Name: lewis structures with formal charge
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 42 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Formal Charges

חלק מהמולקולות או היונים הפולי-אטומיים יכולים להיות מיוצגים על ידי כמה מבני לואיס, אך איך אפשר להחליט איזה מבנה הוא הדומיננטי? באמצעות חישוב של המטענים הרשמיים של האטומים, ניתן לקבוע מהו מבנה לואיס הקרוב ביותר למולקולה. כל אטום מקבל מטען היפותטי הנקרא מטען רשמי, שהיה המטען על האטום אם לכל האטומים האחרים במולקולה הייתה אותה אלקטרושליליות.ההנחה היא שכל אלקטרון קושר משותף באופן שווה על ידי שני אטומים. לדוגמה, מימן כלורי. כדי לקבוע את המטען הרשמי של כל אטום, חברו ראשית את מספר האלקטרונים הבלתי קושרים למחצית מספר האלקטרונים הקושרים ואז החסירו את הערך שקיבלתם ממספר אלקטרוני הערכיות.חיבור של כל המטענים הרשמיים במולקולה או ביון שווה למטען הכולל של המולקולה או היון. לדוגמה, ניתן לייצג את תחמצת החנקן בשלושה מבני לואיס אפשריים, אחד עם שני קשרים כפולים, אחד עם שלושה קשרים בין אטומי החנקן ואחד עם קשר משולש בין החנקן לחמצן, כולם משלימים אוקטט. ניתן לזהות את מבנה לואיס הטוב ביותר על ידי חישוב המטען הפורמלי.לחנקן יש חמישה אלקטרוני ערכיות ולחמצן יש שישה אלקטרוני ערכיות. החישוב, המבוסס על מספר האלקטרונים הבלתי קושרים ועל מחצית האלקטרונים הקושרים, נותן את המטען הרשמי של כל מבנה. כיוון שתחמוצת חנקן היא מולקולה ניטרלית, סך כל המטענים הרשמיים צריך להיות אפס.באופן כללי, המטענים הרשמיים של האטומים האינדיבידואליים במבנה לואיס הדומיננטי הם אלו שקרובים לאפס. לכן ניתן להתעלם מהמבנה השלישי עם המטען הרשמי הגבוה יותר. בנוסף לכך, מטען רשמי שלילי, אם קיים, צריך להיות של האטום האלקטרושלילי ביותר.כיוון שחמצן הוא אלקטרושלילי יותר מחנקן, המבנה השני, עם המטען הרשמי השלילי של החמצן מזוהה כמבנה הדומיננטי של תחמוצת חנקן. מטענים רשמיים הם לא המטענים בפועל של מולקולות או אטומים, אלא מוסכמת רישום. המטען בפועל של המולקולה תלוי בכמה גורמים, כולל ההבדל באלקטרושליליות בין האטומים המרכיבים אותה.

מטענים פורמליים

<div style="direction:rtl">

 	במקרים מסוימים, יש לכאורה יותר מאחד ממבני לואיס תקפים עבור מולקולות ויונים פוליאטומיים. ניתן להשתמש במושג מטענים פורמליים כדי לעזור לחזות את מבנה לואיס המתאים ביותר כאשר קיים יותר ממבנה סביר אחד. 
 	<h4>חישוב המטען הפורמלי</h4> 

 	המטען הפורמלי של אטום במולקולה הוא המטען ההיפותטי שיהיה לאטום אם האלקטרונים בקשרים מחולקים באופן שווה בין האטומים. לחלופין, מטען פורמלי נוצר כאשר מספר האלקטרונים הערכיים של אטום ניטרלי, האלקטרונים הלא-קשורים מצטמצמים, בעקבות מחסור במספר הקשרים המחוברים לאותו אטום במבנה לואיס. 

 	לפיכך, המטען הפורמלי מחושב באופן הבא: 
 	<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065_Equations-01.png" style="height: 60px;"> 

 ניתן לבדוק פעמיים את חישובי המטענים הפורמליים על ידי קביעת סכום המטענים הפורמליים עבור המבנה כולו. סכום המטענים הצורניים של כל האטומים במולקולה חייב להיות אפס; סכום המטענים הפורמליים ביון צריך להיות שווה למטען היון. זכור כי המטען הפורמלי המחושב עבור אטום אינו המטען בפועל של האטום במולקולה. המטען הפורמלי הוא רק הליך שימושי בהנהלת חשבונות; זה לא מצביע על נוכחות של מטענים בפועל. <h4>חישוב מטען פורמלי ממבני לואיס</h4> 

 בצע את השלבים הבאים כדי להקצות מטענים פורמליים לכל אטום ביון האינטרהלוגן ICl4−. 

