The molar mass of Br2 is 2 * 79.904 g/mol = 159.808 g/mol.

Number of moles in 21.8 g of Br2 = 21.8 g / 159.808 g/mol = 0.1365 mol.

Since there are 2 Br atoms per molecule of Br2, the number of Br2 molecules in 0.1365 mol of Br2 = 0.1365 mol * Avogadro's Number (6.022 x 10^23) = 8.24 x 10^22 molecules.

Therefore, 21.8 g of bromine gas contains 8.24 x 10^22 molecules of Br2.