Answer :
Answer:
229
Explanation:
A general rule that comes with equations in chemistry is that the reactants must always equal the products. Whether something loses electrons, or mass is transferred, the object it is being transferred to must be accounted for, such that the numbers of each element on both sides equal each other. This law is more commonly known as the conservation of mass, where matter cannot be destroyed, therefore making sure that it is noted where the matter must go.
For this question, the bottom number follows this rule; 90 is turned into 88 and 2, which add up to 90. This means that for the number on top, we need to find a number that adds to 4 to reach 233, as represented by x. We can find this number by subtracting 233 by 4, which gives us 229, the mass number of the Ra atom as per our mass conservation law.
(Note that this can be solved algebraically: 233 = x + 4 ⇒ x = 229)
