# What is the effective atomic number of tetraaquadichlorochromium(II)?

##### 1 Answer

See here, here, and here for extra practice.

The **effective atomic number** is the number of electrons in the central transition metal, plus the number of contributed electrons by the ligands.

You can choose an electron-counting method, but I'll use the **donor pair method**, which assumes the ligands donate electron pairs and assigns the charges accordingly.

Thus, for the aqua and chloro ligands, we get:

#"H"_2"O":# #-># #2# donated electrons, neutral

#"Cl":^(-)# #-># #2# donated electrons,#-1# charge

In this method, we then take the central metal to have whatever oxidation state leaves it with the charge of the complex. As the aqua ligands are neutral, chromium then has a

Its neutral electron configuration was

#[Ar] 3d^5 4s^1# ,

so its

#[Ar] 3d^4 4s^0#

and it thus has

#color(blue)("EAN") = overbrace(22)^(Cr^(+2)) + overbrace(4 xx 2)^(4 xx H_2O) + overbrace(2 xx 2)^(2 xx Cl^(-)) = color(blue)(34)#