Here's why:
* Isotopes: Fluorine has only one naturally occurring isotope, meaning all fluorine atoms have the same number of protons (9) and electrons (9). However, they can have different numbers of neutrons.
* Mass Number: The mass number is the total number of protons and neutrons in an atom's nucleus. While all fluorine atoms have 9 protons, some have 10 neutrons (making their mass number 19), and some have 9 neutrons (making their mass number 18). These are called isotopes of fluorine: Fluorine-19 and Fluorine-18.
* Mass Differences: Fluorine-19 is the most common and stable isotope, making up almost 100% of naturally occurring fluorine. Fluorine-18 is much rarer and is radioactive. This means it decays over time into other elements, releasing energy.
Therefore, two atoms of fluorine can have different masses due to the presence of different numbers of neutrons (isotopes). This difference in mass can also lead to slight variations in some physical properties, such as their reactivity and how they interact with other elements. However, they will still share the same chemical properties because they have the same number of protons and electrons.
