## 3.9 and 3.11 in the homework? I am confused as to what it is they are asking

Posts: 96
Joined: Fri Sep 29, 2017 7:04 am

### 3.9 and 3.11 in the homework? I am confused as to what it is they are asking

So 3.9 asks "Which M2+ ions (where M is a metal) are predicted to have the following ground-state electron configurations: a) [Ar] 3d7 b) [Ar]3d6 and so on.
So for a) I got Co2+ and b) Fe2+ because those are the neutral electron configurations for those metals. And then I check the answer in the book and it was correct.

Then I get to 3.11 and it is similar:
Which M3+ ions (where M is a metal) are predicted to have the following ground state electron configurations:
a) [Ar] 3d6 b) [Ar] 3d5 c) [Kr] 4d5 d) [Kr] 4d3

So I put down a) Fe 3+ b) Mn3+ c) Tc 3+ d) Nb 3+ because those are the elements with the ground state electron configurations that match the question? But then the solutions state a) Co3+ b) Fe 3+ c) Ru 3+ d) Mo 3+ and I do not understand why

Jonathan Tangonan 1E
Posts: 50
Joined: Sat Jul 22, 2017 3:01 am

### Re: 3.9 and 3.11 in the homework? I am confused as to what it is they are asking

For "a" on 3.11 if you look at the ground electron state configuration of Co it is [Ar] 3d7 4s2. As we are looking for a 3+ ion, when 3 electrons are taken from a Co atom they are first removed from the s subshell and then they are lost through the d subshell resulting in the [Ar] 3d6 configuration. For "c" Ru actually has a ground state electron configuration of [Kr] 4d7 5s1. With the same principle in mind, removing the electrons from the s subshell and then the d subshell it results in a [Kr] 4d5 configuration.

William Cryer 1L
Posts: 12
Joined: Fri Sep 29, 2017 7:05 am

### Re: 3.9 and 3.11 in the homework? I am confused as to what it is they are asking

The way I think about this is counting backwards on the periodic. For example, if the electron configuration asks for a 3+, you know that this ion has 3 less electrons than normal, so you can count 3 elements to the right.
Ex) N3+ turns into Be, so write the electron configuration as if you were writing it for Be.

Dayanara Ruiz
Posts: 18
Joined: Fri Sep 29, 2017 7:03 am

### Re: 3.9 and 3.11 in the homework? I am confused as to what it is they are asking

The best way to memorize this is to do the opposite of what the sign is asking for, 3+ion means you take away 3 (-3) electrons from the configuration. This makes it easier to look for the element, which should be three elements to the right.