ðɪ ˈælkəlˌaɪ mˈɛɾəlz ˈɔːl hæv ðə sˈeɪm kɹˈɪstəl stɹˈʌktʃɚ bˈɑːdisˈɛntəd kjˈuːbɪk ænd ðˈʌs ðɪ ˈoʊnli ɹˈɛlᵻvənt fˈæktɚz ɑːɹ ðə nˈʌmbɚɹ ʌv ˈæɾəmz ðæt kæn fˈɪt ˌɪntʊ ɐ sˈɜːʔn̩ vˈɑːljuːm ænd ðə mˈæs ʌv wˈʌn ʌvðɪ ˈæɾəmz, sˈɪns dˈɛnsᵻɾi ɪz dᵻfˈaɪnd æz mˈæs pɜː jˈuːnɪt vˈɑːljuːm.
The alkali metals all have the same crystal structure (body-centred cubic) and thus the only relevant factors are the number of atoms that can fit into a certain volume and the mass of one of the atoms, since density is defined as mass per unit volume.