Chapter 7: Problem 72
Based on the concept of formal charge, what is the central atom in (a) HCN (do not include \(H\) as a possibility)? (b) \(\mathrm{NOCl}(\mathrm{Cl}\) is always a terminal atom)?
Short Answer
Expert verified
Answer: (a) In HCN, the central atom is carbon (C). (b) In NOCl, the central atom is nitrogen (N).
Step by step solution
01
Calculate the formal charges of each atom in HCN
Start by drawing the Lewis structure of HCN. The atoms are connected as follows: H-Cā”N. The formal charge on an atom can be calculated using the formula: Formal charge = (Valence electrons) - (non-bonding electrons) - (bonding electrons)/2.
For carbon (C), formal charge = (4 valence electrons) - (0 non-bonding electrons) - (4 bonding electrons)/2 = 0
For nitrogen (N), formal charge = (5 valence electrons) - (2 non-bonding electrons) - (6 bonding electrons)/2 = 0
Since both carbon and nitrogen have a formal charge of 0 and hydrogen cannot be a central atom, carbon (C) is the central atom in HCN.
02
Calculate the formal charges of each atom in NOCl
For NOCl, draw the Lewis structure in which the nitrogen atom (N) is connected to both the oxygen atom (O) with a double bond and the chlorine atom (Cl) with a single bond: N=O-Cl. Calculate the formal charges for each atom using the same formula as in step 1.
For nitrogen (N), formal charge = (5 valence electrons) - (0 non-bonding electrons) - (6 bonding electrons)/2 = 0
For oxygen (O), formal charge = (6 valence electrons) - (4 non-bonding electrons) - (4 bonding electrons)/2 = 0
For chlorine (Cl), formal charge = (7 valence electrons) - (6 non-bonding electrons) - (2 bonding electrons)/2 = 0
Since all atoms in NOCl have a formal charge of 0, and chlorine is always a terminal atom, the nitrogen (N) atom is the central atom in NOCl.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Understanding Lewis Structures
Lewis structures are visual representations of molecules that show how atoms are bonded together and the distribution of valence electrons around them. They are named after Gilbert N. Lewis, who introduced them in 1916. Constructing a correct Lewis structure is fundamental in predicting the geometry of a molecule, its reactivity, state of hybridization, and much more.
In drawing a Lewis structure, we must first count the valence electrons for each atom, which are the electrons in the outermost shell involved in bond formation. After determining the number of valence electrons, we then distribute them to form bonds between atoms and to fill the octets (or duplets for hydrogen) of each atom. Lone pairs, also called non-bonding electrons, are represented as dots around the atom.
In drawing a Lewis structure, we must first count the valence electrons for each atom, which are the electrons in the outermost shell involved in bond formation. After determining the number of valence electrons, we then distribute them to form bonds between atoms and to fill the octets (or duplets for hydrogen) of each atom. Lone pairs, also called non-bonding electrons, are represented as dots around the atom.
Steps to Draw a Lewis Structure:
- Determine the total number of valence electrons in the molecule.
- Choose the central atom (usually the least electronegative one).
- Connect the atoms with single bonds and start fulfilling the octet rule.
- Place the remaining valence electrons around the atoms to complete their octets as lone pairs.
- If needed, create double or triple bonds to ensure all atoms (except hydrogen) have full octets.
Decoding Valence Electrons
Valence electrons are the highest energy level electrons of an atom and are crucial for determining how an atom will chemically react with others. In simple terms, valence electrons can be considered the 'hands' of atoms that they use to hold onto other atoms to form chemical bonds.
Each element's number of valence electrons is indicated in its group number on the periodic table for groups 1 (IA) to 2 (IIA) and 13 (IIIA) to 18 (VIIIA). For example, carbon in group 14 has four valence electrons, while oxygen in group 16 has six.
Each element's number of valence electrons is indicated in its group number on the periodic table for groups 1 (IA) to 2 (IIA) and 13 (IIIA) to 18 (VIIIA). For example, carbon in group 14 has four valence electrons, while oxygen in group 16 has six.
Importance of Valence Electrons:
- They determine the chemical properties of an element.
- They are used in predicting the types of bonds (single, double, or triple) an atom can make.
- Valence electrons are shared, gained, or lost during chemical reactions to form compounds.
Identifying the Central Atom
In molecular structures, the central atom is typically the one that is least electronegative (with some exceptions) and has the highest capacity to form bonds due to having a greater number of valence electrons. Its identification is crucial for constructing the correct three-dimensional shape of the molecule, and subsequently, for understanding its chemical properties and behavior.
Most molecules tend to have a single central atom that all other atoms (known as terminal atoms) are bonded to. There are some general guidelines to identify it:
Most molecules tend to have a single central atom that all other atoms (known as terminal atoms) are bonded to. There are some general guidelines to identify it:
Guidelines for Central Atom Identification:
- The central atom is often the atom that can make the most bonds.
- Halogens and hydrogen are almost always terminal atoms.
- If carbon is present, it often serves as the central atom since it can form four bonds.