Here's why:

* Valence electrons are the electrons in the outermost shell of an atom, which are involved in chemical bonding.

* Group 13 elements all have their outermost electron configuration as ns² np¹.

For example:

* Boron (B): 1s² 2s² 2p¹ (3 valence electrons)

* Aluminum (Al): 1s² 2s² 2p⁶ 3s² 3p¹ (3 valence electrons)

* Gallium (Ga): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p¹ (3 valence electrons)

* Indium (In): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p¹ (3 valence electrons)

* Thallium (Tl): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p¹ (3 valence electrons)