Compounds are held together with chemical bonds There are two types of bonds, ionic bonds and covalent bonds. Before we talk about them, lets talk about why they form. Octet Rule- Main group elements tend to undergo reactions to achieve a complete outer shell of electrons. Normally this is 8.
Ions If an atom gains electrons, it becomes negatively charged and called an Anion If an atom loses electrons, it becomes positively charged and called an Cation Remember, the number of protons and neutrons stay the same during most chemical reactions
Exceptions to the Octet Rule 1)Elements of periods 1 and 2 cannot have a charge greater than +2 2)Octet rule does not apply to transition elements
Atoms and Ions Atoms and their ions are completely different with different physical and chemical properties Examples:
Naming Ions Most names come from a system developed by the International Union of Pure and Applied Chemistry (IUPAC) These names are called systematic names Some ions are referred to by their common names as well
Naming Cations Elements of Group 1, 2, and 3 form only one type of ion, so we can name the element and add the word “ion” Examples:
Naming Cations Transition Metals form more than one ion so we name the element and add roman numerals to designate the charge Examples: Exception:
Older system (common names) When a metal can form 2 different ions: The suffix “-ous” is used to designate the one with the smaller charge The suffix “-ic” is used to designate the one with the large charge Example: See table 4.2, page 97!!!! (know these)
Naming Anions Monoatomic Anions are named by adding “-ide” to the stem part of the name Examples: Polyatomic anions, anions that contain more than one element, are more complicated. See/Know Table 4.4. page 97
Back to Bonding Atoms bond together in such a way that each atom achieves a valence electron configuration of the nearest noble gas. There are two ways this can happen:
Ionic Bonds 1)Atoms may gain or lose electrons to achieve a filled valence shell, thus becoming ions. An Ionic Bond results from the force of attraction between a cation and an anion
Covalent Bond 2)Atoms may share electrons to fill the valence shell A Covalent Bond results from the force of attraction between 2 atoms that share one or more pairs of electrons
Ionic bonds usually form between metals and nonmetals. Example: Covalent bonds usually form between nonmetals or between a nonmetal and a metalloid. Example:
Electronegativity Electronegativity-a measure of an atoms attraction for the electrons it shares in a chemical bond. Fluorine is the most electronegative element. All other elements are assigned values in relation to fluorine Trend in periodic table NOTE: Ionization energy and electronegativity are different!!!!
Ionic Compounds These are compounds formed by ionic bonds Ionic bonds form by the transfer of one or more valence electrons from an atom of lower electronegativity to the valence of an atom with higher electronegativity The atom that gains the electron becomes an anion The atom that loses the electron becomes a cation
The compound formed by the combination of positive and negative ions is called an ionic compound Ionic bonds usually occur when the difference in electronegativity is 1.9 or more. Example: Single head arrows are used to show the transfer of 1 electron
Ionic compounds do not consist of molecules, but their formula still gives the definite ratio of atoms Ionic compounds are overall neutral so we can predict the formula by balancing the charges Examples –Write the formula for: Lithium Ion and Bromide Ion Barium (Ba) and Iodide Ion
Naming Ionic Compounds Simply give the name of the cation first, then the anion. For Binary Ionic Compounds (contain only two elements) –Example: Binary Compounds that contain metal with multiple ions, use the Roman Numerals
Ionic Compounds containing Polyatomic ions, name positive one first, followed by negative one, as separate words Examples:
Covalent Bonds Covalent bonds- formed when electrons pairs are shared between two atoms These usually occur between two nonmetals or between a nonmetal and metalloid The electron pair is shared by two atoms and at the same time fills the valence of each atom
A bond formed by sharing a pair electrons is called a single bond Example: Lines are used to represent a shared pair of electrons
Two Types of Covalent Bonds Nonpolar Covalent Bonds- electrons in the bond are shared equally Polar Covalent Bonds- electrons in the bond are not shared equally
As a result of polar covalent bonds one atom ends up with a partial negative charge and the other ends up with a partial positive charge. Example: These molecules are said to have a dipole
Drawing Lewis Structures 1)Determine the number of valence electrons in the molecule 2)Decide on the arrangement of atoms in molecule 3)Connect the atoms with single bonds then arrange the remaining electrons in lewis dot pairs so that each atom has a complete outer shell
4) A pair of electrons involved in a covalent bond, called bonding electrons, are shown with a single line. Nonbonding electrons are shown as pairs of lewis dots 5) Single bonds share one pair of electrons and are shown with one line. Double bonds share two pairs of electrons and are shown with two lines. Triple bonds share three pairs of electrons and are shown with three lines.
Naming Binary Molecular Compounds 1) Name the less electronegative element first. 2) Use prefixes mono-, di-, tri-, tetra-, etc, to show the number of atoms of each element. (Note: Mono is omitted for the 1 st word!) Examples:
Bonds Angles and Shapes of Molecules We can predict bond angles using the Valence-Shell Electron-Pair Repulsion Theory, VSEPR. Valence electron repel each other. We treat each single bond, double bond, triple bond, and lone pair as a single electron density region.
Polar and Nonpolar Compounds Polar Compounds- posses an overall dipole over the entire molecule A molecule is polar if: –It has polar bonds –It has a center of partial positive charge and a center of partial negative charge in different locations