 שלב 1. חלקו את זוגות האלקטרונים המחוברים באופן שווה עבור כל קשרי I–Cl: 
 <img alt="איור 1" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065image_1.svg" style="height: 120px;"> 

 שלב 2. הקצה זוגות בודדים של אלקטרונים לאטומים שלהם. לכל אטום כלור מוקצים כעת שבעה אלקטרונים, ולאטום היוד יש שמונה. 

 שלב 3. הורידו את המספר הזה ממספר האלקטרונים הערכיים עבור האטום הניטרלי: 
יוד: 7 – 8 = – 1
כלור: 7 - 7 = 0
סכום המטענים הפורמליים של כל האטומים שווה ל-1, שזהה למטען היון (-1).

 <h4>שימוש במטען פורמלי כדי לחזות מבנה מולקולרי</h4> 

 סידור האטומים במולקולה או יון נקרא המבנה המולקולרי שלה. במקרים רבים, ביצוע השלבים לכתיבת מבנים של לואיס עשוי להוביל ליותר ממבנה מולקולרי אפשרי אחד - קשרים מרובים שונים ומיקומי אלקטרונים בודדים או סידורים שונים של אטומים, למשל. כמה קווים מנחים הכרוכים במטען פורמלי יכולים להיות מועילים להחליט איזה מהמבנים האפשריים הוא הסביר ביותר עבור מולקולה או יון מסוימים: <ol style="padding-right: 35px; margin: 0;"> 
 	<li style="text-align: right;">מבנה מולקולרי שבו כל המטענים הפורמליים הם אפס עדיף על מבנה שבו חלק מהמטענים הפורמליים אינם אפס.</li> 
 	<li style="text-align: right;">אם מבנה לואיס חייב להיות בעל מטענים רשמיים שאינם אפס, ההסדר עם המטענים הרשמיים הקטנים ביותר שאינם אפס עדיף יותר.</li> 
 	<li style="text-align: right;">מבנים של לואיס עדיפים כאשר מטענים סמוכים הם אפס או בסימן ההפוך.</li> <li style="text-align: right;">כאשר בוחרים מכמה מבני לואיס עם התפלגות דומה של מטענים פורמליים, עדיף המבנה עם המטענים הפורמליים השליליים על האטומים האלקטרושליליים יותר.</li> </ol> 

 כדי לראות כיצד הנחיות אלה חלות, שקול כמה מבנים אפשריים עבור פחמן דו חמצני, CO2. ידוע שהאטום הפחות אלקטרושלילי תופס בדרך כלל את המיקום המרכזי, אך מטענים פורמליים עוזרים להבין מדוע זה קורה. ניתן לשרטט שלוש אפשרויות למבנה: פחמן במרכז עם שני קשרים כפולים, הפחמן במרכז עם קשר יחיד ומשולש, וחמצן במרכז עם קשרים כפולים. <img alt="איור 2" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065image_2.svg" style="height: 80px;"> 

 בהשוואה בין שלושת המטענים הפורמליים, ניתן לזהות את המבנה בצד שמאל כמועדף מכיוון שיש לו רק מטענים פורמליים של אפס. 

 כדוגמה נוספת, ליון התיוציאנאט, יון שנוצר מאטום פחמן, אטום חנקן ואטום גופרית, יכולים להיות שלושה מבנים מולקולריים שונים: NCS–, CNS– או CSN–. המטענים הפורמליים הקיימים בכל אחד מהמבנים המולקולריים הללו יכולים לעזור לבחור את הסידור הסביר ביותר של האטומים. מבנים אפשריים של לואיס והמטענים הפורמליים לכל אחד משלושת המבנים האפשריים ליון התיוציאנאט הם - פחמן במרכז עם קשרים כפולים, חנקן במרכז עם קשרים כפולים, וגופרית במרכז עם קשרים כפולים. <img alt="איור 3" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065Image_3.svg" style="height: 95px;"> 

 שימו לב שסכום המטענים הפורמליים בכל מקרה שווה למטען היון (–1). עם זאת, הסידור הראשון של אטומים עם פחמן במרכז עדיף מכיוון שיש לו את המספר הנמוך ביותר של אטומים עם מטענים שאינם אפס. כמו כן, הוא מציב במרכז את האטום שהכי פחות אלקטרושלילי ואת המטען השלילי על היסוד האלקטרושלילי יותר. 

