Hydrogen average atomic mass

The identity of an element depends on the number of protons in the nucleus of the atom. Atoms and ions of a given element that differ in number of neutrons and have a different mass are called isotopes. A nucleus with a specified number of protons and neutrons is called a nuclide, and a nuclear particle, either a proton or neutron, may be called a nucleon. The total number of nucleons is called the mass number and this number is a whole number and is calculated by rounding up the average atomic mass value, for hydrogen, the atomic mass is 1.008 amu (atomic mass units) and is rounded up and the mass number of hydrogen is 1. 1 amu is equivalent to 1 /12th of the atomic mass of carbon. 

The average atomic mass for hydrogen to five significant digits is 1.0079 and that for oxygen is 15.999. 

A table of atomic masses is given on the inside front cover of this book. Hydrogen atoms, with a mass of about 1/12 that of a carbon atom, have an average atomic mass of 1.00797 amu on this relative scale. Magnesium atoms, which are about twice as heavy as carbon, have an average mass of 24.305 amu. The average atomic mass of oxygen is 15.9994 amu. 

It is difficult to imagine just how small an individual atom is. An especially small unit of mass, the atomic mass unit, u, is used to express the masses of atoms. This unit is a mass equal to exactly 1/12 that of an atom of the carbon-12 isotope. An atomic mass unit is only 1.66 X 10 g. An average atom of hydrogen, the lightest element, has a mass of only 1.0079 u. The average mass of an atom of uranium, the heaviest naturally occurring element, is 238.03 u. To place these values in perspective, consider that a signature written by ballpoint pen on a piece of paper... 

The atomic mass of hydrogen is 1.0079. This means that the average atom of hydrogen has a mass of 1.0079 u hydrogen s atomic mass is simply 1.0079 relative to the carbon-12 isotope taken as exactly 12. With this information, it is possible to place hydrogen in the periodic table with the designation shown in Figure 3.3. 

However, one atom in 6500 has a relative mass of 2, an isotope called deuterium. In addition, there is a third isotope of hydrogen, a radioactive form called Uitium, which has a mass of 3. The atomic mass for hydrogen, 1.0079, includes small contributions due to deuterium and tritium. For elements where the natural mixture of isotopes is less strongly biased towards a single species, the average atomic masses deviate considerably from whole numbers. Chlorine (atomic mass 35.453) and copper (atomic mass 65.546) are cases in point. 

Isotope Hydrogen-1 Hydrogen-2 Mass number 1 2 Percentage natural abundance 99.9885 0.0115 Atomic mass (u) 1.007 825 2.014102 Average atomic mass of eiement (u) 1.007 94... 

How do we use the mole in chemical calculations Recall that Avogadro s number is defined such that a 12.01-g sample of carbon contains 6.022 X 10 atoms. By the same token, because the average atomic mass of hydrogen is 1.008 amu (Table 8.1), 1.008 g of hydrogen contains 6.022 x 10 hydrogen... 

Notice that hydrogen has an atomic mass of 1.008 amu helium has an atomic mass of 4.003 amu. This means that, on the average, a helium atom has a mass that is about one-third that of a C-12 atom ... 

At one time, the hydrogen atom with one proton and no neutron was used as the standard to define 1 atomic mass unit (1 amu). Today, chemists use carbon-12, the most abundant isotope of carbon for the standard amu, which is defined as 1/12 of the C-12 atom. Therefore, the actual atomic weight for an element is in average mass units (numbers), taking into account all the isotopes (atoms) of that element. 

The top number in the symbols is the mass number and the bottom number is the atomic number. The atomic mass of the element hydrogen is the weighed average of the mass numbers of the three isotopes according to several tables ... 

The isotope mixture of each element is the reason that the atomic mass (mass number) is not a whole number. It is actually the weighted average of the isotopes. The mass number listed for hydrogen in the periodic table is 1.008. This value is larger than 1, indicating that there are isotopes heavier than H-l. Because 1.008 is just marginally larger than 1, there is only a small amount of these heavier isotopes. 

The isotope 2H is usually referred to as deuterium (D), or heavy hydrogen, but most isotopes of other elements are identified simply by their mass number. The atomic mass listed for each element in the periodic table is a weighted average, the fractional abundance of each isotope times its exact mass, summed over all naturally occurring isotopes. 

As in the case of carbon, the mass for each element given in the table inside the front cover of the book is an average value based on the isotopic composition of the naturally occurring element. For instance, the mass listed for hydrogen (1.008) is the average mass for natural hydrogen, which is a mixture of and 2H (deuterium). No atom of hydrogen actually has the mass 1.008. 

You can find atomic masses on the periodic table, but notice that the values shown are not exact integers. For example, you ll find 12.011 amu for carbon, 1.008 amu for hydrogen, and 4.003 amu for helium. These differences occur because the recorded values are weighted averages of the masses of all the naturally occurring isotopes of each element. 

Until 1900, chemists worked with a scale of relative atomic masses in which the average relative atomic mass of hydrogen was set at 1. At about that time, they changed to a scale in which the average relative atomic mass of naturally occurring oxygen (a mixture of and 0) was set at 16. In 1961, by inter-... 

The atomic mass unit (amu), which is represented with the symbol u, is based on a particular isotope of carbon, called carbon-12. Carbon-12 is considered to have a mass of exactly 12 u, and all of the other elemental isotopes are measured relative to that isotope. The atomic masses, shown on the periodic table, represent a weighted average of the masses of the naturally occurring isotopes of each element. For example, some periodic tables show an atomic mass of 1.00794 u for hydrogen, despite the fact that no particular isotope of hydrogen has a mass number equal to that value. 

Because Avogadro s number of particles (atoms) is 1 mol, it follows that the average mass of one atom of hydrogen is 1.008 amu... 

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