Category: Chemistry Published: March 13, 2014  An atom of sodium from table salt behaves very differently from an atom of sodium from metallic sodium. In general, atoms of the same element are not identical as they can be in different states. Public Domain Image, source: Christopher S. Baird. No. Two atoms of the same chemical element are typically not identical. First of all, there is a range of possible states that the electrons of an atom can occupy. Two atoms of the same element can be different if their electrons are in different states. If one copper atom has an electron in an excited state and another copper atom has all of its electrons in the ground state, then the two atoms are different. The excited copper atom will emit a bit of light when the electron relaxes back down to the ground state, and the copper atom already in the ground state will not. Since the states of the electrons in an atom are what determine the nature of the chemical bonding that the atom experiences, two atoms of the same element can react differently if they are in different states. For instance, a neutral sodium atom (say, from a chunk of sodium metal) reacts with water much more violently than an ionized sodium atom (say, from a bit of salt). Chemists know this very well. It's not enough to say what atoms are involved if you want to fully describe and predict a reaction. You have to also specify the ionization/excitation states of the electrons in the atoms. Even if left alone, an atom often does not come with an equal number of protons and electrons. But what if two atoms of the same element both have their electrons in the same states. Then are they identical? No, they are still not identical. Two atoms of the same element and in the same electronic state could be traveling or rotating at different speeds, which affects their ability to chemically bond. Slower moving atoms (such as the atoms in solid iron) have time to form stable bonds, while faster moving atoms (such as the atoms in liquid iron) cannot form such stable bonds. A slow moving tin atom acts differently from a rapidly moving tin atom. But what if two atoms of the same element both have their electrons in the same states, and the atoms are both traveling and rotating at the same speed. Then are they identical? No. Although two such atoms are essentially chemically identical (they will chemically react in the same way), they are not completely identical. There's more to the atom than the electrons. There's also the nucleus. The nucleus of an atom contains neutrons and protons bonded tightly together. The same chemical element can have a different number of neutrons and still be the same element. We refer to the atoms of the same element with different numbers of neutrons as "isotopes". While the particular isotope involved does not affect how an atom will react chemically, it does determine how the atom will behave in nuclear reactions. The most common nuclear reaction on earth is radioactive decay. Some isotopes decay very quickly into other elements and emit radiation, while other isotopes do not. If you are doing carbon dating, the fact that a carbon-12 atom is not identical to a carbon-14 atom is essential to the dating process. Simply counting the number of carbon atoms in a sample will not give you any information about the age of a sample. You will have to count the number of different isotopes of carbon instead. But what if two atoms are the same element, have electrons in the same state, are traveling and rotating at the same speed, and have the same number of neutrons; then are they identical? No. Just like the electrons, the neutrons and protons in the nucleus can be in various excited states. In addition, the nucleus as a whole can rotate and vibrate at various speeds. Therefore, even if all else is identical, two gold atoms can have their nuclei in different excited states and behave differently in nuclear reactions. To state the case succinctly, it is very hard to have two atoms of the same element be exactly identical. In fact, succeeding in coaxing a group of atoms to be very close to identical was worthy of a Nobel Prize. With that said, don't think that atoms have individual identities beyond what has been mentioned here. If two carbon atoms are in the exact same molecular, atomic, electronic and nuclear states, then those two carbon atoms are identical, no matter where they came from or what has happened to them in the past. Topics: atom, atoms, carbon, chemical, electron, electronic state, element, isotope, nucleus NFL-YET ACADEMY CHEMISTRY-I: atoms.htm FALL, 1999 DR. GERALD A. ROSENTHAL THE CHEMICAL NATURE OF ATOMS THE mass of an atom is concentrated in a very small central portion of the atom which is called the atomic nucleus. The atomic nucleus is made up of electrically positive protons and electrically neutral neutrons. Surrounding the atomic nucleus are the
electrically negative electrons. The masses and charges of these three fundamental constituents of atoms are given below: Particle Charge Mass Electron -1 0 The chemical nature of an atom, that is , the chemical properties of a specific element, is determined by the number of protons in the nucleus. This number of protons is called the atomic number. The mass of the atom, its atomic mass, depends upon both the number of protons and upon the number of neutrons present in the nucleus (remember that the mass of an electron is so small that it is simply ignored for the purpose of establishing the atomic mass). Before moving further into this unit, link to:atoms02.htm. ISOTOPES AND ATOMIC MASSES This is a complex unit that requires attention to detail and careful thought. This is not easy. For many of the chemical elements there are several known isotopes. Isotopes are atoms with different atomic masses which have the same atomic number. The atoms of different isotopes are atoms of the same chemical element; they differ in the number of neutrons in the nucleus. For example, the element hydrogen has atoms that have one proton in the nucleus. This is the
