More Than One Version Of Each Element

The Atomic Mass Mystery


Isotopes exist as the same element but with different mass numbers. All nuclei of any element must contain the same number of protons. They have different masses, because each isotope has a unique number of neutrons.


Examine the entries for each element on the periodic chart and you’ll see the atomic mass of the elements include some fraction smaller than one. If neutrons and protons have masses of approximately one, alone it makes no sense to find fractions of one included in the mass. 



Figure 1 shows an entry for carbon. Instead of finding 12.00 amu as you expect, the entry has 12.01. This results from the fact the atomic mass is composed of carbon atoms which have a mass of 12.00 amu and 13.00 amu . The atomic mass results from the weighted average  of the carbon with a mass number of 12.00 and 99% abundant and carbon with a mass number of 13.00. 


⦿ definition

Isotopes are atoms of the same element which have different mass numbers




carbon on periodic table
Figure 1: Essential information for each element
  • Atomic Mass

Weighted average mass of all the isotopes of a given element


  • Mass Number

Mass of single kind of isotope 

One Size Fits All


Isotopes exist as atoms of the same element. Each isotope has a different total mass. Even though all isotopes of a single element must have the same number of protons, they differ from one another by the number of neutrons.


Consider only the nucleus when you are concerned with isotopes. Electrons play no part in isotopes or the atomic mass, Figure 2. Only protons and neutrons contribute to the mass of an isotope.




Focus on nucleus of atom
Figure 2: Focus on nucleus contains protons and neutron

Isotopes Are Useful


You find isotopes in many roles when they are put to use:


  • BNCT


Boron neutron capture therapy is used to treat tumors which might otherwise be difficult to operate on. It relies on 10B capturing neutrons. The new unstable 11B undergoes α-decay. Then it produces a 7Li ion. The α-particle attacks the cancer cell form within the cell.



  • Radiometric Dating


The most important radiometric dating is carbon dating. It relies on the fact 14C decays to 14N, which has a half-life of about 5,200 years. Carbon dating becomes ineffective earlier than 50,000 years in the past.


  • Nuclear Energy


The most common isotopes used to produce nuclear energy are 235U and Pu239. Both must be extracted from the population of other isotopes of the same element before a sample becomes useful for nuclear reactions.



  • Medical Imaging


99Te is the most common radio nuclide used for radioactive tracers. It finds use to examine brain function without resort to invasive methods.


This list is not exhaustive, but just the tip of the iceberg. It is intended to illustrate a variety of contributions made by isotopes and the need to purify isotopes.


Each Isotope Has A Name


In order to specify a specific isotope among a set of isotopes present for a specific element, you use isotope or nuclide notation. This provides a method to tell the difference between carbon with a mass of 12.00 and carbon with a mass of 13.00.

To identify each isotope among other isotopes of the same element, Figure 3 shows isotope notation.


isotopic symbol notation
Figure 3: Isotope symbol used to denote each unique isotope.


X denotes any possible element. There is no X on the periodic table.


Z means “charge” or the atomic number. As you know, the atomic number gives you the number of protons. 


The superscript A stands for mass number; the sum of the protons and neutrons.

There are two major isotopes of chlorine, 3517Cl and 3717Cl. Notice the mass number is always a whole number. From the isotopic symbol you see for instance 35Cl has 17 protons and 18 neutrons. You find the number of neutrons when you subtract the atomic number from the mass number.


Notice also, you can specify 35Cl without the atomic number, because if you write Cl the only possible atomic number is 17. You should get used to the idea the atomic number and atomic symbol are the same thing. One automatically gives you the other.

Matter: Composition

Matter by Composition


You can describe matter based on composition. This means you describe collections of matter based on what makes it up. The first kind of description refers to whether or not you want to describe a pure substance or mixture.





Cultivate the skill of seeing the microscopic scale of particles when you see or hear about what you observe on the scale you see in ordinary life.




large scale, molecular scale
water as we see it everyday and water the way it would look with a molecular magnifying glass



Pictured above is how we see water in a glass and how we might see water if we pretend to have a magnifying glass strong enough to see individual water molecules.



Pure Substance or Mixture


The components of a pure substance can not be separated based on  their different physical properties.

Mixtures on the other hand, contain more than one pure substance. Each substance within a mixture can be identified by separate physical properties.





pure substances and mixtures
Figure 1: Pure substances and mixtures



In Figure 1 the left column shows pure substances. In the right column you see pure substances when added together and no chemical change happens, they form a mixture.


The first mixture is an example of two compounds: water and carbon dioxide. When they add together, both still remain the same pure substance with the same physical properties. Carbon dioxide mixes with water in order to make carbonated water. You can separate the carbon dioxide from the water and find they remain unchanged.


The second example shows two elements mixed to together to form a mixture. In this case, when copper is mixed with tin in a three to one mixture, we obtain the alloy bronze.


