What will be the atomic number of an element X which is placed in period 2 and group 17?
- A.8
- B.7
- C.10
- D.9
Show answer
Correct answer: D. 9
Explanation
The correct answer is D, 9. Period 2 holds the eight elements from lithium (3) to neon (10), and group 17 is the halogen column, whose members have seven electrons in the outer shell. The element that is both in period 2 and in group 17 is fluorine, with electronic configuration 2, 7, and atomic number 9. You can work it out without memorising: period 2 means two shells, group 17 means seven valence electrons, so the electrons are 2 + 7 = 9, and in a neutral atom the number of electrons equals the atomic number. Fluorine is the most electronegative element and the most reactive non-metal. A, 8, is oxygen, which is in period 2 but in group 16 with six valence electrons. B, 7, is nitrogen, period 2, group 15. C, 10, is neon, period 2, group 18, a noble gas with a full shell. Exam tip: for periods 2 and 3, atomic number = electrons in the inner shells plus the group's last digit; 2 + 7 gives fluorine, 10 + 7 gives chlorine (17).