Lewis structuresalso known as Lewis dot diagramsLewis dot formulas, Lewis dot structureselectron dot structuresor Lewis electron dot structures LEDSare diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. The Lewis structure was named after Gilbert N. Lewiswho introduced it in his article The Atom and the Molecule. Lewis structures show each atom and its position in the structure of the molecule using its chemical symbol.
Lines are drawn between atoms that are bonded to one another pairs of dots can be used instead of lines. Excess electrons that form lone pairs are represented as pairs of dots, and are placed next to the atoms. Although main group elements of the second period and beyond usually react by gaining, losing, or sharing electrons until they have achieved a valence shell electron configuration with a full octet of 8 electrons, hydrogen H can only form bonds which share just two electrons.
The total number of electrons represented in a Lewis structure is equal to the sum of the numbers of valence electrons on each individual atom. Non-valence electrons are not represented in Lewis structures. Once the total number of available electrons has been determined, electrons must be placed into the structure according to these steps:.
Lewis structures for polyatomic ions may be drawn by the same method. When counting electrons, negative ions should have extra electrons placed in their Lewis structures; positive ions should have fewer electrons than an uncharged molecule. When the Lewis structure of an ion is written, the entire structure is placed in brackets, and the charge is written as a superscript on the upper right, outside the brackets. A trick is to count up valence electrons, then count up the number of electrons needed to complete the octet rule or with hydrogen just 2 electronsthen take the difference of these two numbers and the answer is the number of electrons that make up the bonds.
The rest of the electrons just go to fill all the other atoms' octets. Another simple and general procedure to write Lewis structures and resonance forms has been proposed. In terms of Lewis structures, formal charge is used in the description, comparison, and assessment of likely topological and resonance structures  by determining the apparent electronic charge of each atom within, based upon its electron dot structure, assuming exclusive covalency or non-polar bonding.
It has uses in determining possible electron re-configuration when referring to reaction mechanismsand often results in the same sign as the partial charge of the atom, with exceptions. In general, the formal charge of an atom can be calculated using the following formula, assuming non-standard definitions for the markup used:.
The formal charge of an atom is computed as the difference between the number of valence electrons that a neutral atom would have and the number of electrons that belong to it in the Lewis structure. Electrons in covalent bonds are split equally between the atoms involved in the bond. The total of the formal charges on an ion should be equal to the charge on the ion, and the total of the formal charges on a neutral molecule should be equal to zero.
For some molecules and ions, it is difficult to determine which lone pairs should be moved to form double or triple bonds, and two or more different resonance structures may be written for the same molecule or ion. In such cases it is usual to write all of them with two-way arrows in between see Example below. This is sometimes the case when multiple atoms of the same type surround the central atom, and is especially common for polyatomic ions.
When this situation occurs, the molecule's Lewis structure is said to be a resonance structureand the molecule exists as a resonance hybrid. Each of the different possibilities is superimposed on the others, and the molecule is considered to have a Lewis structure equivalent to some combination of these states. However, because the molecule is symmetrical, it does not matter which of the oxygens forms the double bond.
In this case, there are three possible resonance structures. Expressing resonance when drawing Lewis structures may be done either by drawing each of the possible resonance forms and placing double-headed arrows between them or by using dashed lines to represent the partial bonds although the latter is a good representation of the resonance hybrid which is not, formally speaking, a Lewis structure. When comparing resonance structures for the same molecule, usually those with the fewest formal charges contribute more to the overall resonance hybrid.
When formal charges are necessary, resonance structures that have negative charges on the more electronegative elements and positive charges on the less electronegative elements are favored.
Single bonds can also be moved in the same way to create resonance structures for hypervalent molecules such as sulfur hexafluoridewhich is the correct description according to quantum chemical calculations instead of the common expanded octet model. The resonance structure should not be interpreted to indicate that the molecule switches between forms, but that the molecule acts as the average of multiple forms.
Chemical structures may be written in more compact forms, particularly when showing organic molecules.Mitchell1 R obtains wiring diagrams and technical service bulletins, containing wiring diagram changes, from the domestic and import manufacturers.
