Which of the following statement is NOT correct regarding the alkali metals?

All the alkali metals have two valence electrons.

The correct answer is option 4. All the alkali metals have two valence electrons.

Let us go through each statement:

1. All the alkali metals are silvery white, soft, and light metals:

This statement is correct. Alkali metals, which belong to Group 1 of the periodic table, including elements like lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr), are indeed silvery white, soft, and light metals.

2. The ionization enthalpies of the alkali metals are considerably low and decrease down the group:

This statement is correct. Alkali metals have low ionization enthalpies, meaning it requires relatively low energy to remove the outermost electron. The ionization enthalpy tends to decrease down the group.

3. Group 1 metals are known as alkali metals:

This statement is correct. Group 1 elements, which include lithium, sodium, potassium, rubidium, cesium, and francium, are collectively known as alkali metals.

4. All the alkali metals have two valence electrons:

This statement is incorrect. Alkali metals have only one valence electron. The number of valence electrons is determined by the group number in the periodic table, and for alkali metals in Group 1, there is only one electron in the outermost shell.

So, the incorrect statement is 4. All the alkali metals have two valence electrons.