Conceptual Physics

provided only if the atom’s positive charge (and mass) were highly concentrated within

the atom, in a region called the nucleus.

After analyzing the data and seeing how few of the alpha particles actually scattered,

Rutherford concluded that the nuclear radius must be about 10,000 times smaller than

the atomic radius, a figure that is still accepted today. Rutherford’s groundbreaking experiment proved that the atom is mostly empty space. His

atomic model is shown in Concept 3. Note that the diagram is not even close to being drawn to scale; the atomic diameter is far too small

compared to the nuclear size. If the diagram were drawn so that the nucleus were 1 cm wide, roughly the width of your pinky, the atom would

have to be drawn about 100 meters wide, about 10% longer than the length of an American football field.

Nucleus is small compared to orbits of

electrons

38.3 - Components of the nucleus

Rutherford proved that atoms consist of a compact, very dense positively-charged

nucleus surrounded by negatively-charged electrons. In this section, we take a deeper

look at the parts of the nucleus, and introduce some common terminology and notation

that scientists use when talking about elements and nuclei.

The positive charge of the nucleus comes from particles called protons, which are about

1800 times more massive than electrons. The simplest nucleus consists of a single

proton, and the simplest atom is hydrogen, which consists of a proton and an orbiting

electron.

What distinguishes an atom of hydrogen from an atom of gold? A chemical element is

defined by the number of protons in its nucleus. For an atom of a particular element, the

nucleus consists of Z protons, where Z is called the atomic number of the element.

Hydrogen has a single proton, so for hydrogen, Z = 1. Gold has 79 protons, so its

atomic number is 79. Protons and electrons have equal but opposite charges. This

means that an electrically neutral atom has the same number of electrons as protons,

so for example a gold atom has 79 protons and 79 electrons.

There can be more to a nucleus than just protons; there may also be other particles

present, called neutrons. These are uncharged particles that are just a bit more massive

as the proton (about 0.1% more massive). Protons and neutrons are known as

nucleons because they make up the nucleus.

The neutron number,N, states the number of neutrons. Any atom of an element always

has the same number of protons, but it can have different numbers of neutrons. Two

forms of an element with different numbers of neutrons are known as isotopes.

Hydrogen, for instance, always has a single proton, but it can have either zero neutrons

(“common hydrogen”), one neutron (an isotope called deuterium), or two neutrons

(called tritium). Deuterium and tritium are shown in Concept 2.

Different isotopes of an element will have different atomic masses because of the

differing numbers of neutrons. The table in Concept 3 summarizes the charge and mass

properties for protons, neutrons, and electrons.

Atomic masses can be measured using instruments such as the mass spectrometer.

Masses are commonly given in terms of atomic mass units, u, defined such that the

mass of the most abundant kind of carbon atom, carbon-12, has a mass of exactly 12 u.

The value of an atomic mass unit in kilograms is given in Equation 1.

The sum of Z, the number of protons, and N, the number of neutrons, is called A, the

mass number of the atom. Since electrons have little mass compared to protons and

neutrons, the mass number is very close to the entire mass of the atom when it is

expressed in atomic mass units.

An atom with a particular combination of Z and N is called a nuclide. A particular

nuclide always has the same type of nucleus. A widely adopted notation to identify a

nuclide is to write the chemical symbol of the element, with its atomic number

subscripted to the left and its mass number superscripted to the left. This is shown in

Equation 2, as applied to the most common carbon nuclide which has six protons and

six neutrons.

Parts of the nucleus

Protons

Neutrons

·Z= number of protons

·N = number of neutrons

Isotopes

Same number of protons, different

number of neutrons

Neutrons and protons have similar mass

Protons far more massive than electrons

(^700) Copyright 2007 Kinetic Books Co. Chapter 38