These are checked for accuracy and are all redrawn into a consistent format for easy use. All system wiring diagrams are available in black and white format and may be printed depending on your program settings and available printer hardware.
In the past, when cars were simpler, diagrams were simpler. All components were connected by wires, and diagrams seldom exceeded 4 pages in length. Today, some wiring diagrams require more than 16 pages.
It would be impractical to expect a service technician to trace a wire from page 1 across every page to page Removing some of the wiring maze reduces eyestrain and time wasted searching across several pages. Today, the majority of Mitchell1 R diagrams follow a much improved format, which permits space for internal switch details, and component and ground locations.
Components shown with a dashed line instead of a solid line indicate not all circuits are shown in this particular diagram circuits shown in system diagrams are typically applicable to that system only. The remaining circuits connected to that component will be shown in the appropriate system that they apply to. The Data Link Connectors wiring diagrams show the circuits by which the various on-board computers exchange information, and the diagnostic connectors used for diagnosis and their location.
The Ground Distribution wiring diagrams show all vehicle ground points, their location, and the components common to those ground points. The Power Distribution wiring diagrams show the power feed circuits and the components common to those power feeds. Then, go to that system and locate the component within the wiring diagram. For example, if you don't know the specific system in which the ignition switch is located, look up ignition switch in the wiring diagram component location tables and go to the appropriate wiring diagram s which contain either full or partial views of the ignition switch.
The full view of the ignition switch is located in Power Distribution.
The first listing for the component will be the full or most complete view of the component. Additional listings will be partial views of the component. Not all components are used on all models. All components will have a partial view in Ground Distribution and Power Distribution. Data Link Connectors show connecting circuits between modules. Alternate names for components may be listed in wiring diagram component locations tables.
Mitchell1's Wiring Diagrams Mitchell1 R obtains wiring diagrams and technical service bulletins, containing wiring diagram changes, from the domestic and import manufacturers. The diagrams are drawn with the power source at the top of the diagram and the ground point at the bottom of the diagram. Component locations are identified on the wiring diagrams. Any wires that do not connect directly to a component are identified on the diagram to indicate where they go.In G.
In the Lewis theory of acid-base reactions, bases donate pairs of electrons and acids accept pairs of electrons. In other words, a Lewis acid is an electron-pair acceptor.
A Lewis base is any substance, such as the OH - ion, that can donate a pair of nonbonding electrons. A Lewis base is therefore an electron-pair donor. One advantage of the Lewis theory is the way it complements the model of oxidation-reduction reactions. Oxidation-reduction reactions involve a transfer of electrons from one atom to another, with a net change in the oxidation number of one or more atoms. The Lewis theory suggests that acids react with bases to share a pair of electrons, with no change in the oxidation numbers of any atoms.
Many chemical reactions can be sorted into one or the other of these classes. Either electrons are transferred from one atom to another, or the atoms come together to share a pair of electrons. The principal advantage of the Lewis theory is the way it expands the number of acids and therefore the number of acid-base reactions. In the Lewis theory, an acid is any ion or molecule that can accept a pair of nonbonding valence electrons.
This is an example of a Lewis acid-base reaction. The Lewis structure of water suggests that this molecule has nonbonding pairs of valence electrons and can therefore act as a Lewis base.
The Lewis acid-base theroy explains why BF 3 reacts with ammonia. BF 3 is a trigonal-planar molecule because electrons can be found in only three places in the valence shell of the boron atom. As a result, the boron atom is sp 2 hybridized, which leaves an empty 2 p z orbital on the boron atom. BF 3 can therefore act as an electron-pair acceptor, or Lewis acid. It can use the empty 2 p z orbital to pick up a pair of nonbonding electrons from a Lewis base to form a covalent bond.
BF 3 therefore reacts with Lewis bases such as NH 3 to form acid-base complexes in which all of the atoms have a filled shell of valence electrons, as shown in the figure below. The Lewis acid-base theory can also be used to explain why nonmetal oxides such as CO 2 dissolve in water to form acids, such as carbonic acid H 2 CO 3. In the course of this reaction, the water molecule acts as an electron-pair donor, or Lewis base.
The electron-pair acceptor is the carbon atom in CO 2. When the carbon atom picks up a pair of electrons from the water molecule, it no longer needs to form double bonds with both of the other oxygen atoms as shown in the figure below.
