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For instance, if an atom loses an electron and becomes a cation, such as sodium, how does that make the sodium atom positively charged? The sodium's atomic radii decreases due to less electron-electron repulsions which in effect makes the sodium atom more electron dense.

The common response I get from TA's is that positive and negative charges cancel each other out so essentially one proton isn't getting canceled out which results in a net positive charge. However, protons and electrons don't cancel each other out in an atom (effective nuclear charge), so I know that response is incorrect since the electrons closest to nucleus of an atom feel a greater positive charge than the valance electrons.

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  • $\begingroup$ Comparatively, the atomic radius of Sodium in its neutral form is greater than the atomic radius of Sodium in its cationic form so there's a higher probability of Sodiums electrons being closer to the nucleus relative to before which is what I meant by electron density. $\endgroup$ – Shane2020 May 3 '18 at 6:18
  • $\begingroup$ Yes ions are charged, and you cannot hide the fact that if there are more protons than electrons the overall change will be positive and vice versa, no matter about screening or any other fancy thing you can invent. $\endgroup$ – porphyrin May 3 '18 at 7:19
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Yes, ions are actually charged particles and neutral atoms are not. The TA's version is correct: the positive and negative charges cancel each other out. In a positive ion, we have one extra proton that gives the whole thing a net positive charge. In a neutral atom, electrons and protons cancel each other out exactly. How can they not to? After all, their electric charges are precisely identical, just with opposite signs.

Now when we speak about cancelling out like that, we only mean the net charge. Take a mole of neutral atoms and a mole of positive ions (just theoretically, of course, otherwise the consequences might be catastrophic), and even a crude electroscope will tell you the difference between the two right away.

If you specifically intend to ask about one ion, let's look at it from a different perspective. A neutral atom or molecule is, well, neutral, but that doesn't mean it would not participate in electrostatic interactions: there are those small positive and negative charges inside it. They seem to be moving, or at least not being all in one place, and you can feel tiny electric fields resulting from that. The corresponding electric potential decays with distance as $\frac1{r^2}$ for a dipole, and even faster than that for a quadrupole or other configurations of charges. The same applies to an ion, but it also has a net charge, and its potential behaves like $\frac1r$. When looking from a far enough distance, you will only feel the pull of the net charge. Look at a neutral atom from the same distance, and you will feel nothing.

Sure enough, if you look very closely, or even inside an atom (or an ion), you will see a lot of things going on around you, and the net charge will not be all that important.

So it goes.

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The atom would not become more electron dense in your case. Electrons repel each other and that effect + Z number is the main explanation for size in (small, non relativity affected) atoms. The attraction between core and every electron stays the same, it is just the repulsive forces between the screen of electrons and a single electron that is less for each electron lost.

Your TA is right. The net charge of the atom is now positive. Only for very large atoms with a correspondingly large electron "screen" will the screen effectively shield the surroundings from the charge at the core.

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Yes, of course an ion is a charged species. When you say that the charges on electrons and protons can not cancel out each other then, I guess, you are thinking of them far apart from each other. But within an atom, the two charged things (electrons and protons) are sufficiently close to each other to cancel their charges. Even if you go to molecules, it may have different charges on different atoms in it but the overall charge on the molecule will be equal to the sum of charges on each atom. You can think of for example, CO2. In CO2, the C-atom is positively charged and O-atoms are negatively charged but CO2 molecule is neutral. So, charges really cancel out each other and hence after losing an electron an atom becomes a positively charged ion.

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