Ta có:

`2p_M+n_M+3(2p_X+n_X)=238`

`->2(p_M+3p_X)+(n_M+3n_X)=238`

`2p_M-n_M+3(2p_X-n_X)=70`

`->2(p_M+3p_X)-(n_M+3n_X)=70`

Suy ra:

`p_M+3p_X=77->p_M=77-3p_X`

`n_M+3n_X=84->n_M=84-3n_X`

`2p_M+n_M-3-(2p_X+n_X+1)=26`

`->2p_M-2p_X+n_M-n_X=30`

`->2(77-3p_X)-2p_X+84-3n_X-n_X=30`

`->2p_X+n_X=52`

`->n_X=52-2p_X`

`p_X\le n_X\le p_X+3`

`->p_X\le 52-2p_X\le p_X+3`

`->16,33\le p_X\le 17,33 `

`->p_X=17(Cl)`

`->p_M=77-17.3=26(Fe)`

\(Cl:1s^22s^22p^63s^23p^5\)

Ô 17, nhóm VIIA, chu kì 3

\(Fe:1s^22s^22p^63s^23p^63d^64s^2\)

Ô 26, nhóm VIIIB, chu kì 4