Pure Substance


The key feature of  a pure substance is that it possesses a fixed proportion of elements. This proportion is the same no matter what the size of the sample. The fixed proportion remains identical whether you find it in your back yard or on the dark side of the moon. It is a pure substance because it has the same proportion of elements as every other sample of the same pure substance.



Types of Pure Substances




Elements are made of one and only one kind of atom. If there is only one kind of atom, then it has a one hundred percent proportion of only one element. An element cannot be broken down further by any chemical means. This also includes elements which are made of more than one atom like: diatomic elements, molecular elements, network solids, and metals.



kinds of elements
Figure 2: Elements occur in a variety of forms

Figure 2 shows examples of each way you might encounter an element in its wild natural habitat.



To qualify as a compound that is a pure substance, the compound contains more than one element. A compound also has a fixed proportion of elements. The compound is a specific compound because of its specific proportion of elements, (formula).


The fixed proportion of elements (sometimes called fixed composition) makes H2O water, and H2O2 hydrogen peroxide. They make separate pure compounds because they have a different proportion of elements.


Each compound has a unique set of physical properties: boiling point, melting point, density, and refractive index. When you test and record the physical properties of a sample, you characterize a compound.


Because each pure compound has unique physical properties, you use this to separate each pure compound when more than one pure substance resides in a mixture.


Compounds Ionic or Molecular


You can divide compounds into two types: ionic or molecular. The distinction between these is important. Ionic and molecular compounds differ in their physical properties because they are either molecular or ionic.




compound ionic or molecular

On the all important microscopic level, you find ionic compounds are stacks of ions attracted to each other by plus being attracted to minus charges. This attraction goes in all directions and repeats without stop.


In contrast, a molecular compound like carbon dioxide holds together in the bulk phase by attraction between the molecules. The bonds between the atoms: carbon to oxygen hold only the pieces of each individual molecule together.




molecular versus ionic compounds
Figure 4: The atomic level difference between molecular and ionic substances



Figure 4 shows the contrast between sodium chloride, NaCl, and carbon dioxide, CO2. The ions pack together in a hungry mass of opposite charges pressed together. Carbon dioxide has independent molecules loosely held together with weak forces between the molecules.


How loose? 


Sodium chloride has a boiling point of 1465°C, compared to carbon dioxide which has a boiling point (sublimation point) of -78ºC.



A mixture of more than one substance forms a mixture because it has a variable composition. This means instead of one fixed proportion of elements, you can have a sample of air which has twenty percent oxygen or fifteen percent oxygen. It still remains a mixture of air.

A salt solution of water provides another common example where you might have a ten percent salt solution or a one percent salt solution. In both case you are referring to a salt solution though the examples do not have identical compositions.


Heterogeneous or Homogeneous Mixture


Mixtures themselves can have two possible compositions: homogeneous and heterogeneous. Homogeneous mixtures mean all the components of the mixture are evenly distributed throughout the body of the material. Heterogeneous mixtures where the components are thrown together without the requirement the mixture has uniform components.



Two kinds of mixtures



The distinction between these two states of mixture plays into how a mixture can be separated into two or more pure substances.


To visualize the difference between types of mixtures on the molecular scale, Figure 5 shows how two typical mixtures might appear.



Two kinds of mixtures
Figure 5: Homogeneous versus heterogeneous mixture


The part of Figure 5 on the left shows an aqueous solution of sodium chloride. The chloride anions are green and the sodium cations ions are yellow. The important point to notice is that these ions are uniformly distributed through the entire solution. Every part of the mixture has an identical percentage of sodium chloride in the water.



The right side of Figure 5 shows a mixture of water and octane, (a liquid hydrocarbon). The lower layer has a clear separation between itself and the upper level. The mixture of octane and water do not have a uniform distribution.



When a mixture has a uniform distribution of every component substance, the homogeneous mixture is often a solution

Air makes a great example where all the gases in air are equally present throughout a room. Very few people choose their seat in a class room because they are afraid part of the room has less air than any other part of the room.



You label a mixture as heterogeneous if the components jumble together and are not uniformly distributed. This means the material consists of zones and regions. This includes materials like wood, concrete, and composite materials.


The Big Deal 


When you follow a chain of questions about any substance, you will find it easy to correctly classify any lump of matter into its correct category. 


Figure 6 gives a flowchart where you serially ask questions and find the correct composition of matter.




decision flowchart to classify compound
Figure 6: How to decide what kind of compound


If the materiel does not have a uniform distribution, then you have a heterogeneous mixture.


If you can separate the components of a sample by physical means: distillation, filtration, chromatography; then the sample is an homogeneous mixture.


If the sample can be decomposed to other pure substances, the your substance is a compound.


If all these questions result in “no” your final remaining option is that your sample is an element