Thompson developed an atomic model, the raisin pudding model, which described the
atom as being a diffuse positively charged sphere with electrons scattered throughout.
Ernest Rutherford, in 1910, was investigating atomic structure by shooting positively
charged alpha particles at a thin gold foil. Most of the particles passed through with no
deflection, a few were slightly deflected, and every once in a while an alpha particle was
deflected back towards the alpha source. Rutherford concluded from this scattering exper-
iment that the atom was mostly empty space where the electrons were, and that there was
a dense core of positive charge at the center of the atom that contained most of the atom’s
mass. He called that dense core the nucleus.
Subatomic Particles
Our modern theory of the atom describes it as an electrically neutral sphere with a tiny
nucleus at the center, which holds the positively charged protons and the neutral neutrons.
The negatively charged electrons move around the nucleus in complex paths, all of which
comprise the electron cloud. Table 5.1 summarizes the properties of the three fundamental
subatomic particles:
Table 5.1 The Three Fundamental Subatomic Particles
NAME SYMBOL CHARGE MASS (AMU) MASS (G) LOCATION
proton p+ 1 + 1.007 1.673× 10 −^24 nucleus
neutron n^00 1.009 1.675× 10 −^24 nucleus
electron e− 1 − 5.486 × 10 −^4 9.109 ×l0−^28 outside nucleus
Many teachers and books omit the charges on the symbols for the proton and neutron.
The amu (atomic mass unit)is commonly used for the mass of subatomic particles
and atoms. An amu is^1 ⁄ 12 the mass of a carbon-12 atom, which contains 6 protons and
6 neutrons (C-12).
Since the atom itself is neutral, the number of electrons must equal the number of protons.
However, the number of neutrons in an atom may vary. Atoms of the same element (same
number of protons) that have differing numbers of neutrons are called isotopes. A specific
isotope of an element can be represented by the following symbolization:
X represents the element symbol taken from the periodic table. Z is the atomic numberof
the element, the number of protons in the nucleus. A is the mass number, the sum of the
protons and neutrons. By subtracting the atomic number (p) from the mass number
(p +n), the number of neutrons may be determined. For example, (U-238) contains
92 protons, 92 electrons, and (238 −92) 146 neutrons.
Electron Shells, Subshells, and Orbitals
According to the latest atomic model, the electrons in an atom are located in various energy
levels or shells that are located at different distances from the nucleus. The lower the number
of the shell, the closer to the nucleus the electrons are found. Within the shells, the electrons
are grouped in subshellsof slightly different energies. The number associated with the
shell is equal to the number of subshells found at that energy level. For example, energy
92(^238) U
Z
AX
Basics 47