# Why is the valency of the Calcium (Ca) Bohr model, set to 2 electrons, instead of 1 electron for stability? [duplicate]

This question already has an answer here:

This is an Ascii table for the elements on the 4th period (row) of the periodic table, and distribution of electrons in each orbit.

+---------+----------+---------+---------+----------+---------+
|         |          |         |         |          |         |
+---------+----------+---------+---------+----------+---------+
| Element | Atomic # | Shell 1 | Shell 2 | Shell 3  | Shell 4 |
| K       | 19       | 2       | 8       | 8        | 1       |
| Ca      | 20       | 2       | 8       | 8 (9?)   | 2 (1?)  |
| Sc      | 21       | 2       | 8       | 9        | 2       |
| Ti      | 22       | 2       | 8       | 10       | 2       |
| V       | 23       | 2       | 8       | 11       | 2       |
| Cr      | 24       | 2       | 8       | 13       | 1       |
| Mn      | 25       | 2       | 8       | 13       | 2       |
| Fe      | 26       | 2       | 8       | 14       | 2       |
| Co      | 27       | 2       | 8       | 15       | 2       |
| Ni      | 28       | 2       | 8       | 16       | 2       |
| Cu      | 29       | 2       | 8       | 18       | 1       |
| Zn      | 30       | 2       | 8       | 18       | 2       |
| Ga      | 31       | 2       | 8       | 18       | 3       |
| Ge      | 32       | 2       | 8       | 18       | 4       |
| As      | 33       | 2       | 8       | 18       | 5       |
| Se      | 34       | 2       | 8       | 18       | 6       |
| Br      | 35       | 2       | 8       | 18       | 7       |
| Kr      | 36       | 2       | 8       | 18       | 8       |
+---------+----------+---------+---------+----------+---------+


I know all elements must have 4 orbitals due to the period (row #) of 4, and the outer most orbital (or shell), 4, is the valence.

Cr and Cu, have their valance set to 1, instead of 2, because, the electrons closer to the nucleus, requires less energy to sustain itself, and is thus more stable, but at the same time the each orbit (or shell) must have at least 1 electron on it. In regards to Cr and Cu, I thought there would be a precedence for orbit 3, the orbit preceding the valence, to be more full and within 18 or less electrons.

This forum post (link), hosted on a college's server, unfortunately fails to address the issue with Ca.

## marked as duplicate by Mithoron, Karl, Buttonwood, Jon Custer, Todd MinehardtAug 31 at 16:08

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

• You should use the standard electron configuration nomenclature instead of numbering "shells" and "orbitals". The configuration of Ca is $[Ar] 4s^2$, chromium $[Ar] 3d^5 4s^1$, copper $[Ar] 3d^{10} 4s^1$ – Karl Aug 29 at 18:56
• @Karl instructor went straight to drawing the Bohr model, rather than familiarize us with SPDF filling patterns and the square notations. – gimmegimme Aug 30 at 1:54
• Well, Bohr is only step one. It seems you have progressed now, keep it up! – Karl Aug 30 at 6:33

## 1 Answer

After following Mithoron's link, I familiarized my self with the writing on the following ChemGuide site (link).

I went into Excel spreadsheet, and had each 'sub orbital' it's own column, and came up with the following.

+---------+---------+----+---------+----+----+---------+-----+
| Shell 1 | Shell 2      | Shell 3           | Shell 4       |
+---------+---------+----+---------+----+----+---------+-----+
| 1s      | 2s      | 2p | 3s      | 3p | 3d | 4s      | SUM |
| 2       | 2       | 6  | 2       | 6  | 0  | 1       | 19  |
| 2       | 2       | 6  | 2       | 6  | 0  | 2       | 20  |
+---------+---------+----+---------+----+----+---------+-----+


This is what the overall table (link) looks like in excel; I can't attach the image directly to my post.

While googling, to get a clear understanding of S,P,D,F I came across ThoughtCo's article (link). This let me know the stability precedence, i.e. electron distribution, more clearer, as follows:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f

4s is filled before, 3d. 3s and 3p, can hold a combined maximum of 8 electrons.

The following (link) has a diagram with downward/diagonal arrows, showing how one can better remember the stability precedence.