4.1 Atomic Theory and Bonding The very small

Definitions Atom: smallest particle of an element, has a neutral charge. PEN = proton(s) + electron(s) + neutron(s) ELEMENT: made up of 1 type of atom (eg.oxygen O) IONS: atoms with charges (eg. oxygen is O2-) MOLECULES: groups of covalently bonded atoms (eg. oxygen molecules are O2) COMPOUNDS: are made up of at least 2 atoms bonded together. Hydrogen and oxygen are atoms/elements H2O is a compound

Structure of an atom Atomic mass for proton is 1 AMU, neutron is 1 AMU, and electron is 1/1836

numbers to remember Protons = Atomic Number (see periodic table in data booklet) Neutrons = Mass number – Atomic number (Mass # - proton #). Neutrons + Protons = Mass # Electrons in an atom = atomic number (also proton #) Electrons in an ion = atomic number – ion charge

Families of the periodic table Columns of elements are called groups, or families Group 1 = alkali metals (1+, highly reactive) All elements in a family have… Group 2 = alkaline earth metals (2+, reactive) similar properties Group 17 = the halogens (1-, very reactive) bond with other elements in similar ways Group 18 = noble gases (0, unreactive) have the same number of valence electrons Periods are horizontal rows on the periodic table. Family names (on the periodic table!):

The Periodic Table Where are the following? See page 172 of text Atomic number Period Group/Family Metals Non-metals Transition metals Metalloids Alkali metals Alkaline earth metals Halogens Noble gases See page 172 of text

Bohr diagrams Bohr diagrams show how many electrons appear in each electron shell around an atom. (2, 8, 8, 18, 18) Valence electrons: electrons in the outermost shell ONLY. If the valence shell is full = stable If the valence shell is not full = reactive

Bohr diagrams Electrons appear in shells in a very predictable manner. There is a maximum of two electrons in the first shell, eight in the 2nd shell, and eight in the 3rd shell. The period number = the number of shells in the atom. Except for the transition elements, the last digit of the group number = the number of electrons in the valence shell. The noble gas elements have full electron shells and are very stable.

bohr diagrams What element is this? It has 2 + 8 + 8 = 18 electrons, and therefore 18 protons It has 8 electrons in the outer (valence) shell (wait for it) It’s argon!

Lewis Diagrams Only valence electrons are shown Dots representing valence electrons are placed around the element symbols (on 4 sides, imagine a box around the symbol) Electron dots are placed singularly, then they are paired

Periodic Table & Ion Formation Ions: Atoms that gain or lose electrons to become stable (full valence shells). Ions are now electrically charged particles, unlike atoms 1. Cations: metals lose electrons & form positive ions (Na+) Multivalent: Some metals can have more than one charge (Fe2+ or Fe3+). 2. Anions: Non-metals gain electrons & form negative ions (O2-) Atoms gain and lose electrons in an attempt to have the same number of valence electrons (electrons farthest from the nucleus) as the nearest noble gas in the periodic table.

Bonds with bohr Ionic bonds with Bohr Formed between Metals (cations) & non-metals (anions). Valence electrons are transferred from metal to nonmetal. Eg. Li2O Covalent bonds with Bohr Formed between two or more non-metals Valence electrons are shared between atoms Eg. HF

ionic bonds with lewis Remove or add electron dots to make full valence shells. Square brackets and the charge are placed around each ion Nitrogen ion

covalent bonds with lewis valence electrons are drawn to show sharing of electrons. Remember: All atoms “like” to have a full valence shell The shared pairs (“bonding pairs”) of electrons are usually drawn as a straight line “lone pairs” are the electrons not shared Valence electrons are represented by lines for electron pair bonds and dots for unbonded electrons.

Steb-by-step guide to drawing lewis diagrams Try NaCl and CO2 1. Find the total number of electrons: Tabulate the total number of outer energy level electrons for all atoms in the molecule. For each atom, read the group number. 2. Draw a first tentative structure: The element with the smallest atomic number is usually the central element. Draw a tentative molecular and electron arrangement attaching other atoms with single bonds as the first guess. Single bonds represented with a line represent 2 electrons 3. Add electrons as dots to get octets around atoms: When counting electrons for the octet around an atom, count both electrons in a bond for each atom and any lone pair electrons. Hydrogen, of course, gets only 2 electrons. 4. Count the total number of electrons in the final structure to see if the total agrees with the number tabulated in step #1. If not, then move a lone pair of electrons into a double bond. Or add more lone pairs of electrons. 5. Cycle through steps 3 and 4 several times until you get it right by trial and error.

lewis diagrams of diatomic molecules Diatomic molecules, like O2, are also easy to draw as Lewis diagrams. Several non-metals join to form diatomic molecules. Valence electrons are shared, here in two pairs. This is drawn as a double bond.