fluorine has an atomic number of 9 and a mass number of 19. how many electrons are needed to complete the valence shell of a fluorine atom?
The Correct Answer and Explanation is :
The correct answer is: one electron
Fluorine (F) has an atomic number of 9, meaning it has 9 protons and, in its neutral state, 9 electrons. The mass number of 19 indicates the total number of protons and neutrons in the nucleus, which means fluorine has 10 neutrons (19 – 9 = 10).
The electron configuration of a fluorine atom is (1s^2 2s^2 2p^5). This configuration shows that fluorine has two electrons in its first energy level (the 1s orbital) and seven electrons in its second energy level (the 2s and 2p orbitals). The second energy level can hold a maximum of eight electrons (according to the octet rule), which states that atoms tend to bond in such a way that they each have eight electrons in their valence shell, resembling the electron configuration of noble gases.
Since fluorine has seven electrons in its second energy level, it needs one additional electron to complete its valence shell and achieve a stable configuration. When fluorine gains one more electron, it achieves the electron configuration of neon, a noble gas, which is (1s^2 2s^2 2p^6).
Fluorine’s strong electronegativity—its ability to attract electrons—makes it very reactive, often forming bonds by gaining or sharing electrons. When fluorine reacts with other elements, it typically gains one electron, resulting in the formation of a fluoride ion (F⁻) with a complete octet.
In summary, fluorine needs one electron to complete its valence shell, allowing it to achieve stability and complete its octet. This tendency to gain electrons is what drives its high reactivity, particularly with alkali metals and other elements that readily lose electrons.