A negative ion. anion One-half the distance between the nuclei of identical atoms that are bonded together. atomic radius A positive ion. cation The energy change that occurs when an electron is acquired by a neutral atom. electron affinity A measure of the ability of an atom in a chemical compound to attract electrons. electronegativity An atom or group of bonded atoms that has a positive or negative charge. ion The energy required to move on electron from a neutral atom of an element. ionization energy Any process that results in the formation of an ion. ionization The electrons available to be lost, gained, or shared in the formation of chemical compounds. valence electrons State the general period and group trends among main-group elements with respect to each of the following properties: A) atomic radii B) first ionization energy C) electron affinity D) ionic radii E) electronegativity A) Generally decreases across periods and increases down groups. B) Generally increases across periods and decreases down groups C) Generally increases across periods among groupd 13-17 and decreases down groups but many execptions are observed. D) Generally decrease across periods and increase down groups. E) Gradually increases across periods and either decreases or remains the same down groups. Among the main-group elements, what is the relationship between group number and the number of valence electrons among group members? For groups 1-2 the number of valence electrons is equal to the group number. For groups 13-18, the number of valence electrons is equal to the group number minus 10.