The element with nucleon number 19 and proton number 9 is fluorine

Step-by-step solution

Element Y has:

• Proton number (atomic number) Z=9: this defines the element as fluorine (F).
• Nucleon number (mass number) A=19: this matches the common isotope ¹⁹F.

Fluorine has the electronic configuration [He] 2s²2p⁵, which gives a total of 7 valence electrons in the outer shell.

Elements with 7 valence electrons are placed in Group 17, which in older notation is called Group VII (Group VIIA), the halogens.

Therefore, Element Y (fluorine, Z = 9) belongs to Group VII (Group 17, halogens) → option c.

Explanation of each option

Option a: Group I

Group I (modern Group 1) elements are the alkali metals like lithium, sodium, potassium.

These elements have 1 valence electron and are highly reactive metals, not non-metals like fluorine, which has 7 valence electrons and is a halogen.

So, Element Y (fluorine) cannot be in Group I.

Option b: Group III

Group III in old notation usually corresponds to Group 13 (the boron family: B, Al, Ga, etc.).

These elements have 3 valence electrons, whereas fluorine has 7 valence electrons and belongs to a different group.

So, Element Y cannot be in Group III.

Option c: Group VII (correct)

Group VII (old notation) corresponds to Group 17, the halogens: F, Cl, Br, I, At, Ts.

Halogens all have 7 valence electrons, and fluorine (Z = 9, A ≈ 19) is the first element of this group.

Thus, Element Y with proton number 9 must be in Group VII (Group 17) → correct answer.

Option d: Group VIII

Group VIII (old notation) corresponds mainly to Group 18, the noble gases: He, Ne, Ar, etc.

Noble gases have full outer shells (2 or 8 valence electrons) and are very unreactive, unlike fluorine, which is highly reactive and has 7 valence electrons.

So, Element Y does not belong to Group VIII.

SEO-friendly explanation (article style)

Identifying the element

An element is uniquely identified by its proton number (atomic number), which tells how many protons are in its nucleus.

For Element Y, the proton number is 9, so it must be fluorine (F), which has atomic number 9 and a common nucleon number (mass number) of 19.

Electronic configuration and valence electrons

Fluorine (Z = 9) has the electronic configuration [He] 2s²2p⁵, giving a total of 7 electrons in the outermost shell (2s²2p⁵).

The number of valence electrons determines the group of a main-group element in the periodic table, so 7 valence electrons place fluorine among the halogens.

Group VII (Group 17) halogens

Halogens form Group 17 in modern IUPAC numbering, historically called Group VIIA or Group VII.

This group includes fluorine, chlorine, bromine, iodine and astatine, all of which have 7 valence electrons and very similar chemical properties.

Why the correct answer is Group VII

Since Element Y is fluorine with atomic number 9 and 7 valence electrons, it belongs to the halogen family in Group VII (Group 17), not to alkali metals, boron family or noble gases.

Therefore, for the question “Element Y has a nucleon number of 19 and a proton number of 9. Which group in the Periodic Table does it belong to?”, the correct option is c. Group VII.