Compounds, and Mixtures
Any substance that contains only one kind of an atom is
known as an element. Because atoms cannot be created
or destroyed in a chemical reaction, elements such as
phosphorus (P4) or sulfur (S8) cannot
be broken down into simpler substances by these reactions.
Example: Water decomposes into a mixture of hydrogen and
oxygen when an electric current is passed through the liquid.
Hydrogen and oxygen, on the other hand, cannot be decomposed
into simpler substances. They are therefore the elementary,
or simplest, chemical substances - elements.
Each element is represented by a unique symbol. The
notation for each element can be found on the periodic table
The elements can be divided into three categories that
have characteristic properties: metals, nonmetals, and
semimetals. Most elements are metals, which are found on the
left and toward the bottom of the periodic table. A handful
of nonmetals are clustered in the upper right corner of the
periodic table. The semimetals can be found along the
dividing line between the metals and the nonmetals.
Elements are made up of atoms, the
smallest particle that has any of the properties of the
element.John Dalton, in 1803, proposed a modern theory of the
atom based on the following assumptions.
1. Matter is made
up of atoms that are indivisible and indestructible.
2. All atoms of an element are
3. Atoms of different elements
have different weights and different chemical
4. Atoms of different elements
combine in simple whole numbers to form compounds.
5. Atoms cannot be created or
destroyed. When a compound decomposes, the atoms are
Elements combine to form chemical compounds that are often
divided into two categories.
Metals often react with nonmetals to form ionic
compounds. These compounds are composed of positive and
negative ions formed by adding or subtracting electrons from
neutral atoms and molecules.
Nonmetals combine with each other to form covalent
compounds, which exist as neutral molecules.
The shorthand notation for a compound describes the number
of atoms of each element, which is indicated by a subscript
written after the symbol for the element. By convention, no
subscript is written when a molecule contains only one atom
of an element. Thus, water is H2O and carbon
dioxide is CO2.
Ionic and Covalent Compounds
positive and negative ions (Na+Cl-)
Exist as neutral
suchs as table salt (NaCl(s))
gases (C6H12O6(s), H2O(l),
melting and boiling points
Lower melting and
boiling points (i.e., often exist as a liquid or gas
at room temperature)
force of attraction between particles
force of attraction between molecules
into charged particles in water to give a solution
that conducts electricity
Remain as same
molecule in water and will not conduct electricity
Determining if a
Compound is Ionic or Covalent
Calculate the difference between the electronegativities
of two elements in a compound and the average of their
electronegativites, and find the intersection of these values
on the figure shown below to help determine if the compound
is ionic or covalent, or metallic.
|Practice Problem 1:
each of the following compounds, predict whether you
would expect it to be ionic or covalent.
(a) chromium(III) oxide, Cr2O3
(b) carbon tetrachloride, CCl4
(c) methanol, CH3OH
(d) strontium fluoride, SrF2
here to check your answer to Practice Problem 1
|Practice Problem 2:
the following data to propose a way of distinguishing
between ionic and covalent compounds.
||Melting Point ( oC)
||Boiling Point ( oC)
here to check your answer to Practice Problem 2
A molecule is the smallest particle that has any of
the properties of a compound. The formula for a molecule must
be neutral. When writing the formula for an ionic compound,
the charges on the ions must balance, the number of postive
charges must equal the number of negative charges.
||Balanced formula has 2 positive charges (1
calcium ion with +2 charge) and 2 negative charges (2
chloride ions with a -1 charge)
||Balanced formula has 6 positive charges (2
aluminum ions with a +3 charge) and 6 negative
charges (3 sulfate ions with -2 charge)
Mixtures Vs. Compounds
The law of constant composition states
that the ratio by mass of the elements in a chemical compound
is always the same, regardless of the source of the compound.
The law of constant composition can be used to distinguish
between compounds and mixtures of elements: Compounds
have a constant composition; mixtures do not. Water
is always 88.8% O and 11.2% H by weight regardless of its
source. Brass is an example of a mixture of two elements:
copper and zinc. It can contain as little as 10%, or as much
as 45%, zinc.
Another difference between compounds and mixtures of
elements is the ease with which the elements can be
separated. Mixtures, such as the atmosphere, contain two or
more substances that are relatively easy to separate. The
individual components of a mixture can be physically
separated from each other.
Chemical compounds are very different from mixtures: The
elements in a chemical compound can only be separated by
destroying the compound. Some of the differences between
chemical compounds and mixtures of elements are illustrated
by the following example using raisin bran and
Raisin bran has the following characteristic properties of
- The cereal does not have a constant composition; the
ratio of raisins to bran flakes changes from sample
- It is easy to physically separate the two
"elements," to pick out the raisins, for
example, and eat them separately.
Crispix has some of the characteristic properties of a compound.
- The ratio of rice flakes to corn flakes is constant;
it is 1:1 in every sample.
- There is no way to separate the "elements"
without breaking the bonds that hold them together.