Account for the decreasing values of bond angle from 107º in NH3 to 91º in SbH3 down the group 15 elements of the periodic table.​

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so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer