low electronegativities because they readily lose their valence electrons.
- Electronegativity generally decreases down a group.
p-Block
- p-Block non-metals generally have higher electronegativities than s-block metals.
- Electronegativity generally increases from left to right across a period.
- Electronegativity generally decreases down a group.
- Fluorine has the highest electronegativity on the Pauling scale.
General trend: Electronegativity increases across a period and decreases down a group.
Read more: Electronegativity
9. Electron-Gain Enthalpy
s-Block
- s-Block elements generally have relatively low tendency to accept an additional electron.
- Group 2 elements have particularly low electron affinity because the added electron must enter a higher-energy p-subshell.
p-Block
- Many p-block non-metals have a greater tendency to accept electrons than s-block metals.
- Halogens have highly favorable electron-gain enthalpies.
- Chlorine has a more favorable electron-gain enthalpy than fluorine because the added electron experiences less electron-electron repulsion in chlorine's larger 3p orbital.
- Noble gases have very unfavorable electron-gain enthalpies because an additional electron would have to enter a new shell.
- Group 15 elements show less favorable electron gain than neighboring Group 14 and Group 16 elements because of the relatively stable half-filled p3 configuration.
General trend: The tendency to accept an electron generally increases across a period, although several important exceptions arise from electronic configuration.
Read more: Electron-Gain Enthalpy
10. Hydration Energy
s-Block Cations
- Small, highly charged ions have large hydration energies because they attract water molecules strongly.
- For Group 1 cations, hydration energy generally decreases from Li+ to Cs+ as ionic size increases.
- For comparable ions, M2+ generally has a greater magnitude of hydration energy than M+ because of its higher charge density.
p-Block Cations
- Small, highly charged p-block cations such as Al3+ can have very large hydration energies.
- The magnitude of hydration energy depends mainly on ionic charge density rather than simply on whether an element belongs to the s- or p-block.
General relationship: The magnitude of hydration energy generally increases with increasing ionic charge and decreasing ionic radius.
Read more: Solvation Energy
11. Flame Colouration
s-Block
- Several s-block elements produce characteristic flame colours because their valence electrons can be excited relatively easily.
- Lithium: crimson red.
- Sodium: intense yellow.
- Potassium: lilac or violet.
- Calcium: brick red.
- Strontium: crimson red.
- Barium: apple green.
p-Block
- Most p-block elements are not commonly identified by simple flame tests because many do not produce strong characteristic visible flame colours under ordinary test conditions.
Characteristic flame tests are particularly useful for several Group 1 and Group 2 elements.
12. Photoelectric Effect
s-Block Metals
- Some alkali metals have relatively low work functions and can therefore emit electrons when exposed to electromagnetic radiation of sufficient frequency.
- Caesium has a particularly low work function and has been used in photoelectric devices.
p-Block Metals
- The photoelectric effect is not restricted to s-block elements. Metallic p-block elements can also exhibit photoelectric emission when the incident radiation has sufficient frequency.
- The occurrence of photoelectric emission depends primarily on the metal's work function and the frequency of the incident radiation.
Read more: Photoelectric Effect
13. Polarizing Power and Fajans' Rule
s-Block Cations
- Large, singly charged cations such as K+ and Rb+ have relatively low polarizing power.
- Small cations such as Li+ and Be2+ have relatively high polarizing power.
- High polarizing power can increase the covalent character of compounds, as illustrated by LiCl and BeCl2.
p-Block Cations
- Small, highly charged p-block cations such as Al3+ have high polarizing power.
- Greater polarizing power can lead to increased covalent character in ionic compounds.
General relationship: Polarizing power increases with increasing ionic charge and decreasing ionic size.
Related topics:
Fajans' Rule
Polarization and Polarizability
14. Melting and Boiling Points
s-Block
- Many s-block metals have relatively low melting points compared with transition metals, although the values vary considerably between elements.
- Group 1 metals generally have low melting points, with some irregularities in the trend down the group.
- Group 2 metals generally have higher melting points than the corresponding Group 1 metals, but the trend is not strictly monotonic.
p-Block
- The p-block shows a wide range of melting and boiling points because it contains molecular non-metals, network solids, metalloids, and metals.
- Nitrogen and oxygen are gases at room temperature, whereas elements such as aluminium, tin, and lead are solids.
- The variation depends strongly on bonding type and structure, so a single simple trend cannot be applied to the entire p-block.
Trend: Melting and boiling points show complex variations and depend strongly on bonding, structure, atomic mass, and crystal packing.
Summary of Important Trends
| Property | General Trend Across a Period | General Trend Down a Group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionization energy | Generally increases | Generally decreases |
| Electronegativity | Generally increases | Generally decreases |
| Metallic character | Generally decreases | Generally increases |
| Electropositive character | Generally decreases | Generally increases |
| Polarizing power of cations | Generally increases with increasing charge density | Generally decreases with increasing ionic size |
Conclusion
The s-block and p-block elements show systematic variations in their physical and chemical properties because of differences in electronic configuration, atomic size, effective nuclear charge, and shielding. s-Block elements are predominantly metallic and electropositive, whereas the p-block contains metals, metalloids, and non-metals. Understanding these trends provides a foundation for explaining the chemical behaviour of elements across the periodic table.