Chapter 1 Lesson 1 - Intro to Organic Chemistry

What is Organic Chemistry? Organic Chemistry is the branch of chemistry involving organic compounds. Why carbon? The carbon atom can form four covalent bonds to other atoms. Carbon atoms can link together in chains almost any length, branched chains as well as straight chains or even rings. Carbon can form single, double or triple bonds. Atomic Structure and Ground State Electron Configurations At the center of an atom there is a positively charged nucleus, composed of protons and neutrons. The size of the atom is essentially defined by the size of its electron cloud. Most of the electron cloud is empty space. An atom has no net charge. The number of electrons equals the number of protons. The number of protons in the nucleus is known as the atomic number (Z). If the number of protons and electrons are unequal, then the entire species called an ion bears a charge. Cation more protons than electrons Anion more electrons than protons   Quantum Numbers To completely describe an electron in an atom, four quantum numbers are needed: energy or shell (n), angular movement (l), magnetic movement (m), and spin (ms). The principle quantum number (n) describes the electron shell, or energy level, of an atom. The value of n ranges from 1 to the shell containing the outermost (Valence) electron of that atom. The angular momentum (l) quantum number describes the shape of the orbital. Values of l range from 0 (an s orbital) to n-1. The magnetic moment (ml) quantum number describes the direction of the orbital. Values of ml range from -l to 0 to +l. The electron spin (ms) can have two values, +1/2 and -1/2.   Atomic Orbitals and Shells Atomic orbitals are organized in shells that are defined by the Principle quantum number, n. Atomic orbitals have different shapes defined by the angular momentum quantum number, l. Up to two electrons are allowed in any orbital. With increasing shell number, the size and energy of the atomic orbital increases. Within a given shell, an atomic orbital's energy increases in the following order s, p, d, f, etc Ground State Electron Configurations Valence Electrons and Core electrons The way in which electrons are arranged in atomic orbitals is called the atom's electron configuration •The most stable (i.e lowest energy) electron configuration is called the ground state configuration •Electrons occupying the outermost shell (i.e highest energy) are Valence electrons The rest are core electrons The Three Rules to Obtaining an Atom's Ground State Electron Configuration Pauli's exclusion principle No more than two electrons can Occupy a single orbital two electrons in the same orbital must   have opposite spins 2 Aufbau principle Each successive electron must fill the lowest- energy orbital orbital available 3. Hund's Rule Before a second electron can be paired in the Same orbital all other orbitals at the same energy must contain   a single electron Ground State Electron Configuration of Carbon Atom   •Carbon has 6 protons in its nucleus therefore 2s 1 2p11 the carbon atom has 6 electrons •Two electrons occupy the Is atomic orbital and the remaining four are valence electrons in the Second energy level (n=2) Since only two electrons can pair up to occupy the 25 orbital, the remaining two electrons singly occupy degenerate 2p orbitals The Covalent Bond A covalent bond is characterized by the sharing of valence electrons between two or more atoms A covalent bond is characterized by the sharing of valence electrons between two or more atoms Why is a H2 molecule more stable than two isolated H atoms? bond strength increases; bond length decreases Single bonds are the most common type of bond found in organic molecules The main difference among single, double, and triple bonds is the number of electrons involved Single bond: two shared electrons Double bond: four shared electrons Triple bond: six shared electrons Lewis dot structures (Lewis structures) are a convenient way to show a molecule's connectivity. (a) H₂ (b) CH₄ (c) CO₂ (d) NH₃ Each H has a share of 2e- Carbon's octet consists of 8 shared e- Oxygen's octet consists of 4 shared e- and 4 unshared e- Nitrogen's octet consists of 6 shared e- and 2 unshared e- H-H H-C-H H H O=C=O H-N-H H Each H has a share of 2e- • Steps to Drawing a Lewis Structure Count the total number of valence electrons in the molecule Write the skeleton of the molecule, showing only the atoms and the single bonds required to hold them together Subtract two electrons from the total in Step 1 for each single covalent bond drawn in Step 2   Distribute the remaining electrons as lone pairs If an atom has less than a filled valence shell, convert lone pairs from neighboring atoms into bonding pairs of electrons, creating double and/or triple bonds