The s-block metals are not just laboratory curiosities — sodium, potassium, magnesium and calcium are among the most important elements in living systems. This topic pulls together their biological roles, a side-by-side comparison of Group 1 and Group 2, and a consolidated look at the anomalies and diagonal relationships that recur through the chapter.
Biological importance of sodium and potassium
A typical adult carries roughly 90 g of sodium and 170 g of potassium, far more than the few grams of iron or copper. Na+ ions are found mainly in the fluids outside cells (blood plasma, interstitial fluid), while K+ is concentrated inside cells. This difference is maintained by the sodium–potassium pump, a protein in the cell membrane that pumps Na+ out and K+ in against their concentration gradients, using energy from ATP. The resulting ionic gradients drive the transmission of nerve signals, regulate the flow of glucose and amino acids into cells, control fluid balance and maintain blood pressure. The brief, ordered movement of Na+ and K+ across the membrane is exactly what carries an electrical impulse along a nerve.
Biological importance of magnesium and calcium
Magnesium sits at the centre of the chlorophyll molecule, the green pigment that captures light energy for photosynthesis — without Mg2+, green plants could not make food, so almost the entire food chain depends on it. Mg2+ also activates many enzymes, especially those involved in energy transfer with ATP. Calcium is the most abundant metal ion in the body by mass: about 99% of it is locked into bones and teeth as calcium phosphate and calcium carbonate, giving them rigidity. The small remaining fraction of Ca2+ in body fluids is vital for blood clotting, muscle contraction and the transmission of nerve impulses. A steady level of Ca2+ in blood is tightly controlled by hormones.
Group 1 versus Group 2: a summary
Both groups are reactive s-block metals that are good reducing agents, form ionic compounds (except the first members), and are never found free. The contrasts are systematic: Group 1 (ns1, +1) are larger, softer, more reactive, with lower ionisation enthalpies and lower melting points; Group 2 (ns2, +2) are smaller, harder, less reactive, with higher ionisation enthalpies and higher melting points. The +2 charge on Group 2 ions gives stronger lattice and hydration energies, which is why their salts are often less soluble (e.g. carbonates and sulphates of Ca, Sr, Ba are sparingly soluble) than the corresponding Group 1 salts.
Consolidated anomalies and diagonal relationships
The first element of each group — lithium in Group 1 and beryllium in Group 2 — behaves differently from the rest because it is unusually small with a very high charge density. This is the same reason behind the diagonal relationship, in which the first element resembles the second element of the next group: Li resembles Mg and Be resembles Al, because the increase in charge on moving one group right is roughly cancelled by the increase in size on moving one period down, so the charge-to-radius ratios match. Shared features include nitride formation (Li3N, Mg3N2), sparingly soluble carbonates for Li/Mg, and amphoteric oxides with covalent chlorides for Be/Al.
What is the sodium–potassium pump, and why is it important?
Solution- The Na–K pump is a membrane protein that moves Na+ and K+ across the cell membrane.
- It pumps Na+ out of the cell and K+ into the cell, against their concentration gradients.
- This active transport requires energy supplied by ATP.
- The resulting ion gradients are essential for nerve impulse transmission, fluid balance and uptake of nutrients.
Answer: It is an ATP-powered membrane protein that keeps Na+ high outside and K+ high inside cells; these gradients drive nerve signalling, fluid balance and nutrient transport.
State the biological roles of magnesium and calcium in the human and plant body.
Solution- Magnesium is the central metal ion of chlorophyll, enabling photosynthesis in green plants.
- Magnesium also activates many enzymes, particularly those handling ATP.
- About 99% of body calcium is in bones and teeth as calcium phosphate/carbonate, giving rigidity.
- The Ca2+ in body fluids is essential for blood clotting, muscle contraction and nerve signalling.
Answer: Mg2+ is central to chlorophyll and enzyme activation; Ca2+ builds bones/teeth and is needed for blood clotting, muscle contraction and nerve transmission.
Explain why the first member of each s-block group (Li and Be) shows anomalous behaviour.
Solution- Li and Be are the smallest atoms/ions in their groups.
- A small ion with its charge gives a very high charge density (polarising power).
- High polarising power distorts the electron clouds of anions, introducing covalent character into their bonds.
- This makes their compounds (covalency, solubility, reactions) differ from the larger, more ionic members below them.
Answer: Their very small size gives a high charge density and polarising power, making their compounds more covalent and their chemistry anomalous compared with the rest of the group.
Why does lithium show a diagonal relationship with magnesium rather than resembling sodium?
Solution- On moving from Li one group to the right and one period down, we reach Mg.
- Moving right increases the ionic charge (+1 to +2), which tends to raise charge density.
- Moving down increases the ionic size, which tends to lower charge density.
- These two effects nearly cancel, so Li+ and Mg2+ have similar charge-to-radius ratios and hence similar chemistry.
Answer: The rise in charge from Li to Mg is offset by the rise in size, giving similar charge-to-radius ratios, so lithium resembles magnesium diagonally.
Compare any four properties of Group 1 and Group 2 metals of the same period.
Solution- Valence configuration: Group 1 is ns1 (+1 ions); Group 2 is ns2 (+2 ions).
- Atomic size: a Group 1 atom is larger than the Group 2 atom of the same period (lower nuclear charge).
- Ionisation enthalpy and hardness: Group 2 has higher ionisation enthalpy and is harder.
- Reactivity: Group 1 metals are more reactive than the corresponding Group 2 metals.
Answer: Compared with the Group 1 metal of the same period, the Group 2 metal is smaller, harder, has a higher ionisation enthalpy, forms +2 ions and is less reactive.
Name two pairs of compounds that illustrate the diagonal relationships in the s-block, and give the shared property.
Solution- For Li–Mg: lithium nitride Li3N and magnesium nitride Mg3N2 — both metals react directly with nitrogen.
- Also for Li–Mg: Li2CO3 and MgCO3 are both sparingly soluble and decompose on heating to the oxide and CO2.
- For Be–Al: BeO and Al2O3 are both amphoteric.
- Also for Be–Al: BeCl2 and AlCl3 are both covalent Lewis acids soluble in organic solvents.
Answer: Li3N/Mg3N2 (direct nitride formation) and BeO/Al2O3 (amphoteric oxides) are two pairs illustrating the Li–Mg and Be–Al diagonal relationships.