 # Lewis Dot Diagram For Of2

### General | Latest Info

Thus far in this chapter, we have discussed various types of bonds that form between atoms and / or ions. In all cases, these bonds involve sharing or transfer of valence shell electrons between atoms. In this section, we will explore typical methods for depicting valence shell electrons and chemical bonds, namely Lewis symbols and Lewis structures. Step 1: Calculate the number of valence electrons. Hcn: + = 10 H 3 CCH 3: + = 14 HCCH: + = 10 NH 3: + = 8 step 2. Draw skeleton and connect atoms with single bonds. Remember that H is never a central atom: step 3: Where needed to distribute electrons to terminal atoms: step 4: Where needed to place remaining electrons on the central atom: We can draw the Lewis structure of any covalent molecule by following six steps discussed earlier. In this case, we can condense the last few steps, since not all of them apply. 1. Calculate the number of valence electrons: XeF 2: 8 + = 22XeF 6: 8 + = 50 2. Draw skeleton joining atoms by single bonds. Xenon will be central atom because fluorine cannot be central atom: 3. Distribute remaining electrons. Xef 2: We place three lone pairs of electrons around each F atom, accounting for 12 electrons and giving each F atom 8 electrons. Thus, six electrons remain. These lone pairs must be placed on Xe atom. This is acceptable because Xe atoms have empty valence shell d orbitals and can accommodate more than eight electrons. The Lewis structure of XeF 2 shows two bonding pairs and three lone pairs of electrons around Xe atom: 4. Xef 6: We place three lone pairs of electrons around each F atom, accounting for 36 electrons. Two electrons remain, and this lone pair is place on Xe atom: eight electrons: eight electrons: no electrons Be 2 + eight electrons: no electrons Ga 3 + no electrons Li + eight electrons: O 2: in this case, Lewis structure is inadequate to depict the fact that experimental studies have show two unpaired electrons in each oxygen molecule. H 2 CO: AsF 3: ClNO: SiCl 4: H 3 O +: {matheq}{\text{NH}}_{4}^{+}{endmatheq} {matheq}{\text{BF}}_{4}^{-}{endmatheq} HCCH: ClCN: {matheq}{\text{C}}_{2}^{\text{2+}}{endmatheq} SeF 6: XeF 4: {matheq}{\text{SeCl}}_{3}^{+}:{endmatheq} Cl 2 BBCl 2: 11. Two valence electrons per Pb atom are transferred to cl atoms; resulting Pb 2 + ion has a 6 s 2 valence shell configuration. Two of the valence electrons in HCl molecule are share, and the other six are located on Cl atom as lone pairs of electrons. 13. 15. 17. The Complete Lewis structures are as follow: 19. 100. An 0 - g sample of this compound would contain 85. 7 g C and 14. 3 g H: {matheq}\begin{array}{l}\frac{85.7\text{g}}{12.011{\text{g mol}}^{-1}}=7.14\text{mol C}\ \frac{14.3\text{g}}{1.00794{\text{g mol}}^{-1}}=14.19\text{mol H}\end{array}{endmatheq} this is ratio of 2 H to 1 C, or empirical formula of CH 2 with formula mass of approximately 14. As {matheq}\frac{42}{14}=3,{endmatheq} formula is 3 CH 2 or C 3 H 6. Lewis structure is: 21.

