Question

For the transmutation shown, which of the following is X?

Answer 1

Answer:

Choice A: carbon-12, $^{12}_{\phantom{0}6}\text{C}$.

Explanation:

• The number on the top-left corner of each particle is its mass number.
• The number on the lower-left corner of each particle is its atomic number.

Let

• the mass number of X be A, and
• the atomic number (a.k.a. proton number) of X be Z.

Symbol of X: $^{A}_{Z}\text{X}$.

Consider the two conservation laws:

• Sum of mass numbers on the left-hand side = Sum of mass numbers on the right-hand side.
• Sum of atomic numbers on the right-hand side = Sum of atomic numbers on the right-hand side.

Sum of mass numbers:

• Left-hand side: 9 + 4;
• Right-hand side: A + 1.

Equate the two sides:

$9 + 4 = A + 1$

$A = 12$.

Thus the mass number of X is 12.

Try the steps above to find the atomic number of X. The number $0$ on the lower-left corner of $^{1}_{0}n$ indicates that the atomic number of this neutron is zero.

$Z = 6$.

Thus the atomic number of X is 6.

What element is X? Refer to a modern periodic table. The element with atomic number 6 is carbon. The symbol for carbon is $C$. The mass number of this isotope is 12. The answer will be $^{12}_{\phantom{0}6}\text{C}$.