7.1 Chemical Names and formulas
A chemical formula indicates the relative number of atoms of each kind in a chemical
compound.
Molecular compound – chemical formula reveals the number of atoms of each element
contained in a single molecule of the compound
ex) C8H18
Ionic compound – consists of a lattice of positive and negative ions held together by mutual
attraction. → chemical formula represents one formula unit – the simplest ration of the
compound’s positive ions and its negative ions.
ex)
By gaining or losing electrons, many main-group elements form ions with noble-gas
configurations.
+
ex) G1 metals lose 1 electron → 𝑁𝑎
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Monoatomic ion
○ Ions formed from a single atom
■ Non-metals →anions
■ Not all main-group elements readily form ions → carbon and silicon form
covalent bonds – share electrons with other atoms.
ex) tin and lead → lose 4 X , lose 2 but retain the two electrons in
their outer s orbitals to form 2+ cations. ????
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Naming monoatomic
○ Cation: Just the element’s name – (ex) K+ = potassium cation
○ Anion: -ide (ex) F- = fluoride anion
Binary compound
○ Compounds composed of two elements
○ Total numbers of positive and negative charges must be equal
○ The formula for a binary ionic compound can be written given the identities of the
compound’s ions.
■ Ex. magnesium bromide / ions combined: Mg 2+, Br -, Br- → MgB2
(chemical formula)
■ Crossing over
Nomenclature
○ Naming system – combining the names of the compound’s positive and negative
ions.
■ Cation first, anion next
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○
ex)
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■ ex) Al2O3 – aluminum oxide
For most simple ionic compounds, the ratio of the ions is not given in the
compound’s name, because it is understood based on the relative charges of the
compound ions.
The stock system of Nomenclature
■ Some elements such as iron, form two or more cations with different
charges.
■ Roman numerals → Fe2+ [iron(II)]
➡
Oxyanion
○ Polyatomic ions that contain oxygen
○ Some elements can combine with oxygen to form more than one type of
oxyanion.
■
■
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ex) nitrogen ➪
The name of the ion with the greater number of
oxygen atoms ends in -ate. The name of the ion
with the smaller number of oxygen atoms ends in
-ite.
Some elements can form more than two types of oxyanions.
■ ex) chlorine
■ one fewer oxygen atom than -ite →
prefix hypo■ one more oxygen atom than -ate →
prefix perUnlike ionic compounds, molecular compounds are composed of individual
covalently bonded units, or molecules.
As with ionic compounds, there is also a Stock system for naming molecular
compounds
The old system of naming molecular compounds is based on the use of prefixes.
○ ex)
Some covalent compounds do not consist of individual molecules
Instead, each atom is joined to all its neighbors in a covalently bonded,
three-dimensional network.
Subscripts in a formula for covalent-network compound indicate smallest
whole-number ratios of the atoms in the compound.
○ ex)
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Acid
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A certain type of molecular compound. Most acids used in the lab are either
binary acids or oxyacids.
■ Binary acids: consist of two elements usually hydrogen and a halogen.
■ Oxyacids: acids that contain hydrogen, oxygen, and a third element
(usually a nonmetal)
In the laboratory, the term acid usually refers to a solution in water of an acid
compound rather than the acid itself.
■ ex) hydrochloric acid → water solution of the molecular compound HCl.
Many polyatomic ions are produced by the loss of hydrogen ions from oxyacids.
■
●
ex)
Salt
○
An ionic compound composed of a cation and the anion from an acid
■ Ex) NaCl → contains the anion from hydrochloric acid, HCl
■ CaSO4 → salt containing the anion from H2SO4
■ Bicarbonate ion HCO3 - from H2CO3.
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7.3
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A chemical formula indicates
○ The elements present in a compound
○ The relative number of atoms or ions of each element present in a compound
■ Formula mass
■ Molar mass
■ Percentage composition
Formula mass
○ Mass of any unit represented by chemical formula (H20, NaCl) = formula mass
○ The sum of the average atomic masses of all atoms represented in its formula
■ ex) formula mass of water = H2O
● Av. atomic mass → H: 1.01 amu
● Av. atomic mass → O: 16.00 amu
○ Av. mass of H2O →18.02 u
○ Molecular mass
■ Mass of ionic compound e.g. NaCl = not molecular mass only formula
mass