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### Introduction

Writing Lewis Structures, NASA's Cassini - Huygens mission detected a large cloud of toxic hydrogen cyanide on Titan, one of Saturn's moons. Titan also contains ethane, acetylene, and ammonia. What are Lewis structures of these molecules? Calculate the number of valence electrons. Hcn: + = 10H 3 CCH 3: + = 14HCCH: + = 10NH 3: + = 8 Draw skeleton and connect atoms with single bonds. Remember that H is never central atom: Where needed to distribute electrons to terminal atoms: HCN: six electrons placed on NH 3 CCH 3: no electrons remainHCCH: no terminal atoms capable of accepting electrons. Nh 3: no terminal atoms capable of accepting electrons Where needed to place remaining electrons on the central atom: HCN: no electrons remainH 3 CCH 3: no electrons remainHCCH: four electrons placed on carbon NH 3: two electrons placed on nitrogen Where needed to rearrange electrons to form multiple bonds in order to to obtain octet on each atom: HCN: form two more C - N bondsH 3 CCH 3: all atoms have correct number of electronsHCCH: form triple bond between two carbon atomsNH 3: all atoms have correct number of electrons check Your Learning Both carbon monoxide, CO, and carbon dioxide, CO 2, are products of combustion of fossil fuels. Both of these gases also cause problems: CO is toxic and CO 2 has been implicated in global climate change. What are Lewis structures of these two molecules?

##### Lewis Structures

We also use Lewis symbols to indicate the formation of covalent bonds, which are shown in Lewis structures, drawings that describe bonding in molecules and polyatomic ions. For example, when two chlorine atoms form chlorine molecule, they share one pair of electrons: Lewis structure indicates that each Cl atom has three pairs OF electrons that are not used in bonding and one share pair of electrons. Dash is sometimes used to indicate shared pair of electrons: single shared pair of electrons is called a single bond. Each Cl atom interacts with eight valence electrons: six in lone pairs and two in single bond.

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### Problems

We use Lewis symbols to describe valence electron configurations of atoms and monatomic ions. Lewis symbols consist of an elemental symbol surrounded by one dot for each of its valence electrons: figure 1 shows Lewis symbols for elements of the third period of the periodic table. Lewis symbols can also be used to illustrate formation of cations from atoms, as shown here for sodium and calcium: likewise, they can be used to show formation of anions from atoms, as shown here for chlorine and sulfur: figure 2 demonstrates use of Lewis symbols to show transfer of electrons during formation of ionic compounds.

##### Lewis Structures

For very simple molecules and molecular ions, we can write Lewis structures by merely pairing up unpaired electrons on constituent atoms. See these examples: For more complicated molecules and molecular ions, it is helpful to follow the step - by - step procedure outlined here: determining total number of valence electrons among all atoms. For cations, subtract one electron for each positive charge. For anions, add one electron for each negative charge. Draw skeleton structure of a molecule or ion, arranging atoms around the central atom. Connect each atom to the central atom with a single bond. Distribute remaining electrons as lone pairs on terminal atoms, completing octet around each atom. Place all remaining electrons on the central atom. Rearrange electrons OF outer atoms to make multiple bonds with central atom in order to obtain octets wherever possible. Let us determine Lewis structures OF SiH 4, {matheq}{CHO2-}{endmatheq} NO +, and OF 2 as examples in following this procedure: determine total number OF valence electrons in molecule or ion. For molecule, we add the number OF valence electrons on each atom in molecule: {matheq}\begin{align} &\phantom{+}{SiH4}\ &\phantom{+}\textrm{Si: 4 valence electrons/atom × 1 atom = 4}\ &\underline{\textrm{+H: 1 valence electron/atom × 4 atoms = 4}}\ &\hspace{271px}\textrm{= 8 valence electrons} \end{align}{endmatheq} For negative ion, such as {matheq}{CHO2-}{endmatheq} we add the number OF valence electrons on atoms to the number of negative charges on ion: {matheq}{CHO2-}\ \textrm{C: 4 valence electrons/atom × 1 atom} \hspace{6px}= \phantom{1}4\ \textrm{H: 1 valence electron/atom × 1 atom} \hspace{12px}= \phantom{1}1\ \textrm{O: 6 valence electrons/atom × 2 atoms = 12}\ \underline{+\hspace{100px}\textrm{1 additional electron} \hspace{9px}= \phantom{1}1}\ \hspace{264px}\textrm{= 18 valence electrons}{endmatheq} For positive ion, such as NO +, we add the number OF valence electrons on atoms in ion and then subtract number OF positive charges on ion from total number OF valence electrons: {matheq}{NO+}\ \textrm{N: 5 valence electrons/atom × 1 atom} = \phantom{−}5\ \textrm{O: 6 valence electron/atom × 1 atom}\hspace{5px} = \phantom{−}6\ \underline{\textrm{+ −1 electron (positive charge)} \hspace{44px}= −1}\ \hspace{260px}\textrm{= 10 valence electrons}{endmatheq} since OF 2 is neutral molecule, We simply add number OF valence electrons: {matheq}\phantom{+ }{OF2}\ \phantom{+ }\textrm{O: 6 valence electrons/atom × 1 atom} \hspace{10px}= 6\ \underline{\textrm{+ F: 7 valence electrons/atom × 2 atoms} = 14}\ \hspace{280px}\textrm{= 20 valence electrons}{endmatheq} draw skeleton structure OF molecule or ion, arranging atoms around central atom and connecting each atom to central atom with single bond. When several arrangements OF atoms are possible, as for {matheq}{CHO2-}{endmatheq} we must use experimental evidence to choose the correct one. In general, less electronegative elements are more likely to be central atoms. In {matheq}{CHO2-}{endmatheq} less electronegative carbon atom occupies central position with oxygen and hydrogen atoms surrounding it. Other examples include P in POCl 3, S in SO 2, and Cl. In {matheq}{ClO4-}{endmatheq} exception is that hydrogen is almost never the central atom. Like most electronegative element,ss fluorine also cannot be central atom. Distribute remaining electrons as lone pairs on terminal atoms to complete their valence shells with octet OF electrons. There are NO remaining electrons on SiH 4, SO it is unchanged: Place all remaining electrons on the central atom. For SiH 4, {matheq}{CHO2-}{endmatheq} and NO +, there are NO remaining electrons; We already place all OF electrons determined in Step 1. For OF 2, we had 16 electrons remaining in Step 3, and we Place 12, leaving 4 to be Place on the central atom: Rearrange electrons OF outer atoms to make multiple bonds with the central atom in order to obtain octets wherever possible.

