The atomic mass of carbon would be 12.01 grams per mole of carbon atoms. 2) Sum of Protons and Neutrons for a Single Atom To calculate the atomic mass of a single atom of an element, add up the mass of protons and neutrons. Example: Find the atomic mass of an isotope of carbon that has 7 neutrons. Isotopes of carbon are atomic nuclei that contain six protons plus a number of neutrons (varying from 2 to 16). Carbon has two stable, naturally occurring isotopes. The isotope carbon-12 (12 C) forms 98.93% of the carbon on Earth, while carbon-13 (13 C) forms the remaining 1.07%.
For comparison, the atomic mass of a carbon-12 atom is exactly 12 daltons. Alternately, the atomic mass of a carbon-12 atom may be expressed in any other mass units: for example, the atomic mass of a carbon-12 atom is about 1.998 467 052 × 10−26 kg.
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.
In the original periodic table published by Dimitri Mendeleev in 1869, the elements were arranged according to increasing atomic mass— at that time, the nucleus had not yet been discovered, and there was no understanding at all of the interior structure of the atom, so atomic mass was the only guide to use. Once the structure of the nucleus was understood, it became clear that it was the atomic number that governed the properties of the elements.
Atomic Structure
All matter is formed from basic building blocks called atoms. Atoms are made of even smaller particles called protons,electrons, and neutrons. Protons and neutronslive in the nucleus of an atom and are almost identical in mass. However,protons have positive charges whereas neutrons have no charge. Electronshave a negative charge and orbit the nucleus in shells or electronorbitals and are much less massive than the other particles. Sinceelectrons are 1836 times less massive than either protons or neutrons, most ofthe mass of an atom is in the nucleus, which is only 1/100,000th the size of anentire atom(!).
The organization of a hydrogen atom is as shown below:
Hydrogen atom
The red dot is a proton in the nucleus. It has a positivecharge of +1 unit. The blue dot is an electron. It has a negativecharge of -1 unit. For any normal atom, the number of electrons andprotons is equal, meaning the electrical charge is balanced. Thereis only one orbital for hydrogen. Let's look at a larger atom, carbon.
Carbon atom
There is now a new particle in the nucleus, the neutron(represented by the white dots). There are also 6 protons in the nucleus,for a total of 12 particles. In addition, there are now 6 electronsorbiting the nucleus in two orbitals. The reason the carbon atom needs asecond orbital are complex and are beyond the scope of this geology class. But the rules that govern atoms say that the first orbital can only have twoelectrons, the second orbital is allowed eight electrons, the third orbital onlyeight electrons, etc. (See page 27 of your text to the numbers ofelectrons in each orbital for the first 20 elements.)
There are 91 naturally occurring elements. Atoms are the smallest pieces of an elements possible, and in fact the word'atom' comes from the Greek word 'a tomos' which means'not cutting' -- i.e. you can't cut it any smaller than that. Weusually represent elements by their atomic symbol. Hydrogen is representedby an 'H'; carbon by a 'C'.
For atoms, changing the number of protons changes the kind ofelement. So, if I were to drop an additional proton into thenucleus of the carbon atom illustrated above, I would no longer have carbon -- Iwould have nitrogen. Similarly, if I took away a proton from the carbonatom, I would have another element, boron. The number of protons in thenucleus of an atom is the same as the atomic number of thatatom. If you add together the number of protons and neutrons, you get the atomicmass number of that particular atom.
Quick quiz: What is the atomic number of the hydrogen atom shownabove? What is it's atomic mass number? What is the atomic numberand atomic mass number of the carbon atom shown above?
Answers: The atomic number of hydrogen is 1 (count theprotons). The atomic mass number of hydrogen is also one (there areno neutrons!). For carbon, the atomic number is 6, and the atomicmass number is 12 (6 protons plus 6 neutrons).
Look at the illustration of the carbon atom again. What if we added aneutron instead of a proton? Would we have the same element? Yes. But, the atom would be different. Adding orsubtracting neutrons from the nucleus of an atom creates isotopes of that atom. For instance, lets add two neutrons to the carbon atom, represented by greendots below:
Carbon isotope
Adding the two neutrons changes our atom. However, becausethe number of protons are the same, it is still carbon but now it is an isotopeof carbon. We represent isotopes by using the chemical symbol('C' for carbon) and a number. The first carbon atom with only 6neutrons would be called 12C or Carbon-12. The new one with 8neutrons would be 14C or Carbon-14. Note that the number'14' is also the atomic mass number for this isotope.
Chemists have worked to organize the elements in a particular way called thePeriodic Table. It is ordered such that elements in each column havecertain chemical and physical properties in common. Below is an image ofthe Periodic Table:
*Image from http://www.chemtutor.com/perich.htm
Each element has an atomic symbol and an atomicnumber.
Quick Quiz: Recall that the atomic number is thenumber of protons in the nucleus. What is the number of protons in aSodium (Na) atom? An Oxygen (O) atom? A Uranium (U) atom?
Answers: Sodium has 11 protons, Oxygen 8protons, and Uranium 92 protons.
Atomic Mass Of Carbon Monoxide
Continue and learn about ions.
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Define Atomic Mass
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