one proton, every helium atom contains two protons, and every lithium atom contains
three protons. The number of protons in the nucleus of an atom determines its identity;
this number is known as the atomic numberof that element.NEUTRONS
The third fundamental particle, the neutron, eluded discovery until 1932. James Chad-
wick (1891–1974) correctly interpreted experiments on the bombardment of beryllium
with high-energy -particles. Later experiments showed that nearly all elements up to
potassium, element 19, produce neutrons when they are bombarded with high-energy
-particles. The neutronis an uncharged particle with a mass slightly greater than that
of the proton.Atoms consist of very small, very dense nuclei surrounded by clouds of electrons at
relatively great distances from the nuclei. All nuclei contain protons; nuclei of all
atoms except the common form of hydrogen also contain neutrons.Nuclear diameters are about 10^5 nanometers (nm); atomic diameters are about 10^1
nm. To put this difference in perspective, suppose that you wish to build a model of an
atom using a basketball (diameter about 9.5 inches) as the nucleus; on this scale, the atomic
model would be nearly 6 miles across!MASS NUMBER AND ISOTOPES
Most elements consist of atoms of different masses, called isotopes.The isotopes of a
given element contain the same number of protons (and also the same number of elec-
trons) because they are atoms of the same element. They differ in mass because they
contain different numbers of neutrons in their nuclei.5-7
5-6
Figure 5-7 A plot of some of
Moseley’s X-ray data. The atomic
number of an element is found to be
directly proportional to the square
root of the reciprocal of the
wavelength of a particular X-ray
spectral line. Wavelength (Section
5-10) is represented by .
184 CHAPTER 5: The Structure of Atoms
Atomic number,Z1020304050AlSiClKCaTiCrVMnFeNiCoCuZnYNbRuAgPd
Mo
Zr^1
This does not mean that elements
above number 19 do not have
neutrons, only that neutrons are not
generally knocked out of atoms of
higher atomic number by -particle
bombardment.