i/The planetary model ap-
plied to a nonmetal, 1, an
unmagnetized metal, 2, and a
magnetized metal, 3. Note that
these figures are all simplified in
several ways. For one thing, the
electrons of an individual atom do
not all revolve around the nucleus
in the same plane. It is also very
unusual for a metal to become so
strongly magnetized that 100%
of its atoms have their rotations
aligned as shown in this figure.by moving charges; we have not yet discussed the mathematics and
geometry of magnetic forces, but it is easy to see how random ori-
entations of the atoms in the nonmagnetic substance would lead to
cancellation of the forces.
Even if the planetary model does not immediately answer such
questions as why one element would be a metal and another a non-
metal, these ideas would be difficult or impossible to conceptualize
in the raisin cookie model.
Discussion Question
A In reality, charges of the same type repel one another and charges
of different types are attracted. Suppose the rules were the other way
around, giving repulsion between opposite charges and attraction be-
tween similar ones. What would the universe be like?8.2.3 Atomic number
As alluded to in a discussion question in the previous section,
scientists of this period had only a very approximate idea of how
many units of charge resided in the nuclei of the various chemical
elements. Although we now associate the number of units of nu-
clear charge with the element’s position on the periodic table, and
call it the atomic number, they had no idea that such a relation-
ship existed. Mendeleev’s table just seemed like an organizational
tool, not something with any necessary physical significance. And
everything Mendeleev had done seemed equally valid if you turned
the table upside-down or reversed its left and right sides, so even if
you wanted to number the elements sequentially with integers, there
was an ambiguity as to how to do it. Mendeleev’s original table was
in fact upside-down compared to the modern one.
j/A modern periodic table, labeled with atomic numbers. Mendeleev’s
original table was upside-down compared to this one.In the period immediately following the discovery of the nucleus,
physicists only had rough estimates of the charges of the various nu-Section 8.2 The nucleus 501