Inside the Atom
Objectives
Describes atomic structure and the evidence for quantization
Defines isotopic mass and the atomic mass of elements
Looks at the construction and use of mass spectrometers
Discusses the electronic configuration of atoms
Introduces the idea of an electron acting as a wave
In Chapter 2 we saw that atoms are not created or destroyed in chemical reactions,
but that they are simply reshuffled to produce new molecules. Experiments show
that the law of conservation of mass is found to apply to all chemical reactions,
even to those carried out at high temperature. Do these observations mean that
atoms are completely indestructible?
This chapter shows how atoms have been forced to reveal their inner secrets.
Atoms are not indestructible – they do indeed have an inner structure, and
although they often behave as if they were solid, they are mostly empty space!
Atomic structure
Pieces of history
In 1897, Sir J. J. Thomson (1856–1940) investigated the way that gases at low
pressure conduct electricity. When high voltages were used, streams of minute
particles were detected on a fluorescent screen. These particles possessed a negative
charge and were even lighter than hydrogen atoms. The particles were called
electrons.
Thomson realized that the electrons must have been squeezed out of the atoms of
gas by the high voltages used in his experiments. (This is one piece of evidence that
atoms are not indestructible. Another piece of evidence is provided by radioactivity,
discussed in Chapter 21.) In other words, electrons are subatomic particles. Thom-
son realized that since atoms are usually observed to be electrically neutral, there
must be positive charges in the atom too. He proposed that atoms consisted of a pos-
itively charged ‘cement’ in which rings of electrons were embedded. Thomson’s
3.1
Contents
3.1Atomic structure 293.2Isotopes 323.3Mass spectrometer 353.4Electronic structure of
atoms 383.5Evidence for the
existence of energy
levels in atoms 403.6More advanced ideas
about electronic
structure 413.6Revision questions 453
UNIT