The valence shell of boron is the second electron shell and contains three electrons. How many covalent bonds can boron form by sharing its electrons?
Covalent bonds involved shared valence electrons. The “co” in covalent means shared while the “valent” refers to valency. Boron is the fifth element in the periodic table, sitting in group 13 and period two. Boron has the atomic number five, meaning that every atom of boron has five protons in its nucleus. And a neutral atom of boron will have five electrons. So boron has two electrons in the inner shell and three electrons in its outer shell. This information reflects what’s mentioned in the question that there are three electrons in the valence shell of boron.
A simple single covalent bond like that between hydrogen atoms in a hydrogen molecule can be drawn like this. Two electrons, one from each atom, come together to form a single bond. This can be drawn as the lowest structure with two dots between the element symbols or with a line. Since it takes one electron from each atom to form a single bond, we can expect boron to form three bonds since it has three valence electrons. We can see this in the molecule borane BH3. Here, boron forms three covalent bonds, using all three of its valence electrons in single bonds to the hydrogens.
Now, the second electron shell has a maximum occupancy of eight electrons. This means that there’s a space for two more electrons in the second electron shell. However, the question asks for the number of covalent bonds boron can form by sharing its electrons. Since boron only has three valence electrons, it can only form three covalent bonds maximum.
Therefore, the number of covalent bonds boron can form by sharing its electrons is three.