Question

When californium-249 is bombarded by oxygen-18, a new isotope and four neutrons are produced.
Express your answer as a nuclear equation.

Answer 1

number preceding symbol = atomic number(a subscript)
number after symbol = mass number(a superscript)
In nuclear equations the subscripts add up and the superscripts add up.

98Cf249 + 8O18 ==> nXm + on1 + on1 + on1 + on1.

So your job is to determine n, X, and m.
Remember the numbers must add up on both sides to the same thing.
For preceding number (the subscripts).
98 + 8 = n + 0 + 0 + 0 + 0 so n = 106
For number after the symbol (superscripts)
249 + 18 = m + 1 +1 +1 +1
m = 249 + 18 - 4 = 263
Now look on the periodic table and identify element X, element # 106. That's Sg. So the equation, in full, is
98Cf249 + 8O18 ==> 106Sg263 + 4 on1.