The species that gains the electron pair is the Lewis acid; for example, the oxygen atom in H3O+ gains a pair of electrons when one of the H—O bonds is broken and the electrons shared in the bond become localized on oxygen.
ðə spˈiːsiːz ðæt ɡˈeɪnz ðɪ ᵻlˈɛktɹɑːn pˈɛɹ ɪz ðə lˈuːɪs ˈæsɪd; fɔːɹ ɛɡzˈæmpəl, ðɪ ˈɑːksɪdʒən ˈæɾəm ɪn ˈeɪtʃ θɹˈiː ˈoʊ plˈʌs ɡˈeɪnz ɐ pˈɛɹ ʌv ᵻlˈɛktɹɑːnz wɛn wˈʌn ʌvðɪ ˈeɪtʃ—ˈoʊ bˈɑːndz ɪz bɹˈoʊkən ænd ðɪ ᵻlˈɛktɹɑːnz ʃˈɛɹd ɪnðə bˈɑːnd bɪkˌʌm lˈoʊkəlˌaɪzd ˌɔn ˈɑːksɪdʒən.