##### The Octet Rule

We will also encounter a few molecules that contain central atoms that do not have fill valence shell. Generally, these are molecules with central atoms from groups 2 and 12, outer atoms that are hydrogen, or other atoms that do not form multiple bonds. For example, in Lewis structures of beryllium dihydride, BeH 2, and boron trifluoride, BF 3, beryllium and boron atoms each have only four and six electrons, respectively. It is possible to draw a structure with a double bond between boron atom and fluorine atom in BF 3, satisfying the octet rule, but experimental evidence indicates bond lengths are closer to that expected for B - F single bonds. This suggests the best Lewis structure has three B - F single bonds and electron deficient boron. Reactivity of compound is also consistent with electron deficient boron. However, B - F bonds are slightly shorter than what is actually expected for B - F single bonds, indicating that some double bond characters are found in actual molecule. Atoms like boron atom in BF 3, which do not have eight electrons, are very reactive. It readily combines with molecule containing atom with a lone pair of electrons. For example, NH 3 reacts with BF 3 because lone pair of nitrogen can be shared with boron atom:

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### Lewis Symbols

In this case, Lewis structure is inadequate to depict the fact that experimental studies have shown two unpaired electrons in each oxygen molecule. 11. Two valence electrons per Pb atom are transferred to cl atoms; resulting Pb 2 + ion has a 6 s 2 valence shell configuration. Two of the valence electrons in HCl molecule are share, and the other six are located on Cl atom as lone pairs of electrons. 21. Each bond includes sharing of electrons between atoms. Two electrons are shared in single bond; four electrons are shared in double bond; and six electrons are shared in triple bond.

