Explanation:

- Elements in the periodic table are organized into groups (columns) and periods (rows) based on their properties.

- Group 17 is known as the halogens, and elements in this group have 7 valence electrons.

- Period 2 consists of elements with two electron shells.

- Fluorine (F), which belongs to Group 17 and Period 2, has an atomic number of 9.

- Option 1: 8 - Incorrect. Oxygen has atomic number 8, but it's in Group 16.

- Option 2: 7 - Incorrect. Nitrogen has atomic number 7, but it's in Group 15.

- Option 3: 10 - Incorrect. Neon has atomic number 10, but it's a noble gas in Group 18.

- Option 4: 9 - Correct. Fluorine fits this description.