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`SCIENCE
`
`A Public Outreach Module:
`Atoms, Elements, and Isotopes
`
`An atom is the basic structure from which all matter is composed, in the same manner as a brick is the basic
`structure from which a wall is built. Although atoms are too small to be seen with our eyes, scientists have long
`had indirect evidence for the existence of atoms. We can now use the world's most powerful scanning tunneling
`microscopes to "see" the magnified images of atoms and to study surface reaction sites on an atom-by-atom
`basis.
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`Figure 1: Just as a brick is basic to the structure of a wall,
`an atom is basic to the structure of matter.
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`Atoms are made of small particles called protons, neutrons, and electrons. Each of these particles is described in
`terms of measurable properties, including mass and charge. Mass is the amount of matter that an object
`contains. The proton and neutron have roughly the same mass and have approximately one thousand times the
`mass of the electron. The proton and electron have equal, but opposite, electrical charges. A neutron does not
`have an electrical charge.
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`Figure 2: Model of Proton, Neutron, and Electron
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`If the proton and neutron were
`enlarged, and each had the
`approximate mass of a
`hippopotamus, the electron,
`enlarged to the same scale, would
`have less mass than an owl.
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`Figure 3: Comparison of Masses
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`In an atom, the protons and neutrons clump together in the center and are called the nucleus. Because the
`protons are positively charged, the nucleus has a positive electric charge. The electrons of the atom move
`rapidly around the nucleus. If we attempt to detect an electron in an atom, we might find evidence of it located
`almost anywhere around the nucleus. However, if we repeat this experiment many times, it will be found that the
`electron is much more likely to be located in certain regions of space surrounding the nucleus than in other
`regions of space. We might think that the electron is rapidly moving around the nucleus and our experiment
`"catches" the electron as an instantaneous "snapshot" of it in motion. The probability of finding the electron in
`any region of space can then be described by a cloud that rapidly thins out as one goes farther from the nucleus.
`The density of the cloud at any point is the probability of finding the electron at that point.
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`Most of an atom is empty space. The
`nucleus of the atom contains almost
`all of the mass of the atom. A greatly
`enlarged atom might look like a
`marble (the nucleus) inside an empty
`football stadium (the electron
`probability cloud).
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`Figure 4: Electron Probability Cloud Around a Nucleus
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`The attractive electric force between the positively-charged protons in the nucleus and the negatively-charged
`electrons around the nucleus holds the atom together. Atoms containing the same number of protons and
`electrons have no net charge. Atoms that have extra electrons or are missing electrons have a net electrical
`charge and are called ions. Ions can interact with other ions due to the electrical attraction between opposite
`charges.
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`Figure 5: Diagram Comparing a Beryllium Atom and a Di-positively-Charged Beryllium Ion
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`Atoms interact with other atoms by sharing or transferring electrons that are farthest from the nucleus. These
`electrons are sometimes called valence electrons. These outer electrons determine the chemical properties of
`the element, such as how readily it interacts with other elements and the allowable ratios for its combinations
`with other substances.
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`Figure 6: Model of Covalent and Ionic Bonds
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`An element is a substance made up of a
`single type of atom. It can't be broken into
`simpler components by chemical processes.
`There are 92 naturally occurring elements.
`They may be solids, liquids, or gases.
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`Figure 7: Modern Periodic Table
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`The elements are distributed unevenly, with
`some much more common than others. The
`ten most abundant elements of the earth's
`crust make up more than 99% of our planet.
`
`Element
`
`Oxygen
`Silicon
`Aluminum
`Iron
`Calcium
`Sodium
`Potassium
`Magnesium
`Titanium
`Hydrogen
`
`Symbol
`O
`Si
`Al
`Fe
`Ca
`Na
`K
`Mg
`Ti
`H
`
`Relative % of Earth's Mass
`46.6
`27.7
`8.1
`5.0
`3.6
`2.8
`2.6
`2.1
`0.4
`0.1
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`Figure 8: The Ten Most Abundant Elements
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`How fine can the dust become and still be
`considered gold?
`
`Gold is one
`example of an
`element.
`
`Figure 9
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`A bar of gold can be shaved into
`gold dust, and still be
`recognizable as gold.
