Question

Which element does not have the same number of electrons in its outermost shell as the other elements in its group?
• hydrogen (H)
• helium (He)
• boron (B)
• bromine (Br)

Answer 1

In the context of the periodic table, elements are grouped based on their electron configurations, particularly focusing on the number of electrons in their outermost shell (valence electrons).

- Hydrogen (H) has 1 electron in its outermost shell.
- Helium (He) has 2 electrons in its outermost shell.
- Boron (B) has 3 electrons in its outermost shell.
- Bromine (Br) has 7 electrons in its outermost shell.

Among the listed elements, **hydrogen (H)** does not belong to a group with other elements that have the same number of valence electrons. Although it is in Group 1, it is unique because it is not a metal and has only 1 electron, while the other groups typically have a different electron configuration in their outer shells (e.g., Group 2, Group 13, etc.).

If considering the other elements:
- Helium is a noble gas and has a full outer shell.
- Boron is in Group 13 and has 3 valence electrons.
- Bromine is in Group 17 and has 7 valence electrons.

Thus, hydrogen is the answer since it has a different number of electrons in its outermost shell compared to boron and bromine, which are in standard groups with other elements.