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Structure+and+Bonding+(1) (1).docx

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University of Toronto St. George
Doug Thomson

Note: Chapter 1 is about 23-25 pages long, I condensed it into • The three about 4 pages, and it has all the main points, every bolded different p word and even pictures when possible. It took me a long time orbitals within a given shell to pick out the necessary parts and make it all neat like this so are oriented in hopefully it’s useful! All images are courtesy of google space along images. mutually perpendicular directions, denoted Structure and Bonding px, y and pz. Organic chemistry is the study of carbon compounds  its Each p orbital is separated position in the periodic table and electronic structure makes it by a region peculiar.As a group 4Aelement, carbon can share four of zero electron density called a node. valence electrons and form four strong covalent bonds. They can chain together, and is able to form a huge array of compounds, from methane to DNA. • 1.1 Atomic Structure: The Nucleus • 1.3Atomic Structure: Electron • Nucleus consists of positive subatomic particles called Configurations protons, which are electrically neutral. There are an even number of negatively charged electrons as the • The lowest-energy protons. arrangement, or - • 10e typical atom is around 200 picometers, or 2 x 10 ground-state electron m in diameter. configuration of an atom is a listing of the orbitals • Atomic number (Z) gives the number of occupied by its electrons…We predict this protons/electrons it contains, and mass number (A) arrangement via: which gives the total number of protons plus neutrons in its nucleus. Rule 1: Lowest energy orbitals fill up first, 1s -> 2s -> • Atoms with the same atomic number but different 2p -> 3s -> 3p -> 4s ->3d, called the aufbau principle. mass numbers are called isotopes. Rule 2: Electrons act as if they spin around an axis; this spin can have two orientations, up or down,( • 1.2 Atomic Structure: Orbitals ↑ or ↓ ) and only two electrons can occupy an orbital, • How are electrons distributed? – According to the of opposite spin, called Pauli exclusion principle. quantum mechanical model, the behavior of a specific electron can be described in a mathematical equation Rule 3: If two or more empty orbitals of equal energy called a wave equation – the solution to the equation is are available, electrons occupy spins parallel until all called a wave function or an orbital noted as the orbitals are half-full, called the Hund’s rule. Ψ Greek letter psi . • We plot Ψ 2in 3-D space, and find that an orbital • 1.4 Development of Chemical Bonding Theory describes a volume of space around a nucleus that an • In 1858,August Kekule andArchibald Couper electron is most likely to occupy. I.e Imagine looking at a photograph of an electron with low shutter speed, independently proposed that carbon is tetravalent and the image would look like a blurry cloud, indicating always forms four bonds when joining other elements where the electron has been. to form stable compounds, and can even form extended chains! • The different orbitals are denoted s, p, d and f, each with a different shape. S orbitals are spherical, p orbitals are dumbbell-shaped, and 4 of 5 d orbitals are cloverleaf-shaped, while the fifth just looks like a dumbbell with a doughnut in the middle. • Orbitals are organized into electron shells centered around the nucleus gaining successive size and energy as the shell #increases. The first shell contains a single s orbital (1s), and thus holds only 2 electrons, the second shell contains one 2s orbital and three 2p orbitals thus 8 electrons. • This view was two dimensional, and until 1874, the third dimension was not proposed, and that the four bonds of carbon are in fact not random, but have specific spatial directions. • Why do atoms bond together, and how can the bonds be described electronically. The why is easy, atoms bond because the resultant compounds are usually more stable and lower in energy  energy, usually heat, always flows out of the chemical system when a bond forms, and is always needed to be put in to break a bond. The how is tricky without knowing more about the electronic properties of atoms. • The atom’s outermost shell, or valence shell, imparts special stability to the noble gases, and that main- group elements tend to take on electron configuration • To describe how electron sharing leads to bonds of the nearest noble gas. i.e) lithium, group 1A, tends between atoms, two models have been developed to to lose an s electron to achieve noble-gas describe covalent bonding: valance bond theory and configuration as a cation. Chlorine, a group 7A element tends to gain a p electron to fill the valence molecular orbital theory. shell and form an anion. The resultant ions, Li Cl are • According to the valence bond theory which is easier to visualize, a covalent bond forms when two atoms held together by electrostatic attraction called an ionic approach each other closely and a singly occupied bond. orbital on one atom overlaps a singly occupied orbital on the other atom. The electrons are now paired in the • Elements in the middle of the periodic table, like overlapping orbitals and are attracted to the nuclei of carbon are different. CH 4 methane, is not ionic, but both atoms, thus bonding the atoms together. rather electrons are shared in something called a • (The H-H bond results from the overlap of two singly covalent bond. The natural collection of atoms held occupied hydrogen 1s orbitals) together by these bonds are called molecules. • The H-H bond is cylindrically symmetrical  such • We can indicate bonds, when formed by the head on overlap of two covalent bonds in atomic orbitals along a line drawn between the nuclei, σ are called sigma bonds. • Since the product, H2has 436 kJ/mol less energy than the two starting H atoms, the product is more stable than the reactants, meaning H-H has a bond strength of 436 kJ/mol. AKAwe need to put that much energy molecules via Lewis Stuctures, or electron-dot into the H-H bond to break it apart. structures, where valence shell electrons are • Bond length refers to the optimum distance between represented as dots. Even simpler is the Kekule nuclei leading to maximum stability. structure, or line-bond structure, where two-electron covalent bonds are indicated as lines between atoms. 3 • 1.6 sp Hybrid Orbitals and the Structure of Methane • Valence electrons that are not used for bonding are called lone-pair electrons, or non-bonding electrons. • Bonding in hydrogen molecules seems to be pretty simple…however the situation becomes complicated in organic molecules with tetravalent carbon atoms. • 1.5 Describing Chemical Bonds: Valance Bond For example, methane, CH , 4as four valence Theory electrons (2s 2p ) and forms four bonds. Two different orbital types means two different bonds, • Imagin
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