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Reading my chemistry 101 again I can not find a graphical difference between an ionic bond (gaining-losing and electron) and a covalent bond (sharing) an electron when I try to think of the orbits/cloud of the electron. Why do they say that one is gained/lost or shared?

I would love to see a graphic example (meaning a drawing, video or something) that represents if there is a difference between different orbits (of NaCl and H2 or something). Since the electron occupies a cloud instead of an orbit in both cases, where is the difference?

May be is not a great question, but I can not visualize it! Thanks for helping starters

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    $\begingroup$ It is hard to visualise, even the idealised cases. Furthermore, the bonds are in between these two concepts, which makes it even harder to grasp. In an ideal ionic bonds, electrons get transferred from one partner to the other, so in approximation the electrons only experience the field of one nuclei. The resulting ions then are attracted to reach other electrostatically. In a covalent bond, the binding electrons are shared, they experience the field of both bonding partners. $\endgroup$ Aug 3, 2020 at 10:26
  • $\begingroup$ In an ionic bond, one atom has lost one electron, given to the second. Afterwards, the atoms are becoming ions, and they are separated. They can move independently from one another. They attract themselves, but they are not necessarily in contact. In a covalent bond, the two atoms are attached one another with their common electrons. They stay together. The distance between their nucleus is constant, or nearly, as the bond can be stretched. $\endgroup$
    – Maurice
    Aug 3, 2020 at 11:36
  • $\begingroup$ Thank a lot to both of you. In NaCl, Maurice, how they are together when they are together? or there is Na with 10 and Cl with 18 in their own orbits (living happily, but close to each other?). Is that it? But what about HF? The hidrogen lives alone without electron, but it is still hidrogen? This might be to knaive, Sorry if it doesn't make sense. $\endgroup$
    – Mike
    Aug 3, 2020 at 12:28
  • $\begingroup$ @ Mike. The sodium atom Na is like a mother who must go to work without her baby. She gives him for a while to a nurse, thinking that later on she will pick him back. This is like an ionic bond. Na gives an electron to a foreign atom acting like the nurse. The baby is the image of the electron. A covalent bond is different. It is like a child sleeping in a bed between mother and father, or walking between his parents who are holding him by the hand. The child belongs to the two parents simultaneously. Not the ionic bond. $\endgroup$
    – Maurice
    Aug 3, 2020 at 13:31

1 Answer 1

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You may map the electron density around an atom will find regions where this probability is relatively high. Similar to a hiking map where lines mark the same elevation in a terrain, contour maps offer a representation in 2D, where the lines mark regions of same electron density. You find these used in quantum mechanics (like the ones below), or in crystallography (example).

These contour maps may be drawn for atoms in vicinity of each other, as shown below. The comparison of multiple electron density maps may reveal you the differences between a non-polar molecular bond, a polar molecular, and the one in the ionic state.

enter image description here

(reference)

For nitrogen ($\ce{N#N}$) shown on the left, the map depicts a symmetrical electron density distribution which is symmetrical along the line you may imagine between the two nitrogen atoms.

Again, similar to a hiking map, these maps are a projection of a terrain as seen from a plane; seen perpendicular to this direction yields a profile like the following for the hydrogen atom (in this figure, read «charge density» as synonym for «electron density»):

enter image description here

(reference)

For a polarized bonding, like in the $\ce{H-Cl}$ molecule, the minimum electron density between the two binding atoms no longer is at half distance between the two atoms. Contrasting to a polarized bond, in an ionic state, like in NaCl, the electron density between the two atoms not only passes a minimum, but may become practically negligible:

enter image description here

(reference, slide #48)

The discern between a «polarized bond» in molecules and the «ionic state» is somewhat arbitrary.

Addition:

@Mike suggested to compare the situations above with the one in a typical metal like sodium, or copper. In the solid state, the spatial arrangement of metal atoms is regular; for example in a bcc lattice (sodium), or a fcc lattice (copper). Similar as in the case of NaCl, the metal atoms again donate the outmost electrons, formally yielding a charged cation. Contrasting to NaCl, however, these donated electrons are shared among all sodium / copper atoms; colloquially, this distribution is called electron gas because these electrons may move freely and contribute for electric conductance.

If you compare the electron density map of solid copper with the one of solid NaCl, passing from one atom to the next one, only the later contains extended volumes with practical zero electron density (marked by the blue arrow) insulating the ions from each other:

enter image description here

(figure slightly modified from this reference)

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  • $\begingroup$ Thanks @Buttonwood you really understood what I could hardly explain. I'm not sure about making another question (nonsense?): When I think about H2 (g-deuterium) I think of two atoms of H together and then space, but when I think about Na2 in solid I imagine that the two atoms are together and next to each other (Na2 in a solid form or another element like H2 but solid). Does Na2 form a crystal or solid structure like ice water does? Do they "chain" like Polyethylene? If it makes sense to ask that, I will create a question. Thanks again $\endgroup$
    – Mike
    Aug 6, 2020 at 18:24
  • $\begingroup$ Thanks @Buttonwood it is really helping. I'm reading Chemistry for dummies and refresing my chemistry from highschool and some of the stuff in your answer is way too advance for me, but the graphs are really helping me to see it. Is there like an hydrogen bonding for NaCl molecules (or they are just solids and they are close to each other)? I also have this question: chemistry.stackexchange.com/questions/138549/…. Thanks again $\endgroup$
    – Mike
    Aug 7, 2020 at 12:12
  • $\begingroup$ @Mike I'm not aware about NaCl molecules. For normal molecular bonds (e.g. H2), or hydrogen bonds (e.g., the dimers of carboxylic acids), there is significant electron density all along the line between the two atoms, there is direction. However, NaCl is kept together as a crystal with 1) Coulomb attraction between opposite Ions ($\ce{Na+}$ and $\ce{Cl-}$) and 2) Coulomb repulsion between Ions of same charge (either $\ce{Na+}$ to $\ce{Na+}$, or $\ce{Cl-}$ to $\ce{Cl-}$). In an analogy: like with the little kitchen magnets, you seek an (energetically speaking) optimal arrangement, but in 3D. $\endgroup$
    – Buttonwood
    Aug 7, 2020 at 19:46

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