Atomic structure orbitals

200 picometers (pm), where 1 pm = 10 m. To give you an idea of how small this is, a thin pencil line is about 3 million carhon atoms wide. Many organic chemists and biochemists, particularly in the United States, still use the unit angstrom (A) to express atomic distances, where 1 A = 100 pm = 10 m, but well stay with the SI unit picometer in this book. 

A specihc atom is described by its atomic number (Z), which gives the number of protons (or electrons) it contains, and its mass number A), which gives the total number of protons plus neutrons in its nucleus. All the atoms of a given element have the same atomic number—1 for hydrogen, 6 for carbon, 15 for phosphoms, and so on— but they can have different mass numbers depending on how many neutrons they contain. Atoms with the same atomic number but different mass numbers are called isotopes. 

The weighted average mass in atomic mass units (amu) of an element s naturally occurring isotopes is called the element s atomic mass (or atomic weight)—1.008 amu for hydrogen, 12.011 amu for carbon, 30.974 amu for phosphoms, and so on. Atomic masses of the elements are given in the periodic table in the front of this book. 

How are the electrons distributed in an atom You might recall from your general chemistry course that, according to the quantum mechanical model, the behavior of a specific electron in an atom can be described by a mathematical expression called a wave equation— the same type of expression used to describe the motion of waves in a fluid. The solution to a wave equation is called a wave function, or orbital, and is denoted by the Greek letter psi (tf). 

By plotting the square of the wave function, in three-dimensional space, an orbital describes the volume of space aroimd a nucleus that an electron is most likely to occupy. You might therefore think of an orbital as looking like a photograph of the electron taken at a slow shutter speed. In such a photo, the orbital would appear as a blurry cloud, indicating the region of space where the electron has been. This electron cloud doesn t have a sharp boundary, but for practical purposes we can set the limits by saying that an orbital represents the space where an electron spends 90% to 95% of its time. 

What shapes do orbitals have There are four different kinds of orbitals, denoted s, p, d, and f Of the four, we ll be concerned primarily with and p orbitals because these are the most important in organic chemistry. The 

Representations of s, p, and d orbitals. The s orbitals are spherical, the p orbitals are dumbbell-shaped, and four of the five d orbitals are cloverleaf-shaped. Different lobes of p orbitals are often drawn for convenience as teardrops, but their true shape is more like that of a doorknob, as indicated fay the computer-generated representation of a 2p orbital on the right. 

The distribution of electrons in an atom. The first shell holds a maximum of two electrons in one Is orbitai the second shell holds a maximum of eight electrons in one 2s and three Ip orbitals the third shell holds a maximum of eighteen electrons in one 3s, three 3p, and five 3d orbitals and so on. The two electrons in each orbital are represented by up and down arrows.  

Shapes of the 2p orbitals. Each of the three mutually perpendicular dumbbell-shaped orbitals has a node between its two lobes. 

The three different p orbitals within a given shell are oriented in space along mutually perpendicular directions, denoted and p-. As shown in 

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Atoms, structures, orbitals 1 component 2 component 3 component Calculation E by [7.8.14] Notes... 

The chemistry of molecules consists of three major modules molecular architecture (structure) molecular dynamics (conformational analysis) and molecular transformation (chemical reactions). The molecular architecture consists of the basic principles of molecular structure and it deals with the atomic structure, orbitals, hybridization and bonding. Molecular dynamics deals with the molecular motion involving rotation around chemical bonds, steric interactions, torsional strain and properties associated with the conformational changes. Molecular transformation accounts for bond formation and bond breaking within the molecule or between molecules, which is generally called the chemical reaction, and consists of two major aspects, reaction mechanism and kinetics. The third module is one of the major areas of chemistry. This aims to understand the reaction mechanism and its manipulation to reduce the reaction barrier, improve stereoselectivity, increase product yield, or suppress undesirable side reactions. 

Atomic Structure The Nucleus Atomic Structure Orbitals 4 Atomic Structure Electron Configurations 6 Development of Chemical Bonding Theory 7 The Nature of Chemical Bonds Valence Bond Theory sp Hybrid Orbitals and the Structure of Methane 12 sp Hybrid Orbitals and the Structure of Ethane 13 sp2 Hybrid Orbitals and the Structure of Ethylene 14 sp Hybrid Orbitals and the Structure of Acetylene 17 Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur 18 The Nature of Chemical Bonds Molecular Orbital Theory 20 Drawing Chemical Structures 21 Summary 24... 

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