Question

which one of the following has 7 valence electrons?
(A) $ N{a^ + } $
(B) $ Al $
(C) $ F $
(D) $ Ti $
(E) $ B{r^ - } $

Answer 1

Hint :Valence electron is an outer shell electron that is associated with an atom which participates in the formation of a chemical bond. When two atoms interact, the electrons in the outermost shells are the first ones to come into contact with each other and are the ones that determine how an atom will react in a chemical reaction.

Complete Step By Step Answer:
Generally for neutral atoms, the number of valence electrons is equal to the atom’s main group number. For example, carbon is in group 4 and has 4 valence electrons. For charged atoms, there is a small deviation from the normal valence electrons. If an atom is negatively charged it means that there are more electrons than the normal valence electrons. If an atom is positively charged it means that there are fewer electrons than the normal valence electron. For example, $ Ca $ normally has 2 valence electrons but $ C{a^{2 + }} $ has zero valence electrons since with +2 it is missing two electrons. Normally $ S $ has 6 valence electrons but $ {S^{2 - }} $ has 8 valence electrons since with -2 it has gained two more electrons.
Generally, any element in the halogen group will have 7 valence electrons.
With this basic information let’s go through the options.
 $ N{a^ + } $ have 8 outermost electrons after losing 1 electron, so the number of valence electrons is 8.
 $ Al $ comes in the group 3 and hence it has 3 valence electrons.
 $ F $ has 7 electrons in the outermost shell. As we discussed earlier, all halogens will have 7 valence electrons. Hence $ F $ have 7 valence electrons.
 $ Ti $ is a transition metal and its atomic number is 22. It contains 4 electrons in the outermost shells, hence the number of valence electrons is 4.
 $ B{r^ - } $ have 8 electrons in the outermost shells after gaining an electron. Hence the number of valence electrons is 8.
Hence $ F $ is the only atom with 7 valence electrons among all the options. So option (C) is the right answer.

Note :
Keep in note that valency and valence electrons are different. Valency of an element is the number of electrons that are shared, lost or gained by an atom during a chemical reaction. The outermost shell of an atom is known as valence shell and the electrons present in it are called valence electrons.
For example, the number of valence electrons in $ F $ is 7 and the valency of $ F $ is 1 because it is easier to gain an electron and satisfy the octet rule and become stable.