LEQ: How do molecules form from atoms? Assignment: Reading 2.1, 32.1 (next unit) Activator: electromagnetism The electrical and magnetic properties of matter are related. What happens when you place “like” ends of a magnet together? Predict: what do you think will happen when you place a like charge near a like charge? Key terms: compound, molecule, ion

Protons determine the element (type of atom) (+) charge Neutrons - nuclear stability and decay: no charge Electrons participate in electromagnetism and bonding (-) charge Oxygen atom (O) Nucleus: 8 protons (+) 8 neutrons outermost energy level: 6 electrons (-) inner energy level: 2 electrons (-) The bonding principles we will consider will focus on two particles in atoms: Protons which are positively charged (+) Electrons which are negatively charged (-) Electrons can move during chemical reactions and are attracted to regions of opposite charge such as the positively charged nucleus of another atom in close proximity.

Dozens of elements are essential to life We can make some generalizations about the elements colored in yellow (and certainly others) if we consider the arrangement of the periodic table. Group 1, 2, and 13-18 are often depicted on charts as Group IA-VIIIA elements (America’s old CAS system; the new IUPAC convention numbers each group 1-18). We will often refer to the CAS system IA-VIIIA to keep track of valence electron number. For example: Group 1 or IA from our class chart has a single valence electron Group 2 or IIA from our class chart has two valence electrons Group 17 or VIIA from our class chart has 7 valence electrons

Compounds consist of atoms of different elements bonded together. water (H2O) carbon dioxide (CO2); shown below Overlap in energy levels indicates bonding A compound is a chemical consisting of two or more elements. Water and carbon dioxide are both chemical compounds held together by covalent bonds (depicted in figure, note the position of electrons implying that the atoms are sharing them). Each formula given suggests that these compounds have a fixed ratio: Water consists of two atoms of hydrogn and one atom of oxygen Carbon dioxide consists of a single carbon atom and two oxygen atoms

Atoms share pairs of electrons in covalent bonds. Covalent bonds are key features of molecules atoms share a pair(s) of electrons. multiple covalent bonds possible covalent bonds Oxygen atom (O) Carbon atom (C) Carbon dioxide (CO2 ) Covalent bonds are a characteristic of chemical compounds that have stable, high energy bonds due to the sharing of electron pairs. We can refer to carbon dioxide as a molecule because it consists of atoms bonded together covalently In our class challenge problem we created a lewis structure for the phosphate ion (PO43-). In some circles it is perfectly fine to describe this ion as a molecule since there are covalent bonds in its structure. Molecules can readily ionize in the chemistry of life under certain conditions!

Ions form when atoms gain or lose electrons. Ionic bonds (compounds) form between oppositely charged ions. Weaker than covalent bonds positive ions: cations negative ions: anions Sodium atom (Na) Chlorine atom (CI) Sodium ion (Na+) Chloride ion (CI-) Na loses an electron to CI ionic bond gained electron An ion is an atom that has acquired an electrical charge due to a difference in proton and electron number. This change in charge is of whole integer values (1, 2, etc.). It is important to note that atoms can readily ionize in close proximity to other ionizable atoms. Sodium (Na) and Chlorine (Cl) are great examples. Sodium (IA element) has an oxidation number of +1 and Calcium (IIA element) has an oxidation number of +2, while sulfur (VIA) has an oxidation number of -2 and chlorine (VIIA) has an oxidation number of -1. Unfortunately the rules for oxidation can get really complicated, but this trend will work for single atoms that ionize in these groups. Ionic bonds form when electrons are transferred from an electron donor, which becomes a cation, to an electron recipient, which becomes an anion. The electrical charge differences between these two ions will attract each other (an any other chemical with an opposite charge) and form a weak electrostatic interaction. It is in appropriate to describe these compounds as molecular if no covalent bonding is involved. Plain old “ionic compound” will do. Ionic bonds are much weaker when compared to covalent bonds.

Summary: Compare and contrast: What are the similarities and differences between covalent bonds and ionic bonds? How does a molecule differ from an atom?