The factors affecting the first ionisation energy are the nuclear charge, the amount of shielding by the inner electrons and the distance from the most loosely held electron from the nucleus, which is always an outer electron in main group elements.
ðə fˈæktɚz ɐfˈɛktɪŋ ðə fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒi ɑːɹ ðə nˈuːklɪɹ tʃˈɑːɹdʒ, ðɪ ɐmˈaʊnt ʌv ʃˈiːldɪŋ baɪ ðɪ ˈɪnɚɹ ᵻlˈɛktɹɑːnz ænd ðə dˈɪstəns fɹʌmðə mˈoʊst lˈuːsli hˈɛld ᵻlˈɛktɹɑːn fɹʌmðə nˈuːklɪəs, wˌɪtʃ ɪz ˈɔːlweɪz ɐn ˈaʊɾɚɹ ᵻlˈɛktɹɑːn ɪn mˈeɪn ɡɹˈuːp ˈɛlɪmənts.