!NEW! Download Periodic Table With Atomic Mass

The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D). The atomic mass is a weighted average of all of the isotopes of that element, in which the mass of each isotope is multiplied by the abundance of that particular isotope. (Atomic mass is also referred to as atomic weight, but the term "mass" is more accurate.)

For instance, it can be determined experimentally that neon consists of three isotopes: neon-20 (with 10 protons and 10 neutrons in its nucleus) with a mass of 19.992 amu and an abundance of 90.48%, neon-21 (with 10 protons and 11 neutrons) with a mass of 20.994 amu and an abundance of 0.27%, and neon-22 (with 10 protons and 12 neutrons) with a mass of 21.991 amu and an abundance of 9.25%. The average atomic mass of neon is thus:

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The original periodic table of the elements published by Dimitri Mendeleev in 1869 arranged the elements that were known at the time in order of increasing atomic weight, since this was prior to the discovery of the nucleus and the interior structure of the atom. The modern periodic table is arranged in order of increasingatomic number instead.

In the modern periodic table, the elements are listed in order of increasing atomic number. The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present). The number of protons determines how many electrons surround the nucleus, and it is the arrangement of these electrons that determines most of the chemical behavior of an element.

In a periodic table arranged in order of increasing atomic number, elements having similar chemical properties naturally line up in the same column (group). For instance, all of the elements in Group 1A are relatively soft metals, react violently with water, and form 1+ charges; all of the elements in Group 8A are unreactive, monatomic gases at room temperature, etc. In other words, there is a periodic repetition of the properties of the chemical elements with increasing mass.

The periodic table can often be presented with an abundance of data about each and every element listed. In it's simplest form (shown below), each entry only has three pieces of information that you will need to know. These three pieces of data are the elemental symbol, the atomic number (typically given the symbol, Z, and the atomic weight. *Note: If you click on the table, you'll launch it into its own window/page on your browser.

Hey you! LOOK again at any periodic table - including the one above. Notice how the atomic weights have no units after them. Maybe you're thinking... "Well, I know the weights are in grams because that is how I learned it in high school". Oof. Sure, you're not wrong. BUT it would be much much better for you to realize that those could be ANY unit of weight/mass you choose and the whole table would still be correct. Relative masses means that they are all corrected relative to each other. You could think in pounds, or kilograms, or ounces, or even tons, or heaven forbid... short tons, long tons, drams, grains, or stones. Not to mention the myriad of masses represented by all the metric prefixes to prepend to "gram". You can work chemistry mass problems in any mass you want and it will still work because the masses are relative to each other. All chemical ratios work just as well with masses as they do with our oh so familiar moles.

So why DO we seem to concentrate on the "gram" as our go to guy on the periodic table for atomic weights and ultimately for molar masses and molecular weights? Well the key here is the way we historically defined the mole. Because of that old definition, we were able to say that all those atomic weights are in grams per mole of substance or abbreviated g/mol. This helps tremendously when having to convert from moles to mass as we often do in chemistry. Counting by number is the molar amount, while measuring by mass is the... well, mass amount (duh). Those atomic weights are the number of grams you will need of that element in order to have exactly 1 mole of that element. It's a nice system.

You could easily shorten that path. How? Well IF the problem is stated in say pounds, and then wants the answer in pounds... there is really no reason to convert to grams first and then back out to pounds later. Just work the problem in pounds - it will work. You'll only have to go to grams IF the number of moles is asked for. Knowing how numbers work and how ratios work is KEY to understanding and working chemistry stoichiometry problems. The periodic table is your ultimate conversion chart for converting any substance into another substance and doing so with exact proper amounts (masses and moles).

A row on the periodic table is called a period. There are seven periods on the periodic table. You might look and think "wait, I counted nine", and that would be technically wrong because those bottom two rows with elements 58-71, and 90-103 are actually from rows (periods!) 6 and 7 from the main table above them. Later, you will find out that those row numbers will match perfectly with the principle quantum number \(n\) from atomic theory. The periodic table has all sorts of cool information just based on its layout.

A column on the periodic table is known as a group or family. The groups are actually numbered up at the top of the table. They go from 1 to 18 which is the more internationally known numbering system and the official one according to IUPAC. But there is often another set of numbers which are split into the "A" and "B" groups. This is the older American system of numbering and there are 1-8 for each group of A and B. The "A" elements are also known as the representative elements (1A-8A) and correspond to groups 1, 2, 13-18 on the IUPAC numbering. Those 10 groups in the middle of the table starting with scandium are the B-groups (IUPAC 3-12) and are known as the d-transition metals.

Images Murray Robertson 1999-2011

Text The Royal Society of Chemistry 1999-2011

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Many scientists worked on the problem of organizing the elements, but Dmitri Mendeleev published his first version of the periodic table in 1869, and is most often credited as its inventor. Since then, the periodic table has evolved to reflect over 150 years of scientific development and understanding in chemistry and physics. Today, with 118 known elements, it is widely regarded as one of the most significant achievements in science.

The periodic table, also known as the periodic table of the elements, arranges the chemical elements into rows ("periods") and columns ("groups"). It is an icon of chemistry and is widely used in physics and other sciences. It is a depiction of the periodic law, which says that when the elements are arranged in order of their atomic numbers an approximate recurrence of their properties is evident. The table is divided into four roughly rectangular areas called blocks. Elements in the same group tend to show similar chemical characteristics.

Vertical, horizontal and diagonal trends characterize the periodic table. Metallic character increases going down a group and decreases from left to right across a period. Nonmetallic character increases going from the bottom left of the periodic table to the top right.

The first periodic table to become generally accepted was that of the Russian chemist Dmitri Mendeleev in 1869; he formulated the periodic law as a dependence of chemical properties on atomic mass. As not all elements were then known, there were gaps in his periodic table, and Mendeleev successfully used the periodic law to predict some properties of some of the missing elements. The periodic law was recognized as a fundamental discovery in the late 19th century. It was explained early in the 20th century, with the discovery of atomic numbers and associated pioneering work in quantum mechanics both ideas serving to illuminate the internal structure of the atom. A recognisably modern form of the table was reached in 1945 with Glenn T. Seaborg's discovery that the actinides were in fact f-block rather than d-block elements. The periodic table and law are now a central and indispensable part of modern chemistry.

The periodic table continues to evolve with the progress of science. In nature, only elements up to atomic number 94 exist;[a] to go further, it was necessary to synthesise new elements in the laboratory. Today, while all the first 118 elements are known, thereby completing the first seven rows of the table, chemical characterisation is still needed for the heaviest elements to confirm that their properties match their positions. It is not yet known how far the table will go beyond these seven rows and whether the patterns of the known part of the table will continue into this unknown region. Some scientific discussion also continues regarding whether some elements are correctly positioned in today's table. Many alternative representations of the periodic law exist, and there is some discussion as to whether there is an optimal form of the periodic table.

Each chemical element has a unique atomic number (Z) representing the number of protons in its nucleus.[3] Most elements have multiple isotopes, variants with the same number of protons but different numbers of neutrons. For example, carbon has three naturally occurring isotopes: all of its atoms have six protons and most have six neutrons as well, but about one per cent have seven neutrons, and a very small fraction have eight neutrons. Isotopes are never separated in the periodic table; they are always grouped together under a single element. When atomic mass is shown, it is usually the weighted average of naturally occurring isotopes; but if there are none, the mass of the most stable isotope usually appears, often in parentheses.[4] ff782bc1db