Chemistry · Ch 3 — Classification of Elements and Periodicity in Properties
The s-Block Elements
The s-Block Elements
The s-Block Elements: An Overview
The s-block of the periodic table consists of two groups: Group 1 (the alkali metals) and Group 2 (the alkaline earth metals). Their name comes from the fact that their outermost (valence) electrons occupy the s-orbital. For Group 1, the general electronic configuration is ; for Group 2, it is . These are the most electropositive metals in the periodic table, meaning they lose electrons very easily.
Because they lose their outermost electron(s) so readily, they form positive ions: Group 1 elements form ions, and Group 2 elements form ions. This tendency to lose electrons is reflected in their low ionization enthalpies. As you move down either group, the atomic size increases, the outermost electron is held less tightly, and the ionization enthalpy decreases further. Consequently, metallic character and chemical reactivity increase down the group.
A direct consequence of this high reactivity is that s-block elements are never found in their pure, elemental form in nature. They always occur as compounds (e.g., chlorides, carbonates, sulfates) because they react vigorously with air, water, and many other substances.
The compounds formed by s-block elements are predominantly ionic in nature. The only major exceptions are the compounds of lithium (the first alkali metal) and beryllium (the first alkaline earth metal), which show a significant degree of covalent character due to their small size and high charge density.
Electronic Configurations of the s-Block Elements
The table below lists the s-block elements, their atomic numbers, and their full electronic configurations. Notice how the outermost or pattern is consistent across each group.
| Group 1 (Alkali Metals) | Atomic Number | Electronic Configuration |
|---|---|---|
| Lithium (Li) | 3 | (or ) |
| Sodium (Na) | 11 | (or ) |
| Potassium (K) | 19 | (or ) |
| Rubidium (Rb) | 37 | (or ) |
| Caesium (Cs) | 55 | (or ) |
| Francium (Fr) | 87 |
| Group 2 (Alkaline Earth Metals) | Atomic Number | Electronic Configuration |
|---|---|---|
| Beryllium (Be) | 4 | (or ) |
| Magnesium (Mg) | 12 | (or ) |
| Calcium (Ca) | 20 | (or ) |
| Strontium (Sr) | 38 | (or ) |
| Barium (Ba) | 56 | (or ) |
| Radium (Ra) | 88 |
The configurations are written using the noble gas core shorthand (e.g., for sodium) to avoid writing out the full inner electron configuration. The outermost or electrons are the ones responsible for the chemical properties.
General Properties and Trends
1. Metallic Character and Reactivity
All s-block elements are metals. They are soft (can be cut with a knife), have low melting and boiling points (compared to transition metals), and are good conductors of heat and electricity. Their most defining chemical property is their low ionization enthalpy.
Ionization Enthalpy Trend:
The value decreases down the group.
This trend is the root cause of their increasing reactivity. Because the outermost electron is held more loosely as you go down, it is easier to remove. This is why caesium (Cs) is more reactive than sodium (Na), and barium (Ba) is more reactive than magnesium (Mg).
2. Nature of Compounds: Predominantly Ionic
The compounds formed by s-block elements are generally ionic. This is because the large difference in electronegativity between the highly electropositive metal and a non-metal (like oxygen or chlorine) leads to a complete transfer of electrons.
The Exception: Lithium and beryllium are the exceptions. Their small size gives them a high charge density (charge/size ratio). This high charge density polarizes the electron cloud of the anion, pulling electron density towards itself. This introduces a significant amount of covalent character into their compounds, as described by Fajan's rules. For example, LiCl is more covalent than NaCl, and BeCl is distinctly covalent.