# Homework Help: Describing bond between elements

1. Jan 23, 2009

### transgalactic

http://img525.imageshack.us/img525/2290/84419342em1.th.gif [Broken]

in case A:
S has a bigger electonegative energy then Ca
so it will "take" 2 electrons from Ca inorder to be a complete gas like (Ar)

so i know why there is +2 on Ca and -2 on S .

Ca has after perfect gas 4S^2
S =[Ne]3s^2 3P^4
but there are dots and cols what they mean??

regarding B:
for Br to become a perfect gas like(Kr)
it needs to take only one electron
why there is -2 ??
and what are all the dots nere it and cols []??

Last edited by a moderator: May 3, 2017
2. Jan 23, 2009

### Staff: Mentor

Eight dots as octet (eight electrons) on the valence shell.

[] just groups atom and electrons together, so that when you write [Ca]<sup>2+</sup> or [Ca:]<sup>0+</sup> it is obvious that charge is that of the atom and the electrons together.

Where do you see -2? There are two Br<sup>-</sup> anions (each with its own octet), that gives -2 in all, but there is no individual entity with this charge in the magnesium bromide.

3. Jan 23, 2009

### transgalactic

on case A
the formula for S is [Ne]3S^2 3p^4

from where in this formula i can see that there is 8 electrons on the valence shell??

4. Jan 23, 2009

### Staff: Mentor

For S2- it becomes [Ne]3s23p6 - hence the octet. [Ne]3s23p4 is a neutral S atom.

5. Jan 23, 2009

### transgalactic

ok i understand case A

regarding case B:
Mg=[Ne]3s^2 so it has 2 electrons on the
valence shell before the merging.

Br=[Ar]3d^10 4s^2 4p^5
so it has 17 electrons on the
valence shell before the merging.

after the merging Br needs to take 1 electron from Mg to become neutral gas.

and for Mg to become a neutral gas we need to take 2 electrons
so we link two Br atoms

but why they present 8 dots around Br??

before it was 17 then we add 2 so we have 19
but 2*8=16

??

6. Jan 23, 2009

### Staff: Mentor

To get stable octet.

We don't add two electrons. We add one electron to each of the two Br atoms. You have to treat each atom separately.

d electrons can be in this case ignored, they don't play any role.

Last edited: Jan 23, 2009
7. Jan 23, 2009

### transgalactic

regarding case B:
before Br has 17 electrons
after the reaction each Br gets 1 electron
so each Br must have 18 electrons

but in the solution
they have only 5
??

8. Jan 23, 2009

### Staff: Mentor

Picture is fuzzy, but they have 8, just like sulfur. : on the left, : on the right (it is "Br:", not "Br."), two above and two below.

9. Jan 23, 2009

### transgalactic

ok even if its 8
before Br has 17 electrons
after the reaction each Br gets 1 electron
so each Br must have 18 electrons

but in the solution
they have only 8

its not 18
??

10. Jan 24, 2009

### Staff: Mentor

I have already explained: d electrons are ignored, they don't play any role.

11. Jan 24, 2009

### transgalactic

why egnoring
D

??
3d^10 4s^2 4p^5

they do play a role
they all
are on the
valence shell

12. Jan 24, 2009

### Staff: Mentor

Octet rule is only a simplification. Obviously it can't work for any atom where d electrons take part in the boding, as there are 10 of them and octet calls for 8 electrons.

In this particular case s&p electrons are enough to explain what is going on, d electrons are not involved - they sit on their orbitals untouched.

13. Jan 24, 2009

thanks
:)