A system of physical chemistry - Index of

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A system of physical chemistry - Index of

RUTHERFORD-BOHR ATOM-MODEL 109

negatively charged hydrogen atom, that is, one which has picked up an

extra electron. In this case n — 2 and N = i, for Ne = the nuclear

charge = ^ in the case of the hydrogen nucleus. On substituting n = 2

and the corresponding value of Sn from the above table in the expressions

(i), (ii), and (iii), we find that this system—which can be represented

by the symbol 1, (2), where (2) refers to the total number of

electrons in the atmosphere and 1 refers to N, the number of unit

charges on the positive nucleus— will give rise to a permanent state

conditioned by the following values of a, w, and W :—

a = 1-33 flo; w = 0-563 W-; W = 1-13 Wo.

Since W is here greater than it is when the atom is neutral, it follows

that the hydrogen atom is actually capable of taking on a negative

charge, for in doing so, the term W is increased from W^ to i-i3 Wo,

and one of the conditions for stability is, as we have seen, a maximum

value for W. The fact that the hydrogen atom can take on a negative

charge probably accounts for the fact that in the periodic table we

usually place hydrogen with the halogens partly because hydrogen can

replace chlorine in organic compounds.

Bohr has also calculated the value of W which would result if the

hydrogen atom acquired two extra electrons, thus becoming (1, (3)).

It is found that W now becomes equal to 0*54 Wq. This value of

W is now less than Wo, its value for the neutral atom, and consequently

still less than for the atom carrying one extra electron. Hence such a

doubly charged hydrogen atom is unstable and will not exist. Experiments

on positive rays show that the hydrogen atom can acquire a single

negative charge, but it is doubtful if it can acquire a double negative

charge.

Helium.

In this case N = 2, and the neutral atom has therefore two electrons.

We have already considered Bohr's theory in relation to the

spectrum of helium. We have now to take up the process whereby first

one, then a second, and finally a third electron can be added to the

'doubly charged heUum nucleus. First, for the formation of the system,

(2, (i)), by bringing an electron from infinity up to the nucleus, since

n = \ and therefore Sn = o, and further since N = 2, the permanent

state is given by— a = o'S cio;

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