Michelson, Physical Review. Similarity between cosmic rays and gamma rays  Nature. On the question of the constancy of the cosmic radiation and the relation of these rays to meteorology Physical Review. The most probable values of the electron and related constants Physical Review. History of research in cosmic rays Nature. Remarks on the history of cosmic radiation Science. Dependence of electron emission from metals upon field strengths and temperatures Physical Review. Relations of Field-Currents to Thermionic-Currents.
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Financial Aid Options. Apply Now! Willamette Promise. College Credit in High School. This text is published under creative commons licensing, for referencing and adaptation, please click here. Up until now we have been discussing only the elemental forms of atoms which are neutrally charged. This is because the number of electrons negative in charge is equal to the number of protons positive in charge. The overall charge on the atom is zero, because the magnitude of the negative charge is the same as the magnitude of the positive charge.sdb.bo/includes/253/riteve-citas-por-internet.php
This one-to-one ratio of charges is not, however, the most common state for many elements. Deviations from this ratio result in charged particles called ions. Throughout nature, things that are high in energy tend to move toward lower energy states. Lower energy configurations are more stable, so things are naturally drawn toward them. For atoms, these lower energy states are represented by the noble gas elements.
These elements have electron configurations characterized by full s and p subshells.
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This makes them stable and unreactive. They are already at a low energy state, so they tend to stay as they are. The elements in the other groups have subshells that are not full, so they are unstable when compared to the noble gases. This instability drives them toward the lower energy states represented by the noble gases that are nearby in the periodic table. There are two ways for an atom that does not have an octet of valence electrons to obtain an octet in its outer shell. One way is the transfer of electrons between two atoms until both atoms have octets.
Because some atoms will lose electrons and some atoms will gain electrons, there is no overall change in the number of electrons, but with the transfer of electrons the individual atoms acquire a nonzero electric charge. Those that lose electrons become positively charged, and those that gain electrons become negatively charged. Recall that atoms carrying positive or negative charges are called ions. If an atom has gained one or more electrons, it is negatively charged and is called an anion.
If an atom has lost one or more electrons, it is positively charged and is called a cation. Because opposite charges attract while like charges repel , these oppositely charged ions attract each other, forming ionic bonds. The resulting compounds are called ionic compounds. The second way for an atom to obtain an octet of electrons is by sharing electrons with another atom. These shared electrons simultaneously occupy the outermost shell of both atoms. The bond made by electron sharing is called a covalent bond.
At the end of chapter 2, we learned how to draw the electron dot symbols to represent the valence electrons for each of the elemental families. This skill will be instrumental in learning about ions and ionic bonding. Looking at Figure 3.
The electron dot symbol for the Nobel Gas family clearly indicates that the valence electron shell is completely full with an octet of electrons. If you look at the other families, you can see how many electrons they will need to gain or lose to reach the octet state. Above, we noted that elements are the most stable when they can reach the octet state. However, it should also be noted that housing excessively high negative or positive charge is unfavorable. Thus, elements will reach the octet state and also maintain the lowest charge possible.
You will note that for the IA, IIA, IIIA and transition metals groups, it is more economical to lose electrons electrons from their valence shells to reach the octet state, rather than to gain electrons. Some atoms, like carbon, are directly in the middle.
The remaining sections of this chapter will focus on the formation of ions and the resulting ionic compounds. Figure 3. A Depiction of St. B In many high voltage applications plasma ionization is an unwanted side effect. Shown is a long exposure photograph of corona discharge on an insulator string of a kV overhead power line. This type of plasma discharge represent a significant power loss for electric utilities. Photograph depicted in a A by: Unknown Author. Photograph depicted in a B by: Nitromethane. The elements on the right side of the periodic table, nonmetals, gain the electrons necessary to reach the stable electron configuration of the nearest noble gas.
Elements on the left side of the periodic table, metals, lose the electrons necessary to reach the electron configuration of the nearest noble gas. Transition elements can vary in how they move toward lower energy configurations. They lose one electron upon ionization, moving into the electron configuration of the previous noble gas. For example as shown in Figure 3. The sodium ion has one fewer electron than it has protons, so it has a single positive charge and is called a cation. It is left with a full octet in the second shell and now has the electron configuration of neon.
