This stems from the filled d subshell providing a much weaker shielding effect on the outermost s electron than the filled p subshell, so that the coinage metals have much higher first ionisation energies and smaller ionic radii than do the corresponding alkali metals.
ðɪs stˈɛmz fɹʌmðə fˈɪld dˈiː sˈʌbʃɛl pɹəvˈaɪdɪŋ ɐ mˈʌtʃ wˈiːkɚ ʃˈiːldɪŋ ɪfˈɛkt ɔnðɪ ˈaʊɾɚmˌoʊst ˈɛs ᵻlˈɛktɹɑːn ðɐn ðə fˈɪld pˈiː sˈʌbʃɛl, sˌoʊ ðætðə kˈɔɪnɪdʒ mˈɛɾəlz hæv mˈʌtʃ hˈaɪɚ fˈɜːst ˌaɪənəzˈeɪʃən ˈɛnɚdʒiz ænd smˈɔːlɚɹ aɪˈɑːnɪk ɹˈeɪdɪˌaɪ ðɐn dˈuː ðə kˌɔːɹɪspˈɑːndɪŋ ˈælkəlˌaɪ mˈɛɾəlz.