One of the oxygen atoms in the intermediate formed when water is added to CO 2 carries a positive charge; another carries a negative charge.
Predict whether the following ions or molecules can act as either a Lewis acid or a Lewis base. Click here to check your answer to Practice Problem 9. When the carbon atom picks up a pair of electrons from the water molecule, it no longer needs to form double bonds with both of the other oxygen atoms as shown in the figure below One of the oxygen atoms in the intermediate formed when water is added to CO 2 carries a positive charge; another carries a negative charge.
Practice Problem 9: Predict whether the following ions or molecules can act as either a Lewis acid or a Lewis base.With the globalization of clinical trials, a growing emphasis has been placed on the standardization of the workflow in order to ensure the reproducibility and reliability of the overall trial. Despite the importance of workflow evaluation, to our knowledge no previous studies have attempted to adapt existing modeling languages to standardize the representation of clinical trials.
This paper's objective is to develop a UML profile to extend the UML Activity Diagram schema into the clinical trials domain, defining a standard representation for clinical trial workflow diagrams in UML. Two Brazilian clinical trial sites in rheumatology and oncology were examined to model their workflow and collect time-motion data.
UML modeling was conducted in Eclipse, and a UML profile was developed to incorporate information used in discrete event simulation software. Ethnographic observation revealed bottlenecks in workflow: these included tasks requiring full commitment of CRCs, transferring notes from paper to computers, deviations from standard operating procedures, and conflicts between different IT systems.
Time-motion analysis revealed that nurses' activities took up the most time in the workflow and contained a high frequency of shorter duration activities. Administrative assistants performed more activities near the beginning and end of the workflow.
Overall, clinical trial tasks had a greater frequency than clinic routines or other general activities. This paper describes a method for modeling clinical trial workflow in UML and standardizing these workflow diagrams through a UML profile. In the increasingly global environment of clinical trials, the standardization of workflow modeling is a necessary precursor to conducting a comparative analysis of international clinical trials workflows. Clinical trials, though historically dominated by the United States and a small subset of countries in North America and Western Europe, are increasingly becoming a global activity with potential implications on health care delivery around the world.
Between andthe number of countries conducting drug research tracked by the Food and Drug Administration  rose from 28 to 79 and by one recent estimate, 24 of the fastest 25 growing countries in clinical trials are in emerging, non-traditional areas .
A study of industry sponsored phase 3 clinical trials in revealed that a majority of the sites were outside the United States .
Although the globalization of clinical trials brings many potential benefits a major challenge is faced regarding the standardization of clinical trials conducted in different parts of the world; the workflows of clinical trials, as well as the standards of care in different countries may vary so much as to invalidate individual trial results .
Ultimately, the relative performance of clinical trials in emerging countries will depend on the internal workflow of these research sites and the establishment of good clinical trials practice guidelines. Such variation in clinical practice guidelines among emerging sites has ethical implications, as well as implications on the trial workflow and validity of results.
This may, at least partly, be due to the lack of standard computational representation for these workflows, which would facilitate an operational comparison of how clinical trials are being conducted around the world.
Additionally, a standard representation would help create more homegeneous clinical trials, which would in turn facilitate the implementation of better meta analyses. Workflow modeling is an established technique of business process re-engineering, and various studies have assessed its potential in re-engineering organizational processes across various quality measures or goals . For example, workflow modeling in business process re-engineering may be used to identify inefficiencies or opportunities for cost reduction inherent in the sequence of tasks .
Yet, the use of workflow modeling in the clinical trials domain is less well established . Few studies have demonstrated the possible use of workflow modeling and analysis towards re-engineering clinical trials and research in this area continues to suffer from the lack of standard representation model.
The variety in representation models may extend from the use of different modeling languages to the use of different representations or vocabularies within a single modeling language . For example, different studies may use different modeling languages or symbols to represent the workflow, as well as different semantic phrases to represent the same activity i.
Hence, the need for standardization applies to both the use of a single modeling language, as well as a standard representation to extend the modeling schema into the clinical trials domain. Clinical trial modeling in Unified Modeling Language UML  provides a potential solution to some of these problems and can serve as a standard format for workflow modeling.
