Elements, Compounds, and Mixtures
Elements
An element is a pure substance composed of a single kind of atom. An atom is the
smallest particle of an element that still has all the properties of the element.
Although smaller (subatomic) particles exist, once an atom is broken apart into
those smaller particles, the properties of that element is lost. Gold is an element. If
you slice and slice a chunk of gold until only one tiny particle is left that can’t be
chopped any more without losing the properties that make gold gold, then you’ve
got an atom.
Atoms of an element cannot be broken apart via physical means or a chemical
reaction. In other words, you cannot separate the subatomic particles (pick out the
protons, neutrons, and electrons) of an atom with an ordinary chemical reaction.
Only nuclear reactions, which actually involve the splitting of the atom and do not
happen under ordinary circumstances, can break apart an atom. Atoms are the
basic unit that form compounds. For example, when carbon reacts with oxygen to
form CO2, the carbon atom does not change in any way, but only bonds the oxygen
atom. For this reason, elements are considered the building block of matter.
Every atom of carbon has six protons.
All atoms of the same element have the same number of protons. The number of
protons is also referred to as the atomic number of an element. It is the number we
use to identify and classify elements, and is the organizing principle of the periodic
table. Protons are subatomic particles – particles that make up an atom. They make
up the nucleus of the atom (along with neutrons) and have a positive charge. The
other main subatomic particles are neutrons, which also make up the nucleus and
are neutral, and electrons, which orbit the nucleus and have a negative charge.
Elements are represented by chemical symbols. For example, gold is represented by
the symbol Au. Oxygen is represented by the symbol O. Sodium is represented by
the symbol Na.
1. What is an element?
2. Define “atom”.
3. Can atoms be broken apart in a chemical reaction? What type of reaction
must happen for an atom to break apart?
4. Why are atom considered the basic building block of matter?
5. Which subatomic particle is the same for every atom of an element?
6. What is another name for the number of protons in an atom?
7. How are elements represented?
Compounds
A compound is composed of two or more elements in a specific, fixed ratio that are
held together with chemical bonds. For example, water (H2O) is a compound made
up of two elements, hydrogen (H) and oxygen (O). These elements are combined in
a very specific way – in a ratio of two hydrogen atoms to one oxygen atom (hence
H2O). A lot of compounds contain hydrogen and oxygen, but only one has that
special 2-to-1 ratio we call water.
Compounds are formed during a chemical reaction, and the atoms that comprise a
compound are held together with chemical bonds. The chemical bonds that hold
together these atoms create a NEW substance that is completely different
characteristics than the elements (atoms) that comprise it. For example, sodium is a
soft, shiny metal, while chlorine is a green, poisonous gas. However, when these two
elements are held together by chemical bonds, they form the compound NaCl, table
salt.
Because the atoms that compose compounds are held together by chemical bonds,
chemists can’t easily separate the atoms of a compound. Compounds can only be
broken down via a chemical reaction. Remember, elements cannot be broken down
by any physical or chemical reactions.
Compounds are always identified by a chemical formula. Chemical formulas tell
you what types of elements make up the compound and in what ratios (or how
many atoms of each element are in the compound). For example, the chemical
formula for water is H2O, the chemical formula for salt is NaCl, and the chemical
formula for sulfuric acid is H2SO4. The formula for sulfuric acid, H2SO4, tell you that
there are two atoms of hydrogen, one atom of sulfur, and four atoms of oxygen per
molecule of sulfuric acid.
1. What is a compound?
2. Do compounds retain the properties of the elements that comprise them?
Why or why not?
3. How are compounds formed and broken down (i.e. what must happen for a
compound to form or break apart)?
4. How do we represent compounds?
5. What information do chemical formulas convey?
Mixtures
Mixtures are physical combinations of pure substances (elements and/or
compounds) that have no definite or constant composition (no fixed ratio) – the
composition of a mixture varies according to who prepares the mixture. Suppose I
ask two people to make a salad. Unless these two people used exactly the same
recipe, these mixtures (salads) would vary somewhat in their relative amounts of
lettuce, tomatoes, cucumbers, etc. They would have produced two slightly different
mixtures. However, each component of a mixture (that is, each element and
compound that makes up the mixture – in the salad example, each ingredient)
retains its own set of physical and chemical characteristics because mixtures are not
held together by chemical bonds but are merely physically blended. Because of this,
it’s relatively easy to separate the various substances in a mixtures. Unlike
compounds, which can only be separated via a chemical reaction, mixtures can be
separated using physical means (i.e. chemical bonds are not broken and reformed).
Mixtures can come in two different forms:
Homogeneous mixtures: Sometimes called solutions, this type of mixture is
relatively uniform in composition; every portion of the mixture is like every other
portion. If you dissolve sugar in water and mix it really well, your mixture is
basically the same no matter where you sample it. Other examples of homogeneous
mixtures are air, blood, paper, and soft drinks.
Heterogeneous mixtures: if you put some sugar in a jar, add some sand, and then
give the jar a couple of shakes, your mixture doesn’t have the same composition
throughout the jar. Because the sand is heavier, there’s probably more sand at the
bottom of the jar and more sugar at the top. In this case, you have a heterogeneous
mixture, a mixture whose composition varies from position to position within the
sample. Other examples of heterogeneous mixtures are salads, a sandwich, and oil
and water.
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What is a mixture?
Do mixtures retain the original component’s properties? Why or why not?
What is the fundamental difference between compounds and mixtures?
What are homogeneous mixtures?
What is another name for homogeneous mixtures?
What are heterogeneous mixtures?
Can only be separated or
combined via a chemical
reaction
Combined and separated
without breaking/forming
chemical bonds
New substance formed
No new substance formed
Can only be combined in
a fixed proportion
Can be combined in any
proportion