When a bond forms between C(H3) and MgBr, why would MgBr have a partial positive charge?

When a bond forms between C(H3) and MgBr (methyl magnesium bromide), MgBr would have a partial positive charge due to the difference in electronegativity between the atoms involved.

To understand why, let's start by looking at the electronegativity values of the atoms in this bond. Carbon (C) has an electronegativity of approximately 2.5, while magnesium (Mg) has an electronegativity of around 1.3. Bromine (Br) has an electronegativity of 2.8.

Electronegativity is a measure of an atom's ability to attract electrons towards itself in a bond. In general, when two atoms with different electronegativity values form a bond, the more electronegative atom will attract the shared electrons more strongly, giving it a partial negative charge, denoted as δ-. The less electronegative atom will have a weaker attraction for electrons and therefore will have a partial positive charge, denoted as δ+.

In the case of C(H3) and MgBr, the carbon atom is more electronegative than the magnesium atom. As a result, the carbon atom will attract the shared electrons in the bond more towards itself, creating a partial negative charge (δ-) on the carbon and a partial positive charge (δ+) on the magnesium atom. This kind of bond, where an electron-deficient atom is bonded to a more electronegative atom, is commonly referred to as a polar covalent bond.

It's important to note that the partial positive charge on the magnesium atom is relatively small compared to the full positive charge of a magnesium cation (Mg2+), which occurs when magnesium loses both of its valence electrons. The partial positive charge arises due to the unequal sharing of electrons in the bond rather than full electron transfer.

In summary, the partial positive charge on the magnesium atom in the bond between C(H3) and MgBr is a result of the difference in electronegativity between the carbon and magnesium atoms. The more electronegative carbon atom attracts the shared electrons more strongly, resulting in a partial negative charge on carbon and a partial positive charge on magnesium.