Figure 1 provides an example of a workflow model in UML UML allows the detailed description of organizational processes in a so-called Activity Diagram ADwhich can be annotated with data to support process analysis . Figure 2 depicts a real world example of an activity diagram representing a clinical process. Various studies have established the use of the UML activity diagram in modeling of business processes and some have even demonstrated its use in the healthcare domain .
In order to define a standard representation for UML models within a domain like clinical trial, one can formulate a UML profile which enables independent developers to generate standardized UML models at different sites .
The UML profile can be loaded into a UML developing environment in order to apply a standard set of data tags attributes to a workflow model; for example, a UML profile defining a standard for colonoscopy workflow might include a standard set of data attributes including the type of endoscope being used, the names of physicians or nurses, the type of sedation being used, and the duration of the procedure.
To this end, a UML profile for clinical trial workflow might specify attributes which facilitate the gathering of data for a time-motion study  to allow comparisons of efficiency in line with the NIH roadmap goal of re-engineering clinical research .Lewis symbols also known as Lewis dot diagrams or electron dot diagrams are diagrams that represent the valence electrons of an atom.
Lewis structures also known as Lewis dot structures or electron dot structures are diagrams that represent the valence electrons of atoms within a molecule. These Lewis symbols and Lewis structures help visualize the valence electrons of atoms and molecules, whether they exist as lone pairs or within bonds.
An atom consists of a positively charged nucleus and negatively charged electrons. Careful investigations have shown that not all electrons within an atom have the same average position or energy. Principal energy levels of gold Au : The figure shows the organization of the electrons around the nucleus of a gold Au atom. The number of electrons in each level is listed on the upper right corner of the figure. Notice that the outermost level has only one electron.
As an example, a neutral atom of gold Au contains 79 protons in its nucleus and 79 electrons. The first principal energy level, which is the one closest to the nucleus, can hold a maximum of two electrons. The second principal energy level can have 8, the third can have 18, and so on, until all 79 electrons have been distributed. The outermost principal energy level is of great interest in chemistry because the electrons it holds are the furthest away from the nucleus, and therefore are the ones most loosely held by its attractive force; the larger the distance between two charged objects, the smaller the force they exert on each other.
Chemical reactivity of all of the different elements in the periodic table depends on the number of electrons in that last, outermost level, called the valence level or valence shell. In the case of gold, there is only one valence electron in its valence level. Atoms gain, lose, or share electrons in their valence level in order to achieve greater stability, or a lower energy state. From this perspective, bonds between atoms form so that the bonded atoms are in a lower energy state compared to when they were by themselves.
Atoms can achieve this more stable state by having a valence level which contains as many electrons as it can hold. For the first principal energy level, having two electrons in it is the most stable arrangement, while for all other levels outside of the first, eight electrons are necessary to achieve the most stable state.
In the Lewis symbol for an atom, the chemical symbol of the element as found on the periodic table is written, and the valence electrons are represented as dots surrounding it.
Only the electrons in the valence level are shown using this notation. The Lewis symbol for carbon : Each of the four valence electrons is represented as a dot. Electrons that are not in the valence level are not shown in the Lewis symbol. The reason for this is that the chemical reactivity of an atom of the element is solely determined by the number of its valence electrons, and not its inner electrons. Lewis symbols for atoms are combined to write Lewis structures for compounds or molecules with bonds between atoms.
The Lewis symbol for an atom depicts its valence electrons as dots around the symbol for the element. In order to write the Lewis symbol for an atom, you must first determine the number of valence electrons for that element. The arrangement of the periodic table can help you figure out this information. Since we have established that the number of valence electrons determines the chemical reactivity of an element, the table orders the elements by number of valence electrons.
Each column or group of the periodic table contains elements that have the same number of valence electrons. Furthermore, the number of columns or groups from the left edge of the table tells us the exact number of valence electrons for that element.Despite the huge variety of possibilities in modern smartphones, they still continue to perform the function for which phones were initially created - mobile communications.
