Mercury-203 undergoes beta minus decay. the subscript in the nuclide notation, hg, is 203 hg →? the element symbol produced is it has a mass number and an atomic number 203 (80) hg → 203 (81) ti + ?

1. The subscript when writing the notation is always dedicated to the atomic number of the element. Since the atomic number for Mercury, Hg, is 80, then the subscript is 80.

2. For the second problem, you just have to balance out the subscripts and superscripts of the reactions.

Superscripts: 203 = 203 + ?; To balance, the missing number would be 0.
Subscripts: 80 = 81 + ?; To balance, the missing number would be -1.
Hence the particle produced is actually an electron, or a beta particle (not an element). The mass number is 0, and the atomic number is 0 (since it does not contain any proton).