##### Lewis Structures

For very simple molecules and molecular ions, we can write Lewis structures by merely pairing up unpaired electrons on constituent atoms. See these examples: For more complicated molecules and molecular ions, it is helpful to follow the step - by - step procedure outlined here: determining total number of valence electrons. For cations, subtract one electron for each positive charge. For anions, add one electron for each negative charge. Draw skeleton structure of a molecule or ion, arranging atoms around the central atom. Connect each atom to the central atom with a single bond. Distribute remaining electrons as lone pairs on terminal atoms, completing octet around each atom. Place all remaining electrons on the central atom. Rearrange electrons OF outer atoms to make multiple bonds with central atom in order to obtain octets wherever possible. Let us determine Lewis structures OF, and as example in following this procedure: determine the total number OF valence electrons in molecule or ion. For molecule, we add the number OF valence electrons on each atom in molecule: SiH 4 Si: 4 valence electrons / atom 1 atom = 4 + H: 1 valence electron / atom 4 atoms = 4 = 8 valence electrons. For negative ion, we add the number OF valence electrons on atoms to the number OF negative charges on ion: CHO 2 - C: 4 valence electrons / atom 1 atom = 4 H: 1 valence electrons / atom 1 atom = 1 O: 6 valence electrons / atom 2 & atoms = 12 + 1 additional electron = 18 valence electrons For positive ion, such as, We add number OF valence electrons on atoms in ion and then subtract number OF positive charges on ion from total number OF valence electrons: NO + N: 5 valence electrons / atom 1 atom = 5 O: 6 valence electrons / atom 1 atom = 6 + - 1 electron = 10 valence electrons Since is neutral molecule, We simply add number OF valence electrons: OF 2 O: 6 valence electrons / atom 1 atom = 6 + F: 7 valence electrons / atom 2 atoms = 14 = 20 valence electrons Draw skeleton structure OF molecule or ion, arranging atoms around central atom and connecting each atom to central atom with single bond. When several arrangements OF atoms are possible, as For, we must use experimental evidence to choose the correct one. In general, less electronegative elements are more likely to be central atoms. In, less electronegative carbon atoms occupy central position with oxygen and hydrogen atoms surrounding them. Other examples include In, In, and In. An exception is that hydrogen is almost never the central atom. Like most electronegative element,ss fluorine also cannot be central atom. Distribute remaining electrons as lone pairs on terminal atoms to complete their valence shells with octet OF electrons.

##### The Octet Rule

Other halogen molecules form bonds like those in chlorine molecule: one single bond between atoms and three lone pairs of electrons per atom. This allows each halogen atom to have a noble gas electron configuration. The tendency of main group atoms to form enough bonds to obtain eight valence electrons is known as the octet rule. The number of bonds that atom can form can often be predicted from the number of electrons needed to reach octet; this is especially true of nonmetals of second period of the periodic table. For example, each atom of group 14 elements has four electrons in its outermost shell and therefore requires four more electrons to reach the octet. These four electrons can be gained by forming four covalent bonds, as illustrated here for carbon in CCl 4 and silicon in SiH 4. Because hydrogen only needs two electrons to fill its valence shell, it is an exception to the octet rule. Transition elements and inner transition elements also do not follow the octet rule: group 15 elements such as nitrogen have five valence electrons in atomic Lewis symbol: one lone pair and three unpaired electrons. To obtain octet, these atoms form three covalent bonds, as in NH 3. Oxygen and other atoms in group 16 obtain octets by forming two covalent bonds:

