What's in the nucleus of an atom?  Recall from Unit D, Module 1: Lesson 3 that the nucleus is very small compared to the entire volume of the atom.  This fact was confirmed by Rutherford in his alpha particle scattering experiments.  The nucleus is only about 10−14 m across, while the entire atom may be as much as 10 000 times wider.  Even though it is very small, the nucleus makes up almost the entire mass of the atom.  The large particles found inside the nucleus are called nucleons.

There are two types of nucleons: protons and neutrons.  Protons carry a charge of +1; neutrons have no net charge.  In a neutral atom, the number of protons is always balanced by the number of electrons.  An ion is created when there is an unequal number of protons and electrons, producing a net positive or negative charge.


Alpha particle: two protons and two neutrons bound together to form a stable particle identical to a helium nucleus


Nucleon: a proton or neutron


Proton: a positively charged particle found in all nuclei

 


Neutron: a neutral particle found in the nucleus

The atomic number uniquely identifies the element.

The periodic table provides important reference information on each element.  The periodic table is ordered by atomic number, the number of protons in the nucleus.  An element is uniquely determined by the number of protons it has - an atom with 92 protons is uranium, regardless of the number of neutrons or electrons present.  If protons are added or removed, the element is no longer uranium.

The number of neutrons in a nucleus is not given on the periodic table, although the atomic mass can be used to calculate the number of neutrons in the most common isotopes .  The number of electrons in an atom is equal to the number of protons and is important for chemists.  The number of electrons is important for beta-positive decay.

In previous science courses you may have used the periodic table to calculate how many of each type of nucleon was in the nucleus.  For example, the atomic mass of sodium, which is 22.99, was rounded to 23 - the total number of nucleons.  Because sodium has 11 protons (the atomic number), it must have 12 neutrons to add up to an atomic mass of 23.

The atomic mass number is the number of nucleons in the nucleus. If the atomic mass number of sodium is 23, why is the atomic mass on the periodic table 22.99?


Atomic number (Z ): the number of protons in the nucleus


Atomic mass: the weighted mean atomic mass number of the element's natural isotopes (This number is given on the periodic table.)


Isotope: an atom that has the same number of protons but a different number of neutrons and, therefore, a different atomic mass number

 


Atomic mass number (A): the number of nucleons in an atom's nucleus

Isotopes

Different atoms of the same element may have different numbers of neutrons and therefore different atomic masses.  Uranium nuclei, for example, have various masses due to variations in the number of neutrons.  The various masses are called isotopes.  Hydrogen exists in three isotopes, with the nuclei having zero, one, or two neutrons.  There are many isotopes of uranium.  The atomic mass value listed on the periodic table is the mean atomic mass of the element that is abundant in nature, which is the value used by chemists who deal with large numbers of atoms at a time.  Physicists tend to deal with individual atoms so the isotope's atomic mass is indicated as a number after the element's name.  For example, review the following table.

Isotope

Name

Atomic Mass

Number of Protons

Number of Neutrons

H-1

Hydrogen 1

1

1

0

H-2

Hydrogen 2 (deuterium)

2

1

1

H-3

Hydrogen 3 (tritium)
unstable

3

1

2

 













All isotopes of one element have the same chemical properties because they have the same number of protons.  The nuclear stabilities, however, may differ dramatically.  Lead, for example, has 35 isotopes, only four of which are stable.

 

Symbolic Notation

Because of the various isotopes of an element such as hydrogen, the chemical symbol H does not provide sufficient information about the nucleus.

The Nuclide Symbol

A = Z + N

Quantity

Symbol

SI Unit

atomic mass number - the number of nucleons

A

--

atomic number

Z

--

neutron number

N

--

chemical symbol

X

--

 

 




















Using this notation, the three isotopes of hydrogen H-1, H-2 and H-3 are expressed as , , and .

As demonstrated with hydrogen, isotopes can be written with the element name followed by the mass number as well as with nuclide symbols.  For example, uranium-238 is the isotope .

Self-Check

  Answer the following self-check question then click the "Check your work" bar to assess your response.

SC 1. How many neutrons are in lead-204?

 

Contact your teacher if your answers vary significantly from the answers provided here.

SC 1. According to the periodic table, the atomic number of lead is 82.

Given

A = 204

Z = 82

Required

neutron number ( N )

Analysis and Solution

N = A - Z

    = 204 - 82

    = 122

Paraphrase

There are 122 neutrons in a nucleus of lead-204 or .


Try This
Complete "Practice Problems" 1 and 2 on page 791 of your physics textbook.

The Atomic Mass Unit

The average mass of a hydrogen atom is 1.01 and it is reported in atomic mass units ( u ), which are defined as exactly of the mass of the carbon-12 atom.

1 u = 1.660539 × 10-27 kg

The value for the atomic mass unit was determined using a mass spectrograph, which is very similar to J.J. Thomson's charge-to-mass ratio experiment you studied in the last module.