Group 17 elements such as fluorine and chlorine tend to form ions that are:

1. Group 17 elements have seven electrons in their outermost shells. 2. One more electron would complete the octet, which is easier than losing seven. 3. So they tend to fill this shell with an electron from other atoms or molecules, making them negatively charged ions. 4. Elements that lose electrons instead become positively charged ions. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 2 'The Chemical Foundation of Life', sections 2.1-2.3_