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### Lewis Structures

For very simple molecules and molecular ions, we can write Lewis structures by merely pairing up unpaired electrons on constituent atoms. See these examples: For more complicated molecules and molecular ions, it is helpful to follow the step - by - step procedure outlined here: determining total number of valence electrons. For cations, subtract one electron for each positive charge. For anions, add one electron for each negative charge. Draw skeleton structure of a molecule or ion, arranging atoms around the central atom. Connect each atom to the central atom with a single bond. Distribute remaining electrons as lone pairs on terminal atoms, completing octet around each atom. Place all remaining electrons on the central atom. Rearrange electrons OF outer atoms to make multiple bonds with central atom in order to obtain octets wherever possible. Let us determine Lewis structures OF SiH 4, {matheq}{\text{CHO}}_{2}^{-},{endmatheq} NO +, and OF 2 as examples in following this procedure: determine total number OF valence electrons in molecule or ion. For molecule, we add the number OF valence electrons on each atom in molecule: {matheq}\begin{array}{l}\ \phantom{\rule{0.8em}{0ex}}{\text{SiH}}_{4}\ \phantom{\rule{0.8em}{0ex}}\text{Si: 4 valence electrons/atom}\times \text{1 atom}=4\ \underline{+\text{H: 1 valence electron/atom}\times \text{4 atoms}=4}\ \ \phantom{\rule{15.95em}{0ex}}=\text{8 valence electrons}\end{array}{endmatheq} For negative ion, such as {matheq}{\text{CHO}}_{2}^{-},{endmatheq} we add the number OF valence electrons on atoms to the number of negative charges on ion: {matheq}\begin{array}{l}\ {\text{CHO}}_{2}^{-}\ \phantom{\rule{0.48em}{0ex}}\text{C: 4 valence electrons/atom}\times \text{1 atom}=4\ \phantom{\rule{0.8em}{0ex}}\text{H: 1 valence electron/atom}\times \text{1 atom}=1\ \phantom{\rule{0.05em}{0ex}}\text{O: 6 valence electrons/atom}\times \text{2 atoms}=12\ \underline{+\phantom{\rule{6.5em}{0ex}}\text{1 additional electron}=1}\ \ \phantom{\rule{15.45em}{0ex}}=\text{18 valence electrons}\end{array}{endmatheq} For positive ion, such as NO +, we add the number OF valence electrons on atoms in ion and then subtract number OF positive charges on ion from total number OF valence electrons: {matheq}\begin{array}{l}\ \ {\text{NO}}^{+}\ \text{N: 5 valence electrons/atom}\times \text{1 atom}=5\ \ \phantom{\rule{0.4em}{0ex}}\text{O: 6 valence electron/atom}\times \text{1 atom}=6\ \phantom{\rule{0.35em}{0ex}}\underline{+{-1 electron (positive charge)}\phantom{\rule{1.8em}{0ex}}=-1}\ \ \phantom{\rule{15.02em}{0ex}}=\text{10 valence electrons}\end{array}{endmatheq} since OF 2 is neutral molecule, We simply add number OF valence electrons: {matheq}\begin{array}{l}\ \phantom{\rule{0.8em}{0ex}}{\text{OF}}_{\text{2}}\ \phantom{\rule{1.25em}{0ex}}\text{O: 6 valence electrons/atom}\times \text{1 atom}=6\ \underline{+\text{F: 7 valence electrons/atom}\times \text{2 atoms}=14}\ \phantom{\rule{16.28em}{0ex}}=\text{20 valence electrons}\end{array}{endmatheq} 2. Draw the skeleton structure of a molecule or ion, arranging atoms around the central atom and connecting each atom to the central atom with a single bond. When several arrangements OF atoms are possible, as for {matheq}{\text{CHO}}_{2}^{-},{endmatheq} we must use experimental evidence to choose the correct one. In general, less electronegative elements are more likely to be central atoms. In {matheq}{\text{CHO}}_{2}^{-},{endmatheq} less electronegative carbon atom occupies central position with oxygen and hydrogen atoms surrounding it. Other examples include P in POCl 3, S in SO 2, and Cl. In {matheq}{\text{ClO}}_{4}^{-}.{endmatheq} exception is that hydrogen is almost never the central atom. Like most electronegative element,ss fluorine also cannot be central atom. 3. Distribute remaining electrons as lone pairs on terminal atoms to complete their valence shells with octet OF electrons. There are NO remaining electrons on SiH 4, SO it is unchanged: For SiH 4, {matheq}{\text{CHO}}_{2}^{-},{endmatheq} and NO +, there are NO remaining electrons; We already place all OF electrons determined in Step 1. For OF 2, we had 16 electrons remaining in Step 3, and we Place 12, leaving 4 to be Place on central atom: 5. Rearrange electrons OF outer atoms to make multiple bonds with central atom in order to obtain octets wherever possible.

