Atomic Structure

Chm 1311 Lecture for 7 June 2000 cont'd.

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Neat pictures of atoms

Sine Qua
Non TV

If it weren't for atoms, there wouldn't be a you to watch TV. Horrors! (Think of the massive unemployment in ad agencies nationwide.)

If it weren't for your TV's tube, we wouldn't know about the composition of atoms! It's neat when the ecological balance is restored like that.

Ancient CRT That "cathode ray tube" consists of electrons, e-, "boiled" off atoms and repelled from a negative pole toward a positive grid and splattering themselves upon phosphors in the tube's face.
J.J. Thompson, without knowing those "beta" rays were electrons, used electrical and magnetic deflection 100 years ago (1897) to calculate the charge to mass ratio (1.8x1011 C/kg) of the rays, and postulated the existence of electrons from experiments on his cathode ray tube.

Oil Drop

While the e/m (charge to mass) ratio was certainly interesting, at least one of those two values would have to be obtained independently in order to discover the magnitude of both. This Robert Millikan did in 1909 with an apparatus which attracted sinking oil droplets by electrical charges. By knowing what force was necessary to counter gravity, the droplet's charge could be found. Millikan was pleased to discover that difference between any two oil drop charges was never less than 1.6x10-19 Coulombs, implying the charge on a single e- was that.


From those two, it was a cinch to find me = 9x10-31 kg, or 2000 times lighter than the smallest atom (hydrogen)! Clearly, atomic masses were not governed by the weights of their electrons.

the Atom

While the electrons coming out of atoms had a uniform negative charge, the atoms themselves were electrically neutral. So there had to be exactly enough positive charge to neutralize all of an atom's electrons.
Several models were postulated for the nature of this positive charge. It might have existed as a uniform sea in which the electrons swam. The opposite extreme would be a congealed positive kernel about which they flew instead. And you could imagine cases in between.
Atoms needed to be probed, and Rutherford used a clever probe, what was then known as the alpha known as a bare helium nucleus, He2+. While the electrons (beta particles) were negatively charged, Rutherford knew alphas were positively charged. He knew alphas were much heavier than betas; so alphas would just brush betas aside. Not so with the atom's positive parts; the lightest of them were 2000 times heavier than betas, comparable and even exceeding alphas in weight. They'd not be so easily bullied.
Surprise! So assuming the positive sea was spread out, Rutherford expected to see very modest deflection of alphas. For the most part, he did; the alpha particles just flew through the thin gold foil, unperturbed by gold's positive parts. But some alphas bounced directly back, prompting one observer to say it was as unlikely as a bullet being stopped in its tracks by tissue paper!


So the "tough nut" kernel theory of the atom's positive charge got the nod. Even more convincing, the weak deflections were perfectly described by the mathematics of charge repulsion assuming both alpha particles and the nucleus were subatomic-sized charge centers. The positively charged entities at the center of atoms were called protons.

Making up

With the protons tucked into the nucleus and each having the same magnitude but precisely opposite-signed charge as the electrons, atomic structure seemed elegantly simple. However, the weight of the nuclei wasn't simply the weight of the constituent protons. Whatever made up that difference couldn't be charged, or atoms would lose their electrical neutrality.
That was bad news since the progress described above was simplified by using charged species (or electric and magnetic fields) to deflect other charged species, and calculating their charges and masses from the deflections. These electrical neutral creatures, neutrons, were going to be more elusive.


While Rutherford had found the proton by 1909, it wasn't until 1932 that Chadwick "found" the using non-observations. By then, atomic particle collision experiments were commonplace. And the charged particles announced themselves by bending in response to electrical or magnetic fields or leaving trails of ionization (charge extraction) in their wake. What Chadwick saw was a nuclear reaction at point A which triggered another at point B with no charged particles intervening between the points! The connection had to have been by neutral particles, the elusive neutrons!
The actual experiment was

a + Be  arrow right C + n  arrow right C + p+ + e- + v

But until the neutron, n, decayed (11 minute half-life) into the p+, it's track was invisible. (v is the neutrino particle which was thought not to have any mass . . . until 1998.)


