Module 2. How Does Molecular Structure Impact Compound Properties PDF

Module 2. How does molecular structure have an impact on the properties of compounds? Matter and sustainability 2.1 Lewis structure and molecular geometry Lewis structure Also called Lewis diagram or Electron-dot diagram A simple diagram or notation to represent the valence ele...

Module 2. How does molecular structure have an impact on the properties of compounds? Matter and sustainability 2.1 Lewis structure and molecular geometry Lewis structure Also called Lewis diagram or Electron-dot diagram A simple diagram or notation to represent the valence electrons of a single atom or those of all the atoms in a molecule. The information it provides is useful when predicting the geometry and properties of a substance. Lewis structure of neutral atoms Each valence electron of an atom is represented as a dot or cross, around the elemental symbol; the maximum is 2 electrons per side , for a total of 8 electrons , because according to the "octet rule", that is the maximum of electrons in a valence shell. Examples: Sodium (Na) has 1 valence electron, so its Lewis structure is: Chlorine (Cl) has 7 valence electrons, so its Lewis structure is: Neon (Ne) is a noble gas with 8 valence electrons, so its Lewis structure is: Learning check Draw the lewis structure for each of the following elements: - Potassium, K group: ______ valence electron: ______ - Carbon, C group: ______ valence electron: ______ - Oxygen, O group: ______ valence electron: ______ Lewis structure for ionic compounds A simple ion → an atom that has acquired a net positive or negative charge by losing or gaining one or more electrons. The ionic bond occurs when the valence electrons of an atom are transferred to a second atom, so it is that the atom that loses the valence electrons (cation) gets a positive charge and its Lewis structure has zero dots. Example: Sodium lost 1 electron to form an octet, is a cation (+). Examples Magnesium Mg, has two valence electron s which it loses to form a simple ion with a +2 electrical charge. The ion is written as Mg 2+. The Lewis diagram for a positive ion is drawn without any dots, only with the charge. 2+ → Mg Zero dots Lewis structure for ionic compounds On the other hand, the atom that gains electrons (anion) gets a negative charge , completes 8 electrons in the valence shell and so its Lewis structure has 8 dots. Example: Chlorine gained 1 electron to form an octet, is an anion (-). Examples Nitrogen, N , has five valence electrons. It tends to gain three electrons to form a simple ion with a -3 electrical charge. The ion is written as N 3-. The Lewis diagram for a negative ion is drawn with all 8 dots, and the charge too. → 8 dots Lewis structure of ionic compounds Drawing both ions one in front of the other one, where the cation is on the left , anion on the right , both include their corresponding charges and subscripts that represent the proportion in the compound. Lewis structure of ionic compounds Drawing both ions one in front of the other one, where the cation is on the left , anion on the right , both include their corresponding charges and subscripts that represent the proportion in the compound. Lattice structure of ionic compounds Lewis structure of ionic compounds represents the transfer of electrons but does not show how the ions look like in real life. The geometry of all ionic compounds consists in a lattice structure which occurs because the forces of attraction between ions cause them to surround themselves with ions of opposite charge. Lattice structure of ionic compounds The ionic compound takes on a predictable three-dimensional crystalline structure known as ionic lattice, which is made of a very large number of ions that can grow indefinitely but keeps a fixed ratio of positive and negative ions. The lattice geometry varies, depending mainly on the sizes of the ions , but it always involves this fixed arrangement of repeating ions pattern. Lattice structure of ionic compounds The molecular formula of ionic compounds is a simple expression of the ratio of ions present in the lattice structure of the substance. For example: in the sodium chloride (NaCl) lattice, each Na+ ion is surrounded by six Cl- ions and each Cl- ion is surrounded by six Na+ ions; thus the formula expresses the 1:1 ratio of elements in the structure. Lewis structure of covalent molecules Covalent bond occurs when two non-metallic atoms share electrons to achieve the octet. It is that two atoms can share 2, 4 or 6 electrons forming single , double or triple bonds , respectively. The Lewis structure consists in the representation of all valence electrons in a molecule, including both, bonding (shared) electrons, and non-bonding electrons (also known as lone-pairs ). Non-bonding electrons play an important role in shape and properties of a molecule. Lewis structure of covalent molecules Lewis structure of covalent molecules 1. Calculate the total number of valence electrons in the molecule by multiplying the valence electrons of each element times the number of atoms in the molecule and totalling these. 2. Draw the skeletal structure of the molecule by writing the symbols of the atoms in the way they are linked to each other. 3. Electron pairs must be represented by a pair of crosses, dots or a single line. Put a pair of electrons for each bond between atoms. 4. Add electron pairs to complete octets (8 electrons) around all atoms. Start adding electrons by the outside atoms in the structure and continue with the central atom. Do not exceed the total of valence electrons calculated in step 1. 5. If not enough electrons to complete all the octets, form double or triple bonds by moving non-bonding electrons to sharing positions. Example CCl4 Molecular Geometry of Covalent Compounds Molecular geometry is the three-dimensional arrangement of atoms in a molecule, and which spacial distribution contributes to properties such as polarity, reactivity, aggregate state, magnetism, color, among others. The position of atoms in a molecule occurs because of the combination of attraction and repulsion forces among atoms and electrons. Valence Shell Electron Pairs Repulsion Theory This is a theory to predict the molecular geometry; it is based on the valence shell electrons of a central atom in a molecule , which repel each other because of their negative charges, thus, the molecule will achieve a geometry that minimizes the repulsion between valence electrons of such atom by taking the more distance possible between them. Types of molecular geometry Angle Molecular between Characteristics geometry atoms Linear 180° Molecules of 2 or 3 atoms Molecules of 3 atoms that have only 2 electron domains and the central atom has zero lone-pairs of electrons. Trigonal planar 120° Molecules of 1 central atom and 3 atoms around. 3 electron domains and the central atom has zero lone-pairs of electrons. Trigonal pyramidal