The atomic number of three elements A, B and C are 9, 10 and 13 respectively. Which of them will form a cation?

The element that will form a cation is the one that tends to lose electrons to achieve a stable electron configuration.

1. Element A has an atomic number of 9, which means it has 9 electrons. The electron configuration is 2,7. It needs to gain 1 electron to achieve a stable configuration.

2. Element B has an atomic number of 10, which means it has 10 electrons. The electron configuration is 2,8. It already has a stable configuration.

3. Element C has an atomic number of 13, which means it has 13 electrons. The electron configuration is 2,8,3. It needs to lose 3 electrons to achieve a stable configuration.

Therefore, Element C is the one that will form a cation.