<p>Two p-block elements x (outer configuration&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>n</mi><msup><mi>s</mi><mn>2</mn></msup><mo>&#160;</mo><mi>n</mi><msup><mi>p</mi><mn>3</mn></msup><mo>)</mo></math>&nbsp;and z (outer configuration&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>n</mi><msup><mi>s</mi><mn>2</mn></msup><mi>n</mi><msup><mi>p</mi><mn>4</mn></msup></math>) occupy neighbouring positions in a period. Using this information which of the following is correct with respect to their ionisation potential&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>I</mi><mi>x</mi></msub><mo>&#160;</mo><mi>a</mi><mi>n</mi><mi>d</mi><mo>&#160;</mo><msub><mi>I</mi><mi>z</mi></msub></math>.</p>