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`Figure 10
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`The smallest particle that would still have the properties
`associated with gold is an atom. To get an idea of how small an
`atom is, consider that a small gold coin may contain over
`20,000,000,000,000,000,000,000 atoms.
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`Figure 11
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`The mass of an atom or particle is expressed in atomic mass units or amus. One atomic mass unit is a very
`small amount of mass. An amu is 1/12 the mass of one atom of 12C, or about 1.66 x 10-27 kg.
`Atomic mass values for elements are almost never an integer. The only exception is carbon, whose mass was
`used as a standard reference. The mass of a 12C carbon atom is specified as exactly 12. The mass of an atom
`of another element is the ratio of its mass to the mass of a carbon atom. Even the masses of protons, neutrons,
`and electrons are ratios of their mass to carbon. These ratios are not integers. Since atoms are made of various
`numbers of these particles, it is unlikely that the mass of an atom other than carbon would add up to exactly a
`whole number.
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`Figure 12: Diagram Comparing Hydrogen, Deuterium, and Tritium Atoms
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`When an element has atoms that differ in the number of neutrons in the nuclei, these atoms are called different
`isotopes of the element. All isotopes of one element have identical chemical properties. This means it is difficult
`to separate isotopes from each other by chemical processes. However, the physical properties of the isotopes,
`such as their masses, boiling points, and freezing points, are different. Isotopes can be most easily separated
`from each other using physical processes.
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`Most atoms of the element hydrogen contain only one proton in their nuclei. Each of these atoms has a mass of
`1.008 amu. There exist atoms of hydrogen that have either one or two neutrons in the nucleus in addition to the
`single proton. These are called deuterium or tritium, having masses of 2.014 amu and 3.016 amu respectively.
`Deuterium and tritium are isotopes of hydrogen. An atom of deuterium has two particles in its nucleus, and tritium
`has three. Since atoms of both deuterium and tritium have only one proton in their nuclei, they only have one
`electron. They behave, chemically, like other hydrogen atoms.
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`Figure 13: Mass Number and Atomic Mass of Hydrogen,
`Deuterium, and Tritium Atoms
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`The sum of the number of protons and neutrons in the nucleus of an atom is called that element's mass number.
`This is not the same as the element's mass. Since different isotopes of an element contain different numbers of
`neutrons in the nuclei of their atoms, isotopes of the same element will have different atomic masses. This was
`shown above for the three isotopes of hydrogen. The symbol for an isotope is the symbol for the element
`followed by the mass number. Hydrogen is symbolized as H1, while deuterium is symbolized as H2.
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`What would we call an atom that had three particles in its nucleus, like tritium, but two were protons and one was
`a neutron? This would be an uncommon isotope of a different element, helium (He3). Because there were two
`protons in this nucleus, there would also be two electrons in the probability cloud around it. Since it is the outer
`electrons that determine the chemical properties of an atom, this would be a different kind of atom than
`hydrogen. The presence of two rather than one electron would cause it to have distinctive chemical properties.
`Thus, this must be a different element, and it is named helium. The most common isotope of helium (He4) has
`two protons and two neutrons in the nucleus of each atom.
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`Figure 14: Diagram of Tritium, 3He, and 4He Atoms
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`To distinguish between elements, we often refer to their atomic numbers. The atomic number is the number of
`protons in the nucleus of an atom of that element (which is equal to the number of electrons around that atom's
`nucleus). Hydrogen's atomic number is 1, while helium's atomic number is 2. Gold has an atomic number of 79,
`which means it has 79 protons in its nucleus. The modern periodic table of the elements shows the different
`elements arranged in increasing order of atomic number.
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`There are 92 elements found in nature and several more exotic, manmade elements. Based on their chemical
`and physical properties, scientists have invented a tool to show relationships among these elements. It is known
`as the periodic table of the elements.
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`Figure 15: Periodic Table of the Elements, Los Alamos National Laboratories
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`Observations show that the same elements exist throughout the known universe. We organize information about
`the elements in the form of a periodic table. The elements and their interactions are studied in all disciplines of
`science, as chemicals form the basis of life science, physical science, and earth and space science. As the
`American Association for the Advancement of Science wrote in their Project 2061, "Science for All Americans"
`publication, "All humans should participate in the pleasure of coming to know their universe better."
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