Note that it still has the same number of protons 11 as the original sodium atom and retains the identity of sodium. Upon losing that electron, the sodium ion now has an octet of electrons from the second principal energy level. The equation below illustrates this process. The electron configuration of the sodium ion is now the same as that of the noble gas neon.
The sodium ion is isoelectronic with the neon atom. Consider a similar process with magnesium and with aluminum:. In this case, the magnesium atom loses its two valence electrons in order to achieve the same noble-gas configuration. The aluminum atom loses its three valence electrons. For most elements under typical conditions, three electrons is the maximum number that will be lost or gained. Only larger atoms, such as lead and uranium, can typically carry larger charge states. This gives them the electron configuration of the noble gas that comes before them in the periodic table.
While hydrogen is in the first column, it is not considered to be an alkali metal, and so it does not fall under the same classification as the elements below it in the periodic table. This is because hydrogen only has an s- subshell and can only house a total of 2 electrons to become filled and obtain the electron configuration of helium. Thus, hydrogen can form both covalent bonds and ionic bonds, depending on the element that it is interacting with.
Note, that hydrogen only has one electron to begin with, so when it loses an electron in the ionized state, there is only a single proton left in the nucleus of the atom. It can also be ionized, forming a -1 anion. In this case, the H — anion is named using standard convention forming the hydride ion. Elements on the other side of the periodic table, the nonmetals, tend to gain electrons in order to reach the stable electron configurations of the noble gases that come after them in the periodic table.
Group VIIA elements gain one electron when ionized, obtaining a -1 charge.
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This gives it a single negative charge, and it is now a chloride ion Cl — ; note the slight change in the suffix -ide instead of -ine to create the name of this anion. Fig 3. On the left, a chlorine atom has 17 electrons. Note that the chloride ion has now filled its outer shell and contains eight electrons, satisfying the octet rule. Group VIA elements gain two electrons upon ionization, obtaining -2 charges and reaching the electron configurations of the noble gases that follow them in the periodic table.
Whereas, Group VA elements gain three electrons, obtaining -3 charges and also reaching the electron configurations of the noble gases that follow in the periodic table. When nonmetal atoms gain electrons, they often do so until their outermost principal energy level achieves an octet.
This process is illustrated below for the elements fluorine, oxygen, and nitrogen. All of these anions are isoelectronic with each other and with neon. They are also isoelectronic with the three cations from the previous section. Under typical conditions, three electrons is the maximum that will be gained in the formation of anions. It is important not to misinterpret the concept of being isoelectronic. A sodium ion is very different from a neon atom because the nuclei of the two contain different numbers of protons. One is an essential ion that is a part of table salt, while the other is an unreactive gas that is a very small part of the atmosphere.
Neon atoms and sodium ions are isoelectronic. Neon is a colorless and unreactive gas that glows a distinctive red-orange color in a gas discharge tube. Sodium ions are most commonly found in crystals of sodium chloride, ordinary table salt. The transition metals are an interesting and challenging group of elements. Predicting how they will form ions is also not always obvious. Many transition metals cannot lose enough electrons to attain a noble-gas electron configuration.
In addition, the majority of transition metals are capable of adopting ions with different charges. According to the Aufbau process, the electrons fill the 4 s sublevel before beginning to fill the 3 d sublevel. However, the outermost s electrons are always the first to be removed in the process of forming transition metal cations.
This is the case for iron above. This is because a half-filled d subshell d 5 is particularly stable, which is the result of an iron atom losing a third electron.
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It has been known since ancient times as green vitriol and was used for centuries in the manufacture of inks. Some transition metals that have relatively few d electrons may attain a noble gas electron configuration. Scandium is an example. However, secondary transfer of these particles to other surfaces can occur from contact with the surfaces or persons on whom the particles have deposited, as with handshaking or contact with clothing. Movement of persons following the shooting, or even scene investigation by forensic scientists, may alter GSR distribution.
Further tertiary or even quaternary transfer is possible. Law enforcement personnel may carry particles from prior shooting events. Blakey et al, The amount and pattern of GSR deposited may vary by the gun used to fire the bullet. There is greater particle numbers with revolvers than with automatic rifles, for example. Particle numbers are greater with nonjacketed bullets, mainly due to an increase in particles composed of lead. A faster burning rate of propellant powder reduces the distance of GSR particles travelled.