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### Glossary

##### Lewis Structures

For very simple molecules and molecular ions, we can write Lewis structures by merely pairing up unpaired electrons on constituent atoms. See these examples: For more complicated molecules and molecular ions, it is helpful to follow the step - by - step procedure outlined here: determining total number of valence electrons. For cations, subtract one electron for each positive charge. For anions, add one electron for each negative charge. Draw skeleton structure of a molecule or ion, arranging atoms around the central atom. Connect each atom to the central atom with a single bond. Distribute remaining electrons as lone pairs on terminal atoms, completing octet around each atom. Place all remaining electrons on the central atom. Rearrange electrons OF outer atoms to make multiple bonds with central atom in order to obtain octets wherever possible. Let us determine Lewis structures OF SiH 4, CHO 2, NO +, and OF 2 as examples in following this procedure: determine the total number OF valence electrons in molecule or ion. For molecule, we add the number OF valence electrons on each atom in molecule: {matheq}\begin{array}{r r l} \text{SiH}_4 & & \ {matheq}1em] & \text{Si: 4 valence electrons/atom} \times 1 \;\text{atom} & = 4 \ {matheq}1em] \rule[-0.5ex]{21em}{0.1ex}\hspace{-21em} + & \text{H: 1 valence electron/atom} \times 4 \;\text{atoms} & = 4 \ {matheq}1em] & & = 8 \;\text{valence electrons} \end{array}{endmatheq} For negative ion, such as CHO 2 −, we add the number OF valence electrons on atoms to the number of negative charges on ion: {matheq}\begin{array}{r r l} {\text{CHO}_2}^{-} & & \ {matheq}1em] & \text{C: 4 valence electrons/atom} \times 1 \;\text{atom} & = 4 \ {matheq}1em] & \text{H: 1 valence electron/atom} \times 1 \;\text{atom} & = 1 \ {matheq}1em] & \text{O: 6 valence electrons/atom} \times 2 \;\text{atoms} & = 12 \ {matheq}1em] \rule[-0.5ex]{21.5em}{0.1ex}\hspace{-21.5em} + & 1\;\text{additional electron} & = 1 \ {matheq}1em] & & = 18 \;\text{valence electrons} \end{array}{endmatheq} For positive ion, such as NO +, we add the number OF valence electrons on atoms in ion and then subtract number OF positive charges on ion from total number OF valence electrons: {matheq}\begin{array}{r r l} \text{NO}^{+} & & \ {matheq}1em] & \text{N: 5 valence electrons/atom} \times 1 \;\text{atom} & = 5 \ {matheq}1em] & \text{O: 6 valence electrons/atom} \times 1 \;\text{atom} & = 6 \ {matheq}1em] \rule[-0.5ex]{21em}{0.1ex}\hspace{-21em} + & -1 \;\text{electron (positive charge)} & = -1 \ {matheq}1em] & & = 10 \;\text{valence electrons} \end{array}{endmatheq} since OF 2 is neutral molecule, We simply add number OF valence electrons: {matheq}\begin{array}{r r l} \text{OF}_{2} & & \ {matheq}1em] & \text{O: 6 valence electrons/atom} \times 1 \;\text{atom} & = 6 \ {matheq}1em] \rule[-0.5ex]{21em}{0.1ex}\hspace{-21em} + & \text{F: 7 valence electrons/atom} \times 2 \;\text{atoms} & = 14 \ {matheq}1em] & & = 20 \;\text{valence electrons} \end{array}{endmatheq} draw skeleton structure OF molecule or ion, arranging atoms around central atom and connecting each atom to central atom with single bond. When several arrangements OF atoms are possible, as for CHO 2 −, we must use experimental evidence to choose the correct one. In general, less electronegative elements are more likely to be central atoms. In CHO 2 −, less electronegative carbon atoms occupy central position with oxygen and hydrogen atoms surrounding them. Other examples include P in POCl 3, S in SO 2, and Cl in ClO 4 −. The exception is that hydrogen is almost never the central atom. Like most electronegative element,ss fluorine also cannot be central atom. Distribute remaining electrons as lone pairs on terminal atoms to complete their valence shells with octet OF electrons. There are NO remaining electrons on SiH 4, SO it is unchanged: Place all remaining electrons on the central atom. For SiH 4, CHO 2 −, and NO +, there are NO remaining electrons; We already place all OF electrons determined in Step 1.