So we got e- on the outside (where Chemistry really takes place), and p+ (in equal numbers for electrical neutrality) and n (neutrons) tucked away in a really small (1/10000 of an atomic-sized) package at the center. As we look how larger atoms build themselves (except for hydrogen, built at the Big Bang, dying stars build the rest of the Periodic Table), we find some atoms differing from one another ONLY in the number of their neutrons! No charge differences involved; no chemical differences involved; just mass. These atoms are called isotopes of one another.

Some atoms, like Be (beryllium), are lucky. 49Be has 4 electrons, 4 protons, and 5 neutrons. And no Be atom has anything different. In other words, there is only one isotope of Be. But the smallest atom, H (hydrogen), comes in three flavors; all have one e- but differ as follows: 11H (p+), 12H (p+ + n), and 13H (p+ + 2n).


You've probably caught on by now that 49Be means that Be has 4 p+ and 9-4=5 neutrons. In fact ZAE means element E has Z protons (called the atomic number) and (A-Z) neutrons (where A is the mass number . . . no, not the mass, the mass number, which is really just a count of how many nucleons [protons and neutrons] reside in its nucleus).
F. A. Aston's
Original Paper
on Mass
While the weights of protons (1.6726x10-27 kg) and neutrons (1.6750x10-27 kg) are known, these don't help in determining the weights of the atoms! Even if you add in the weights of the electrons (each 9.1x10-31 kg), the total atomic weights will still be different from what's measured experimentally (in mass spectrometers). The reason is not simple, but it's easy. (Figure that paradox out!)


The reason is that when protons and neutrons are together in such intimate proximity (remember nuclei are far smaller than atoms), the energies of their association are ENORMOUS. You can easily believe that when you think of the huge repulsions expected from essentially adjacent positive charges; those energies quadruple with every halving of their when separation is near zero, the repulsion energies must be astronomical. Why don't nuclei just blow apart?

The neutrons mediate and stabilize those repulsions utilizing non-electrical forces. What has this to do with mass? Everything! If you're Einstein. Remember he told us that E=mc2 meaning that nuclear energies come at a price of changes in mass. So dropping nucleons into that sort of black hole of the nucleus changes their mass as the energies rise. That means, we can't simply add up nucleon masses. Instead, we have to measure them.

Mass Unit

We need a standard weight. We send 612C through a mass spectrometer, call its measured "weight" 12 amu (atomic mass units), and compare all other atomic weights to it. Using that standard, 1 amu becomes 1.66056520x10-27 kg, and all of the other atoms and all of their isotopes can be recorded by comparison in tables. That table contains not only the amu weights of all the isotopes but also their "natural abundance" on Earth.

So 3579Br has mass 78.918336 amu and is found as 50.686% of all bromine atoms. While 3581Br has mass 80.916289 amu and natural abundance of 49.314%. We expect, therefore, that the average atomic mass of (a large number of) bromine atoms will be

(0.50686)x(78.918336) + (0.49314)x(80.916289) = 79.904 amu

and so it is! All atomic weights are determined thus.


Molecular weights are LOTS simpler than atomic weights! The amu masses of the atoms DO add up to the weight of the molecules of which they are a part. There's no "mass defect" as one sees in trying to predict atomic masses from the sum of their proton and neutron weights. That tells you something interesting: chemical energies do not begin to approach nuclear ones (otherwise the Einstein mass defect would show up). So however energetic our chemical reactions become, we never have to worry about loss or gain of atoms or their masses! In contrast to the "careless" nuclear physicists, whose nuclei are losing weight all the time in nuclear reactions, we chemists are absolutely confident that no matter what chemical reaction we study, the same number and kind of atoms come out as went in. They'll rearrange themselves, perhaps, into different molecules (that's what makes Chemistry interesting), but they'll never disappear or weigh any more or less. Comforting, no?

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Last modified 6 June 2000. Chris Parr