The number of electrons in each element's electron shells, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior.
ðə nˈʌmbɚɹ ʌv ᵻlˈɛktɹɑːnz ɪn ˈiːtʃ ˈɛlɪmənts ᵻlˈɛktɹɑːn ʃˈɛlz, pɚtˈɪkjʊlɚli ðɪ ˈaʊɾɚmˌoʊst vˈeɪləns ʃˈɛl, ɪz ðə pɹˈaɪmɚɹi fˈæktɚɹ ɪn dɪtˈɜːmɪnɪŋ ɪts kˈɛmɪkəl bˈɑːndɪŋ bᵻhˈeɪvjɚ.