The functionality of this function is provided by a SIM card inserted into the special slot and even several. However, it is not uncommon for new and already used smartphones to have problems with communication - Xiaomi does not see the SIM card. This problem has several causes, and their most accurate detection is very important in this case, since further actions depend on them. These cases are good because most often the user can independently deal with this problem without resorting to the help of specialists.
Hardware nuances arise on new devices most often this is a normal marriage and on already used perhaps the phone had time to fly somewhere or swim.
Some nuances can be eliminated on their own, but more serious ones require knowledge of the case. Does the SIM itself work?How to Draw the Lewis Dot Structure for S 2- (Sulfide ion)
Perhaps she had been lying idle for a long time, and the operator turned it off. Contact the call center to solve this problem. The sim card can be very old, the contacts are damaged or oxidized.
Try to clean them with a cotton swab dipped in medical alcohol. Also check the contacts on the smartphone itself - if necessary, perform the same operation with them, using the minimum amount of alcohol to prevent it from swirling inside. Is the card firmly in the slot? Replace the card from the operator or repair the slot by contacting a specialist.
Perhaps you have a version of the device, in which the modem does not support local mobile frequencies. This often happens when ordering a phone directly from China, which, accordingly, have a hardware configuration different from the export models. There's nothing you can do - negotiate with the seller. Often there is a nuance on new devices - a normal marriage. Please contact the store where you bought the phone, let experts know about it if the phone, of course, under warranty.Acids and bases are an important part of chemistry.
However, this theory is very restrictive and focuses primarily on acids and bases acting as proton donors and acceptors. Sometimes conditions arise where the theory does not necessarily fit, such as in solids and gases. InG. Lewis from UC Berkeley proposed an alternate theory to describe acids and bases. His theory gave a generalized explanation of acids and bases based on structure and bonding.
Through the use of the Lewis definition of acids and bases, chemists are now able to predict a wider variety of acid-base reactions. Lewis' theory used electrons instead of proton transfer and specifically stated that an acid is a species that accepts an electron pair while a base donates an electron pair. A coordinate covalent bond is just a type of covalent bond in which one reactant gives it electron pair to another reactant. In this case the lewis base donates its electrons to the Lewis acid.
When they do react this way the resulting product is called an addition compound, or more commonly an adduct. Lewis acids accept an electron pair. Lewis Acids are Electrophilic meaning that they are electron attracting. Lewis Bases donate an electron pair. An atom, ion, or molecule with a lone-pair of electrons can thus be a Lewis base. Each of the following anions can "give up" their electrons to an acid, e.
Complex ions are polyatomic ions, which are formed from a central metal ion that has other smaller ions joined around it. The aluminum ion is the metal and is a cation with an unfilled valence shell, and it is a Lewis Acid. Water has lone-pair electrons and is an anion, thus it is a Lewis Base.
The Lewis Acid accepts the electrons from the Lewis Base which donates the electrons. Thus, Lewis Acid and Base Theory allows us to explain the formation of other species and complex ions which do not ordinarily contain hydronium or hydroxide ions. One is able to expand the definition of an acid and a base via the Lewis Acid and Base Theory. Therefore, by defining a species that donates an electron pair and a species that accepts an electron pair, the definition of a acid and base is expanded.
As of now you should know that acids and bases are distinguished as two separate things however some substances can be both an acid and a base. You may have noticed this with water, which can act as both an acid or a base. This ability of water to do this makes it an amphoteric molecule.
Water can act as an acid by donating its proton to the base and thus becoming its conjugate acid, OH. You may have noticed that the degree to which a molecule acts depends on the medium in which the molecule has been placed in.
Lewis Concept of Acids and Bases
Water does not act as an acid in an acid medium and does not act as a base in a basic medium. Thus, the medium which a molecule is placed in has an effect on the properties of that molecule. Other molecules can also act as either an acid or a base. For example. Note how the amphoteric properties of the Al OH 3 depends on what type of environment that molecule has been placed in.
Lewis Acids Lewis acids accept an electron pair. Various species can act as Lewis acids. All cations are Lewis acids since they are able to accept electrons. Molecules where the central atom can have more than 8 valence shell electrons can be electron acceptors, and thus are classified as Lewis acids e.
Molecules that have multiple bonds between two atoms of different electronegativities e.