The alkali metals have the lowest first ionisation energies in their respective periods of the periodic table because of their low effective nuclear charge and the ability to attain a noble gas configuration by losing just one electron.
ðɪ ˈælkəlˌaɪ mˈɛɾəlz hæv ðə lˈoʊɪst fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒiz ɪn ðɛɹ ɹᵻspˈɛktɪv pˈiəɹɪədz ʌvðə pˌiəɹɪˈɑːdɪk tˈeɪbəl bɪkˈʌz ʌv ðɛɹ lˈoʊ ɪfˈɛktɪv nˈuːklɪɹ tʃˈɑːɹdʒ ænd ðɪ ɐbˈɪləɾi tʊ ɐtˈeɪn ɐ nˈoʊbəl ɡˈæs kənfˌɪɡjɚɹˈeɪʃən baɪ lˈuːzɪŋ dʒˈʌst wˈʌn ᵻlˈɛktɹɑːn.