most common form of hydrogen. Most of the atoms of hydrogen found in the Universe are 1H. That is, they have an atomic number of one due to the presence of single proton in the nucleus, and a mass of one due to the presence of a single proton. Later, we shall consider isotopes again when we consider the important question of atomic mass Moles of Atoms Historical background. The atomic mass of an element is a relative quantity. Originally the atomic mass of hydrogen, the lightest of the elements, was taken to be one and
the atomic masses of all other elements were measured in relation to the atomic mass of hydrogen. This later proved to have been a poor choice. Not only does hydrogen naturally consist of more than one isotope, but there was the additional question (particularly among early chemists) as to whether monatomic hydrogen or diatomic hydrogen should be taken as having atomic mass one. After some effort, and one major false start with oxygen, chemists and physicists agreed on a common scale of relative atomic mass. Carbon of isotopic mass twelve was assigned an
atomic mass of exactly twelve, and all other atomic masses whether of isotopes or of elements were specified relative to carbon of atomic mass twelve. This had the effect of making the relative atomic mass of hydrogen 1.0079...rather than exactly 1.0000.... The difference of less than 1% is too small to matter in many approximate chemical calculations, but it is large enough to be significant when accurate work must be done. Scientists needed a single unit to enable them to compare the exact same number of atoms or molecules of one element or compound to another. Eventually, they agreed upon using the concept of the mole (mol) for this purpose. By definition, one mole of atoms is that number of atoms which exist in exactly twelve grams of carbon of isotopic mass twelve (12C). Remember, scientists have agreed on the isotope carbon-12 for determining all relative atomic masses. The number of atoms in a mol of any element or compound is called the Avogadro number; the best current determination of its value is 6.0221 x 1023. YOU MUST MEMORIZE AVOGADRO'S NUMBER AND HAVE AN ABSOLUTELY CLEAR UNDERSTANDING OF EXACTLY WHAT IT MEANS AND WHY IT IS SO IMPORTANT IN MODERN CHEMISTRY Molar Atomic Masses of Elements The molar mass of an atom is simply the mass of one mole of its identical atoms. However, most of the chemical elements are found on earth not as one isotope but as a mixture of isotopes, so the atoms of one element do not all have the same mass. Chemists therefore distinguish the molar atomic mass of an isotope, which is the mass of one mole of the identical atoms which form that isotope, from the molar atomic mass of an element, which is the mass of one mole of the atoms of the various isotopes of that element having the natural abundance as they are found on earth. I know that this is very difficult, but
if you truly understand this concept, then the following is clear to you: This property has open been called the atomic weight or chemical atomic weight of the element. The weighted mean molar atomic mass of an element as it naturally occurs will be referred to simply as the atomic mass of the element from now on. Example. The relative abundance's of the isotopes 6Li and 7Li in naturally occurring lithium can be computed as follows. Their atomic masses are 6.0151214 and 7.0160030, respectively. The atomic mass of naturally occurring lithium given in the table of atomic mass or weight is 6.941. If the relative abundance of 6Li is 7.5% and the relative abundance of 7Li is 92.5%, then the atomic weight of Lithium (for the entire population of lithium atoms) is: (.075 x 6.0151214) + (.925 x 7.0160030) = 0.451 + 6.49 or 6.941Remember that this is a weighted average in which the contribution of each isotope is taken into account. The final atomic mass of Li is very close to the atomic mass of the 7Li isotope because so much of this isotope (as compared to 6Li) makes up the natural population of lithium atoms. Thus, the atomic mass of Li is taken to be 6.941 because this value takes into account the natural abundance of the isotopes of Lithium. Special thanks to James A. Plambeck of the University of Alberta in Canada who authored some of this material. What happens to the atom when there are varying numbers of neutrons?If you change the number of neutrons, you create isotopes. Isotopes are basically just lighter or heavier versions of an average element. In fact, the way we calculate the mass number of a given element on the periodic table is to average the light, medium, and heavy versions of that element.
Do all the atoms of the same element have the same number of neutrons?Atoms of same element must have same number of protons, same number of electrons but may also have same or different number of neutrons (in case of isotopes).
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Atoms of the same element with varying number of neutrons
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