Understanding Alkali Metals and Periodic Table Groups

The question asks about the group assignment of alkali metals in the Modern Periodic Table. The periodic table organizes elements based on their atomic structure and chemical properties. Elements in the same vertical column are called groups, and they generally have similar chemical behavior because they have the same number of valence electrons (electrons in the outermost shell).

Locating Alkali Metals in the Periodic Table

Alkali metals are a specific series of elements found in one of the groups. Let's look at the groups mentioned in the options:

• Second group: These are the Alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra). They have 2 valence electrons.
• Eighteenth group: These are the Noble gases (He, Ne, Ar, Kr, Xe, Rn). They have a full outermost electron shell (usually 8 valence electrons, except Helium with 2) and are very unreactive.
• Third group: In modern IUPAC numbering, the third group is Group 3 (Sc, Y, Lu, Lr). Historically, sometimes Group IIIA (Boron group) was referred to as the third main group. However, in the context of alkali metals, we are referring to the main groups on the left side of the table.
• First group: This group contains Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), and Francium (Fr). Hydrogen (H) is also in Group 1 but is generally not considered an alkali metal due to its unique properties.

Why Alkali Metals are in the First Group

Elements in the First group have one electron in their outermost energy shell. For example:

• Lithium (Li): Electron configuration is $1s^2 2s^1$. One valence electron.
• Sodium (Na): Electron configuration is $1s^2 2s^2 2p^6 3s^1$. One valence electron.
• Potassium (K): Electron configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^1$. One valence electron.

This single valence electron is easily lost, which explains the high reactivity and characteristic properties of alkali metals, such as forming $+1$ ions.

Conclusion

Based on the definition and location of alkali metals in the Modern Periodic Table, they are assigned to the First group.

Revision Table: Alkali Metals

Additional Information: Properties of Alkali Metals

Alkali metals are highly reactive elements due to their tendency to easily lose their single valence electron. This reactivity increases as you go down Group 1 because the valence electron is further from the nucleus and more easily removed.

Some key properties include:

• They are soft metals that can be easily cut with a knife.
• They have low melting and boiling points compared to most other metals.
• They are good conductors of heat and electricity.
• They react vigorously with water, producing hydrogen gas and the corresponding metal hydroxide (an alkali). This reaction is exothermic and can be quite violent, especially for the heavier alkali metals.
• They are stored under oil or inert gas to prevent reaction with air and moisture.
• They are never found in nature as free elements, only in compounds.