Core structure example:

As (33 electrons):
1s², 2s², 2p⁶, 3s², 3p⁶ | 4s², 3d¹⁰, 4p³
            core                |   valence

Gd (64 electrons):
1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶,  5s³, 4d¹⁰, 5p⁶|
                                    core electrons                        |

| 6s², 4f⁸
|  valence

The core ends with the last filled "p".  That is, the last p⁶.