The overwhelming majority of elements occur in chemical combination with other elements.
Noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) – occur in air as separate atoms.
Oxygen (O), nitrogen (N) and sulfur (S) occur both in compounds and in the most common elemental form as the molecules O 2 , O 3 , N 2 , S 8 .
Carbon (C) occurs both in compounds and in vast, nearly pure deposits of coal.
Gold (Au), silver (Ag), copper (Cu), and platinum (Pt) may also occur uncombined with other elements.
It is the electrons of the atoms of interacting elements that are involved in compound formation.
Elements combine in two general ways: <ul><li>Transferring electrons from the atoms of one element to those of another to form ionic compounds . </li></ul><ul><li>Sharing electrons between atoms of different elements to form covalent compounds . </li></ul>
Electrons of interacting elements generate chemical bonds, the forces that hold the atoms of elements together in a compound.
Ions are charged particles that form when an atom or small group of atoms gains or loses one or more electrons.
Ionic compounds are neutral, they possess no net charge.
For ionic compounds to be neutral, they must contain equal numbers of positive and negative charges but not necessarily equal numbers of positive and negative ions.
Metals loses electrons and nonmetals gain electrons to form ions with the same number of electrons as in the nearest noble gas.
Be careful not to get the false impression that the elements in Group 7A are closer to the noble gases than the elements in Group 1A.
A sample problem on predicting the ion an element forms.
Covalent compounds form when elements share electrons. It usually occurs between nonmetals.
Covalent bond is a pair of electrons mutually attracted by the two nuclei at some optimum distance between them.
Simple cases of covalent compounds are H 2 , O 2 , N 2 , F 2 , Cl 2 , Br 2 , P 4 , S 2 , S 8 , Se 8
Covalent substances may be gases, liquids, or solids at room temperature.
Key distinctions between covalent and ionic substances: <ul><li>Most covalent substances consist of molecules; no molecules exist in a sample of an ionic compound. </li></ul><ul><li>Covalent bonding involves mutual attraction between two nuclei and two electrons; ionic bonding involves attraction between positive and negative ions. </li></ul>
Polyatomic ion is a cation or an anion derived from a small group of atoms. These atoms, two or more, are bonded covalently.
Chemical formula is a denotation of chemical composition of a substance. It is comprised of the element symbols and numerical subscripts that show the type and number of each atom present in the smallest unit of the substance.
Types of chemical formulas: <ul><li>An empirical formula indicates the relative number of atoms of each element in the compound. It is the simplest type of formula. </li></ul><ul><li>A molecular formula shows the actual number of atoms of each element in a molecule of the compound. </li></ul><ul><li>A structural formula shows the number of atoms and the bonds between them, that is, the relative placement and connections of atoms in the molecule. </li></ul>
Some points to note about ion formulas: <ul><li>Members of a periodic table have the same ionic charge; e. g. Li, Na, and K are all in Group 1A and all have a 1+ charge. </li></ul><ul><li>For A-group cations, ion charge equals the group number; e. g. Na + is in Group 1A, Ba 2+ in Group 2A. </li></ul><ul><li>For anions, ion charge equals the period number minus 8; e.g. S is in Group 6A (6-8=-2), so the ion is S 2- . </li></ul>
All ionic compound names give the positive ion (cation) first and the negative ion (anion) second.
A sample problem on naming binary ionic compounds.
Ionic compounds are arrays of oppositely charged ions rather than separate molecular units. There are no molecules in ionic compounds.
A formula for the formula unit not for molecules is written in case of ionic compounds. Thus, ionic compounds generally have only empirical formulas.
What formula CaBr 2 means: <ul><li>The subscript refers to the element preceding it – two bromine nuclei. </li></ul><ul><li>The subscript 1 is understood from the presence of the element symbol alone – one calcium nucleus. </li></ul><ul><li>The charge (without the sign) of one ion becomes the subscript of the other – Ca 2+ Br - gives Ca 1 Br 2 or CaBr 2 . </li></ul>
A sample problem on determining formulas of binary ionic compounds.
A sample problem on determining names and formulas of ionic compounds of elements that form more than one ion.
A sample problem on determining names and formulas of ionic compounds containing polyatomic ions.
A sample problem on recognizing incorrect names and formulas of ionic compounds.
A sample problem on determining names and formulas of anions and acids.
How to name a binary covalent compound: <ul><li>The element with the lower group number in the periodic table is the first word in the name; the element with the higher group number is the second word. </li></ul><ul><li>If both elements are in the same group, the one with the higher period number is named first. </li></ul>
How to name a binary covalent compound: <ul><li>The second element is named with its root and the suffix -ide . </li></ul><ul><li>Covalent compounds have Greek numerical prefixes to indicate the number of atoms of each element in the compound. The first word has a prefix only when more than one atom of the element is present; the second word usually has a numerical prefix. </li></ul>
A sample problem on determining names and formulas of binary covalent compounds.
A sample problem on recognizing incorrect names and formulas of binary covalent compounds.
Molecular mass equals sum of all atomic masses. The number of atoms should be multiplied by the respective subscripts.
A sample problem on calculating the molecular mass of a compound.
A sample problem on naming compounds from their depictions.