3.2 – 3.4 Chemical Bonds, Formulas, Molecular Models, and Atomic Views of Elements and Compounds

Chemical Bonds What is a chemical bond? An attractive force that exists between charged particles; more specifically, between protons and electrons of elements in a compound. Name three types of chemical bonds, and explain the difference between all three. Ionic bonds: a electrostatic force of attraction between a cations and anions. Usually, this is the attraction between metals and nonmetals. Here electrons are transferred, and this results in an ionic compound (solid lattice structure) Covalent bonds: a force of attraction between nonmetals. Here some electrons are shared, and the result is a molecule. The molecule whose structure shows the most equal distribution of charge is the most stable, and results in that structure having lowest potential energy. In a covalent bond, shared electrons are attracted to both positively charged nuclei of the bonding atoms. Metallic bonds: a force of attraction of metal cations and the mobile electrons between metal atoms.

Chemical Formula What is a chemical formula and how is it written (which element comes first)? Give an example. A way to represent a compound by the elements that make it up. When writing a chemical formula, the most metallic element (least electronegative) is written first. MgCl 2 is an example of a chemical formula of an ionic compound.

Chemical Formulas (Continued) There are generally three types of chemical formulas: 1.) Empirical formulas, 2.) Molecular formulas, and 3.) Structural formulas. Describe each and give an example. Empirical formulas: Give the relative number (simplified or reduced) of atoms in an compound. Example: The empirical formula of glucose is CH 2 O Molecular formulas: Gives the actual number of the atoms of each element that make up a compound. Example: The molecular formula of glucose is C 6 H 12 O 6 Structural formula: uses lines to represent covalent bonds and shows how the atoms in a molecule are connected or bonded to each other. Example: Carbon tetrachloride (CCl 4 ), has the following structural formula: Cl I Cl– C–Cl I Cl

Molecular Models A more accurate way to represent a compound. Two types of molecular models are ball-and-stick models and space-filling molecular models. In a ball-and-stick models, color coded ball represent different atoms, and sticks represents the bonds that hold the atoms together. This model also shows the geometry of the molecule. In a space-filling molecular model, atoms fill the space between each other to give a better indication of what a molecule may look like if scaled to visible size. Examples of both models on next pg. 

An Atomic Level View of Elements and Compounds Atomic elements: most elements in nature exist with a single atom as their base unit (monatomic). Example: He Molecular elements usually exist in nature as molecules (most as diatomic molecules). Which elements are diatomic in nature? – The –gens (hydrogen, oxygen, nitrogen, and the halogens)

Compounds What is the difference between ionic and molecular compounds? Ionic compounds are made up of cations and anions bound by ionic bonds. The basic unit of an ionic compound is called a formula unit. Example: CaBr 2 Molecular compounds are made up of two or more covalently bonded nonmetals. The basic unit are molecules. Example: water is made up of H 2 O molecules

Polyatomic Ions Ions composed of two or more covalently bonded atoms with an overall charge. List three polyatomic ions: CO 3 2- (carbonate) SO 4 2- (sulfate) PO 4 3- (phosphate)

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