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There are two main reasons. The first is that the masses of protons and neutrons are not 1 unit but slightly greater. The mass of an atom is the mass of a whole number of protons, a whole number of neutrons as well as the same number of electrons as protons. Overall, therefore, the mass should be greater than the number of protons and neutrons (electrons have very little mass). But some of the mass is converted to energy which is used to hold the positively charged nucleus together. As a result the mass of carbon12 is an exact whole number and that is the only element for which that is true.
The second, and possibly more relevant reason is that most atoms are found in the form of isotopes which have different numbers of neutrons and so different masses. The Atomic Mass listed for an element is an average of the masses of all these isotopes, weighted together according to their abundance of earth.
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Who invented piriodic table?
Dmitri Mendeleev created the first periodic table according to atomic mass in 1869. Others before Mendeleev had organized the elements according to their properties and were able to discern periodicity, although Mendeleev is generally accepted as the creator of the table.With the information he gathered about the elements, he was able to see that there were missing elements that hadn't been discovered. He could figure out the atomic masses of the missing elements by averaging the atomic masses of the elements above and below the missing one. One such element, which he called "eka-silicon" (eventually Ge) was missing, but with understanding of the patterns the periodic table made, he predicted the elements appearance, melting point, atomic mass, density, formula of oxide, and formula of chloride.Throughout the years, other scientists were able to find or create these missing elements to form the table we know today. Today's periodic table is ordered by atomic number instead of atomic mass (as Mendeleev had started). Henry Moseley was the first to order the elements by atomic number so elements would fit together in groups/families and periods better in 1913.
How do you get the atomic mass and and atomic number?
it is simple. all you need to do is look in the periodic table the numbers above and under the letter.Atomic number is determined from the number of protons in the nucleus. Atomic mass is determined from the number of protons AND neutrons in the nucleus. Since elements have multiple isotopes with differing numbers of neutrons, the atomic mass reported on periodic charts is usually the average atomic mass. As the first person who answered this question pointed out, you can usually find the atomic number by looking for a number above the atomic symbol for the element in the the table (H for hydrogen, C for carbon, Fe for Iron, Au for gold, etc.). Sometimes the number will be below the symbol but in bold font. When the periodic table gives an atomic mass, it is usually given under the atomic symbol and is almost always some kind of decimal number that is always greater than the atomic number, for example: Carbon has an atomic number of 6and an atomic mass of 12.0107 amu.
What is the atomic mass on the periodic table?
Each element on the periodic table has two numbers: the atomic number and the relative atomic mass. The atomic number is the number of protons in the nucleus, and the relative atomic mass is the total number of protons and neutrons (so the difference between them is the number of neutrons). The relative atomic mass is always the higher of the two.
How are the elements arranged by atomic mass?
They are NOT normally arranged by atomic mass; instead they are arranged in the periodic table by their atomic number. The latter is the number of protons (or electrons) in a single atom of the element and it is a constant for the element. The atomic mass is an average of the isotopes of the element (in proportion to their abundance).While the atomic number is always an integer, increases in steps of 1 and elements that are a specific "distances" apart share similar chemical characteristic. On the other hand the atomic mass is rarely an integer, and occasionally it reverses the order laid out be the periodic table: for example, element 18, argon has an atomic mass of 39.948 which is greater than that of element 19, potassium with a mass of 39.098: this is because the predominant isotope of argon has 22 neutrons while that for potassium has 20 neutrons.
The atomic mass is determined by?
atomic mass is determined by adding the number of protons and neutrons.this can be seen in the periodic table as the average atomic mass below the name of the element