GSR may be expelled ahead of the bullet, along with the bullet, and after the bullet. Though the amount of residue deposited tends to decrease with increasing range of fire, the actual deposits can be highly variable for ranges up to 20 cm. Brown, Cauchi, et al, GSR has been reported to be found at distances from 6 to 18 meters forward of the shooter, and up to 6 meters laterally. However, climatic conditions significantly influence recovery rates for GSR.
Dalby et al, Use of atomic force microscopy AFM for detection of particle size in relation to range of fire has been described.
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Mou, Lakadwar, and Rabalais, The major methods for detection of primer residues are analytical and qualitative. Scanning electron microscopy with energy dispersive analysis by x-ray detector SEM-EDX and atomic force microscopy AFM are used to identify the primer residue qualitatively. An X-ray analyzer can be beamed directly onto the particles visualized with SEM, so that the energy dispersive pattern can be generated, giving the elemental composition of the particles. For these methods, samples must be obtained from the skin surfaces of a victim at the scene. Delay in obtaining residues, movement, or washing of the body prior to autopsy will diminish or destroy gunshot residues.
Molina et al, A rapid loss in numbers of GSR particles occurs from 1 to 3 hours post firearm discharge, though maximum recovery times of 1 to 48 hours have been reported. Detectable GSR particles may be identified 5 days after firearm discharge Blakey et al, The method of collection for residue is quite simple and easily carried out in the field. The two widely used methods incude collection onto a carbon-coated adhesive stub or with an alcohol swab. Of the two, the stub has fewer false negatives from greater collection efficiency. The swab method may have usefulness when the surface to be tested is smooth, or if propellant analysis is required.
The stub can be directly applied to the surface skin or other material to be tested. The stub, with the residue on the surface, can be directly prepared for examination in the SEM device. A major advantage of SEM is that it can reveal the actual surface details of the particles examined, for comparison with known examples of gunshot residue, and pictures can be taken. The large particles of partially burned powder and the spheres of residue can be distinguished from contaminant materials. Reid et al, Any hand or body part that was close to the fired weapon may have residue appearing consistent with having fired the weapon.
Clothing should always be retained on the body up to autopsy, as this may modify entrance wounds, need examination for gunshot residues, or aid in interpretation of the scene. The type of weapon can influence the distribution of GSR. For handguns, variables include: barrel length and caliber affecting the plume or cone of gases emitted with their GSR particle; nature of the ejection port of pistols; and barrel-drum gap of revolvers. Ditrich, Gunshot residue analysis requires careful evaluation. False positives may be caused by contamination or transfer of GSR to the body by mishandling, or when the body is heavily contaminated by GSR from previous shooting.
However, the number of particles from secondary environmental contamination is low. Berk et al, False positives from neutron activation analysis or from atomic absorption spectroscopy assays can be avoided with SEM because of the ability to identify the morphology of particles. False negatives result from washing of the hands when this area is sampled or by victim wearing gloves.
A rifle or shotgun may not deposit GSR on hands, bur more likely in the crook of the support arm. SEM may also have usefulness for examination of bullets, as embedded materials from the target such as bone fragments may aid in reconstruction of the scene DiMaio VJ et al, SEM has been used to study tool marks made by the firing pin impressions in the primers of spent cartridges.
Such findings could be useful to determine which gun was used to fire the cartridge. In a study of wound samples microwave-digested and analyzed using ICP-MS to detect all elements present at measurable levels in GSR, shot versus unshot tissues could be distinguished. Additionally, jacketed and nonjacketed bullet types could also be distinguished. Udey et al, The presence of GSR may vary from entrance to exit wounds, for the entrance wound will usually have more than the exit, or the exit will have none.
At close range, macroscopic examination of the entrance wound is in concordance with microscopic appearance of GSR in all cases, but for distant range gross detection of GSR is negative in a third of cases, though microscopically present. A fifth of exit wounds, though lacking grossly detectable GSR, have microscopic evidence of GSR, thus confounding distinction of entrance and exit wounds by microscopy alone. Perez and Molina, Residue is lacking in entrance wounds with airguns Denton et al, Cohle et al, The alizarin red S stain can be utilized in microscopic tissue sections to determine the presence of barium as part of GSR Tschirhart, Noguchi, Klatt,