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### Fullerene Chemistry

The first law of thermodynamics states that energy can be transferred or transform, but cannot be created or destroy. Thermodynamics is study of heat energy and other types of energy, such as work, and various ways energy is transferred within chemical systems. Thermo - refers to heat, while dynamics refers to motion. The first law of thermodynamics deals with the total amount of energy in the universe. Law states that this total amount of energy is constant. In other words, there has always been, and always will be, exactly the same amount of energy in the universe. Energy exists in many different forms. According to the first law of thermodynamics, energy can be transferred from place to place or change between different forms, but it cannot be created or destroy. Transfers and transformations of energy take place around the US all the time. For instance, light bulbs transform electrical energy into light energy, and gas stoves transform chemical energy from natural gas into heat energy. Plants perform one of the most biologically useful transformations of energy on Earth: they convert energy of sunlight into chemical energy stored within organic molecules. Thermodynamics often divides the universe into two categories: system and its surroundings. In chemistry, system almost always refers to giving chemical reaction and the container in which it takes place. The first law of thermodynamics tells US that energy can neither be created nor destroy, so we know that energy that is absorbed in endothermic chemical reaction must have been lost from surroundings. Conversely, in exothermic reaction, heat that is released in reaction is given off and absorbed by surroundings. State mathematically, we have: We know that chemical systems can either absorb heat from their surroundings, if the reaction is endothermic, or release heat to their surroundings, if the reaction is exothermic. However, chemical reactions are often used to do work instead of just exchanging heat. For instance, when rocket fuel burns and causes the space shuttle to lift off from the ground, chemical reaction, by propelling rocket, is doing work by applying force over distance. If youve ever witnessed video of the space shuttle lifting off, chemical reactions that occur also release tremendous amounts of heat and light. Another useful form of the first law of thermodynamics relates to heat and work for change in energy of the internal system: {matheq}\Delta E_{sys} = Q + W{endmatheq} While this formulation is more commonly used in physics, it is still important to know for chemistry. Both heat and work refer to processes by which energy is transferred to or from substance. When energy is exchanged between thermodynamic systems by thermal interaction, transfer of energy is called heat. Units of heat are therefore units of energy, or joules. Heat is transferred by conduction, convection, and / or radiation. Heat is transferred by conduction occurs when an object with high thermal energy comes into contact with an object with low thermal energy.

##### Lewis Structures

We also use Lewis symbols to indicate the formation of covalent bonds, which are shown in Lewis structures, drawings that describe bonding in molecules and polyatomic ions. For example, when two chlorine atoms form chlorine molecule, they share one pair of electrons: Lewis structure indicates that each atom has three pairs of electrons that are not used in bonding and one share pair of electrons. Dash is sometimes used to indicate shared pair of electrons: single shared pair of electrons is called single bond. Each atom interacts with eight valence electrons: six in lone pairs and two in single bond.

##### Lewis Symbols

We use Lewis symbols to describe valence electron configurations of atoms and monatomic ions. Lewis symbols consist of an elemental symbol surrounded by one dot for each of its valence electrons: Figure 1 shows Lewis symbols for elements of the third period of the periodic table. Lewis symbols can also be used to illustrate formation of cations from atoms, as shown here for sodium and calcium: Likewise, they can be used to show formation of anions from atoms, as shown below for chlorine and sulfur: Figure 2 demonstrates use of Lewis symbols to show transfer of electrons during formation of ionic compounds.

##### The Octet Rule

Halogens form a class of compounds called interhalogens, in which halogen atoms covalently bond with each other. Write Lewis structures for interhalogens BrCl 3 and {matheq}{\text{ICl}}_{4}^{-}.{endmatheq}

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