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Chromium is element number 24, so a neutral atom would have 24 protons and 24 electrons. It also has 28 neutrons, which gives it an atomic mass of 24 (protons) plus 28 (neutrons), equalling 52 AMU's.
Chromium is an exception to the usual standard notation rules. A neutral Cr atom would have the standard electron notation:
1s2 2s2 2p6 3s2 3p6 3d54s1
Which can be abbreviated as: [Ar] 3d54s1
For your question, you specify 26 electrons, so here we are working with the -2 ion. We add the two extra electrons so that they fill the empty spaces in the last two shells. Cr-2's notation would be:
1s2 2s2 2p6 3s2 3p6 3d64s2
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