 טקסט זה מותאם מ <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, Chemistry 2e, Chapter 7.4 Formal Charges and Resonance.</a></div>

במקרים מסוימים, יש לכאורה יותר ממבנה לואיס תקף אחד עבור מולקולות ויונים פוליאטומיים. הרעיון של חיובים רשמיים יכול לשמש כדי ...

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lewis structures with formal charge

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry.

Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then subtracting the number of bonds on that atom in the Lewis structure. This is a bookkeeping tool, not the actual charge on the atom.

The formal charge of a full structure should also be checked as a whole. For a neutral molecule, the sum of all formal charges must be zero. For an ion, the sum of all formal charges should match the ion’s overall charge.

A common example is the interhalogen ion ICl4−. To assign formal charges, the bonding electron pairs are divided equally among the I–Cl bonds. Then lone pairs are assigned to their atoms. In this structure, each chlorine has seven electrons assigned to it, and iodine has eight. That gives iodine a formal charge of −1 and each chlorine a formal charge of 0, which adds up to the −1 charge of the ion.

Formal charge also helps predict molecular structure when several Lewis structures are possible. A structure with all formal charges equal to zero is preferred over one with nonzero charges. If nonzero charges are needed, the best choice has the smallest charges possible. Structures are also preferred when adjacent formal charges are zero or opposite in sign. When several choices are still possible, the negative charge should be on the more electronegative atom.

Carbon dioxide shows why these rules matter. One possible structure has carbon in the center with two double bonds. Other possibilities place carbon between a single and a triple bond, or place oxygen in the center with double bonds. The structure with carbon in the center and two double bonds is preferred because all of its formal charges are zero.

The thiocyanate ion, NCS−, gives another clear example. It can be written with carbon, nitrogen, or sulfur in the center. The formal charges in each possible Lewis structure help identify the most likely arrangement of atoms. The carbon-centered structure is preferred because it has the fewest atoms with nonzero formal charges. It also places the least electronegative atom in the center and the negative charge on the more electronegative element.

Lewis Structures with Formal Charge

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formal charges

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry. Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then...

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Learn how formal charge helps compare Lewis structures, check total charge, and choose the best arrangement for molecules and ions like CO2 and NCS−.

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Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10&#8722;15 meters, a nucleus is ...

Nuclear Binding Energy

The difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium-4, the mass defect ...

Radioactive Decay and Radiometric Dating

Radioactivity is a spontaneous disintegration of an unstable nuclide and is a random process, as all the nuclei in the sample do not decay simultaneously. ...

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

Nuclear transmutation is the conversion of one nuclide into another. It can occur by the radioactive decay of a nucleus, or the reaction of a nucleus with ...

Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

PS.Chm.1.2

Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

electron-configuration-of-multielectron-atoms

Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

Use models to explain how electrons are distributed in atoms.

PS.Chm.1.3

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

PS.Chm.2.1

atomic-radii-and-effective-nuclear-charge

Atomic Radii and Effective Nuclear Charge

The elements in groups of the periodic table exhibit similar chemical behavior. This similarity occurs because the members of a group have the same number ...

Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy ...

Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



This process can be either ...

Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

Types of Chemical Bonds

Chemical bonding theories were pioneered by American chemist Gilbert&#xA0;N. Lewis. He developed a model called the Lewis model to explain the type and ...

Lewis Symbols and the Octet Rule

Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

Lattice Energy&#xA0;

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a ...

Trends in Lattice Energy: Ion Size and Charge

An ionic compound is stable because of the electrostatic attraction between its positive and negative ions. The lattice energy of a compound is a measure ...

Covalent Bonding and Lewis Structures

Compared to ionic bonds, which results from the transfer of electrons between metallic and nonmetallic atoms, covalent bonds result from the mutual ...

Electronegativity

Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.&#xA0;

Electronegativity values of ...

Bond Polarity, Dipole Moment, and Percent Ionic Character

Bond Polarity

The absolute value of the difference in electronegativity (&Delta;EN) of two bonded atoms provides a rough measure of the polarity to be ...

Lewis Structures of Molecular Compounds and Polyatomic Ions

To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

In some cases, there are seemingly more than one valid Lewis structures for molecules and polyatomic ions. The concept of formal charges can be used to ...

Exceptions to the Octet Rule

Many covalent molecules have central atoms that do not have eight electrons in their Lewis structures. These molecules fall into three categories:


	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

Intermolecular forces (IMF) are electrostatic attractions arising from charge-charge interactions between molecules. The strength of the intermolecular ...

Intermolecular Forces

Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions ...

comparing-intermolecular-forces-melting-point-boiling-point

Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

Intermolecular forces are attractive forces that exist between molecules. They dictate several bulk properties, such as melting points, boiling points, ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Lewis Structures with Formal Charge

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