Connect each atom to the central atom with a single bond (one electron pair). Write the Lewis symbols for each of the following ions: Many monatomic ions are found in seawater, including the ions formed from the following list of elements. How do they differ. Determine the total number of valence (outer shell) electrons in the molecule or ion. Chemistry by Rice University is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted. (Generally, the least electronegative element should be placed in the center.) Lone pairs, unpaired electrons, and single, double, or triple bonds are used to indicate where the valence electrons are located around each atom in a Lewis structure. Most structures—especially those containing second row elements—obey the octet rule, in which every atom (except H) is surrounded by eight electrons. Xenon is a noble gas, but it forms a number of stable compounds. The tendency of main group atoms to form enough bonds to obtain eight valence electrons is known as the octet rule. For example, each atom of a group 14 element has four electrons in its outermost shell and therefore requires four more electrons to reach an octet. This allows each halogen atom to have a noble gas electron configuration. [latex]\begin{array}{r r l} \text{SiH}_4 & & \\[1em] & \text{Si: 4 valence electrons/atom} \times 1 \;\text{atom} & = 4 \\[1em] \rule[-0.5ex]{21em}{0.1ex}\hspace{-21em} + & \text{H: 1 valence electron/atom} \times 4 \;\text{atoms} & = 4 \\[1em] & & = 8 \;\text{valence electrons} \end{array}[/latex], [latex]\begin{array}{r r l} {\text{CHO}_2}^{-} & & \\[1em] & \text{C: 4 valence electrons/atom} \times 1 \;\text{atom} & = 4 \\[1em] & \text{H: 1 valence electron/atom} \times 1 \;\text{atom} & = 1 \\[1em] & \text{O: 6 valence electrons/atom} \times 2 \;\text{atoms} & = 12 \\[1em] \rule[-0.5ex]{21.5em}{0.1ex}\hspace{-21.5em} + & 1\;\text{additional electron} & = 1 \\[1em] & & = 18 \;\text{valence electrons} \end{array}[/latex], [latex]\begin{array}{r r l} \text{NO}^{+} & & \\[1em] & \text{N: 5 valence electrons/atom} \times 1 \;\text{atom} & = 5 \\[1em] & \text{O: 6 valence electrons/atom} \times 1 \;\text{atom} & = 6 \\[1em] \rule[-0.5ex]{21em}{0.1ex}\hspace{-21em} + & -1 \;\text{electron (positive charge)} & = -1 \\[1em] & & = 10 \;\text{valence electrons} \end{array}[/latex], [latex]\begin{array}{r r l} \text{OF}_{2} & & \\[1em] & \text{O: 6 valence electrons/atom} \times 1 \;\text{atom} & = 6 \\[1em] \rule[-0.5ex]{21em}{0.1ex}\hspace{-21em} + & \text{F: 7 valence electrons/atom} \times 2 \;\text{atoms} & = 14 \\[1em] & & = 20 \;\text{valence electrons} \end{array}[/latex], Creative Commons Attribution 4.0 International License, Write Lewis symbols for neutral atoms and ions, Draw Lewis structures depicting the bonding in simple molecules. Draw a skeleton structure of the molecule or ion, arranging the atoms around a central atom. In some hypervalent molecules, such as IF5 and XeF4, some of the electrons in the outer shell of the central atom are lone pairs: When we write the Lewis structures for these molecules, we find that we have electrons left over after filling the valence shells of the outer atoms with eight electrons. Solution For example, when two chlorine atoms form a chlorine molecule, they share one pair of electrons: The Lewis structure indicates that each Cl atom has three pairs of electrons that are not used in bonding (called lone pairs) and one shared pair of electrons (written between the atoms). Cations are formed when atoms lose electrons, represented by fewer Lewis dots, whereas anions are formed by atoms gaining electrons. The other halogen molecules (F2, Br2, I2, and At2) form bonds like those in the chlorine molecule: one single bond between atoms and three lone pairs of electrons per atom. Xenon will be the central atom because fluorine cannot be a central atom: 3. We examined XeF4 earlier. Valence electronic structures can be visualized by drawing Lewis symbols (for atoms and monatomic ions) and Lewis structures (for molecules and polyatomic ions). For example, when two chlorine atoms form a chlorine molecule, they share one pair of electrons: The Lewis structure indicates that each Cl atom has three pairs of electrons that are not used in bonding (called lone pairs) and one shared pair of electrons (written between the atoms). An atom like the boron atom in BF3, which does not have eight electrons, is very reactive. All other atoms do not have charges. The halogens form a class of compounds called the interhalogens, in which halogen atoms covalently bond to each other. Two arrangements of atoms are possible for a compound with a molar mass of about 45 g/mol that contains 52.2% C, 13.1% H, and 34.7% O by mass. (credit: United States Department of Energy), Carbon soot has been known to man since prehistoric times, but it was not until fairly recently that the molecular structure of the main component of soot was discovered. Stoichiometry of Chemical Reactions, 4.1 Writing and Balancing Chemical Equations, Chapter 6. An entire class of compounds, including spheres and tubes of various shapes, were discovered based on C60. Draw a skeleton structure of the molecule or ion, arranging the atoms around a central atom and connecting each atom to the central atom with a single (one electron pair) bond. Check Your Learning The transition elements and inner transition elements also do not follow the octet rule: Group 15 elements such as nitrogen have five valence electrons in the atomic Lewis symbol: one lone pair and three unpaired electrons. To draw the Lewis structure for an odd-electron molecule like NO, we follow the same six steps we would for other molecules, but with a few minor changes: We will also encounter a few molecules that contain central atoms that do not have a filled valence shell. These lone pairs must be placed on the Xe atom. For example, each atom of a group 14 element has four electrons in its outermost shell and therefore requires four more electrons to reach an octet. As the most electronegative element, fluorine also cannot be a central atom. In this case, we can condense the last few steps, since not all of them apply. To obtain an octet, these atoms form three covalent bonds, as in NH3 (ammonia).

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