which comprise a single molecule of a compound.
Empirical formulas refer to the smallest whole number ratios of atoms in
substance is actually a collection of molecules with different sizes but the same
empirical formula. For example, carbon is commonly found as a collection of three
dimensional structures (carbon chemically bonded to carbon). In this form, it is most
easily represented simply by the empirical formula "C" (the elemental name).
Sometimes the molecular formulas are drawn out as structural formulas to
give some idea of the actual chemical bonds which unite the atoms.
Structural formulas give an idea about the connections between atoms, but they
The nucleus of an atom (containing protons and neutrons) remains unchanged
after ordinary chemical reactions, but atoms can readily gain or lose electrons.
If electrons are lost or gained by a neutral atom, then the result is that a
For example, Sodium (Na) has 11 protons and 11 electrons. However, it can
overall net charge of 1+ (the units are electron charge). The ionic state of an atom or
compound is represented by a superscript to the right of the chemical formula: Na
(note the in the case of 1+, or 1-, the '1'is omitted). In contrast to the Na atom,
are simple ions, in contrast to polyatomic ions such as NO
atoms, but have a net positive or negative charge.
The chemical properties of an ion are greatly different from those of the atom
from which it was derived.
The noble gasses are generally chemically non-reactive, they would appear to
have a stable arrangement of electrons.
Other elements must gain or lose electrons, to end up with the same
arrangement of electrons as the noble gases, in order to achieve the same kind of
and 7 electrons. If Nitrogen gained three electrons it would have 10 electrons, like the
Noble gas Neon (10 protons, 10 electrons). However, unlike Neon, the resulting
Nitrogen ion would have a net charge of N
(7 protons, 10 electrons).
This is mainly true for the elements on either side of the chart.
Ions form when one or more electrons transfer from one neutral atom to
another. For example, when elemental sodium is allowed to react with elemental
chlorine an electron transfers from a neutral sodium to a neutral chlorine. The result
is a sodium ion (Na
) and a chlorine ion, chloride (Cl
The oppositely charged ions attract one another and bind together to form NaCl
It should be pointed out that the Na
ions are not chemically bonded
together. Whereas atoms in molecular compounds, such as H
O, are chemically
Pure ionic compounds typically have their atoms in an organized three
dimensional arrangement (a crystal). Therefore, we cannot describe them using
molecular formulas. We can describe them usingempirical formulas.
If we know the charges of the ions comprising an ionic compound, then we can
always electrically neutral overall.
In the NaCl example, there will be one positively charged Na
ion for each
) and the Chlorine
) + 2 (Cl
) = neutral charge
Resulting empirical formula: BaCl
The atomic nucleus is the central area of the atom. It is composed of two kinds
of subatomic particles: protons and neutrons.
Diagram showing the atomic structure
Atoms are the building blocks of all matter. Everything you can see, feel and
touch is all made of atoms. There are even things you cannot see, feel, hear or touch
that are also made of atoms. Basically, everything is made up of atoms.
In 1909, Ernest Rutherford led Hans Geiger and Ernest Marsden through what
is known as the Gold Foil Experiments. During the experiments they would shoot
particles through extremely thin sheets of gold foil. In 1911, Rutherford came to the
conclusion that the atom had a dense nucleus because most of the particles shot
straight through, but some of the particles were deflected due to the dense nucleus of
the gold atoms. This theory would eliminate the idea that the atom was structured
more like plum pudding. The plum pudding model was the leading model of atomic
structure until Rutherford's findings.
neutrons in the atom define what type of atom or element it is. An element is a bunch
of atoms that all have the same type of atomic structure. For instance, hydrogen is an
element. Every hydrogen atom is made up of 1 proton, 0 neutrons, and 1 electron.
The composition of the atomic nucleus gives us lots of information about the
element it represents. The number of protons inside the nucleus gives us theatomic
neutral charge, the electrons (-) need to balance it out with their negative charge.
Therefore, in a neutral atomthere are just as many protons as electrons. So, if you
know the atomic number and know the charge of the atom then the number of
electrons is easy to find. For instance, hydrogen has 1 proton, 1+, so in order for the
hydrogen atom to be neutral it must have 1- charge. Therefore, hydrogen has 1
Where do the neutrons fit in all of this? Well,neutrons are neutral. To keep it
all straight I use the first letters: Neutrons are Neutral, and Protons arePositive. I then
remember Electrons through the process of Elimination.
Although the neutrons do not give the atom any charge, they still hold their
own weight in the importance of the atomic structure. The neutron is the largest of
the subatomic particles. When put the neutrons and protons together we get the
electrons are not inside of the nucleus; instead they are flying around like crazy on
the outside of the nucleus.
Since the atomic number gives us the number of protons in an atom and the
atomic mass gives us the number of protons and neutrons, we can find the number of
neutrons by subtracting the atomic number from the atomic mass.
Atomic mass - atomic number = number of neutrons.
The atomic number of an atom gives each element its identity. You can find
out which element it is by its atomic number and reverse the process to find out what
the atomic number is if you know which element you are working with.
Let's run through all of the numbers with an element, oxygen.
Atomic Mass: 16 
The ability of atoms to lose or to gain electrons.
the position of the element on the periodic table. They are the tendency or ability of
atoms tolose electrons and the tendency or ability to gain electrons.
First, let's consider the ability to lose electrons. This is related to ionization
the amount of energy that it takes to remove an electron from an atom. You have
learned that the ionization energies are lowest for the elements down and on the left
hand side of the periodic table and increase as you go up and all the way across to the
right including the inert gases.
The ionization energy measures how hard it is to lose or remove an electron.
High ionization energy means that it is hard to lose electrons. Low ionization energy
means that it easy to lose electrons. The elements on the left side lose their electrons
fairly easily and the elements on the right side of the periodic table do not lose their
electrons very easily. Taking vertical position on the table into account, the elements
that are lower on the table lose electrons more easily and the elements that are higher
have a harder time losing electrons. Thus the overall trend is from most easily losing
electrons on the lower left to least easily losing electrons on the upper right. Keep
that trend in mind.
Ability to Lose Electrons
You should recall that as you go from left to right on the periodic table, the attraction
for electrons increases and the ability to gain electrons increases. This is true all the
way across the periodic table except/em> for the inert gases. There is an abrupt drop
in the ability to gain electrons when we get to the inert gases. This is because their
energy level is full and any additional electrons will have to start a new energy level.
Ability to Gain Electrons
Types of Chemical Bonds
A group of atoms bonded to one another form a molecule.
If the molecule has more than one type of element present it is a compound.
These 2 types of bonds are 1. ionic and 2. covalent.
Atoms start off with the same number of positive protons and negative
electrons. This way the opposite charges cancel each other out.
Charged atoms, or ions, can form when atoms lose or gain electrons- remember
that atoms will gain or lose electrons in order to have a full outer shell.
If an atom starts off with 9 protons and 9 electrons, the positive and negative
charges are balanced out. However, this atom only has 7 electrons in its outer shell,
so it wants 1 more electron to have 8 and be happy. But when the atom gains an extra
negative electron, it now has 10 negative electrons and 9 positive protons. Therefore
its overall charge is -1. If an atom has one electron in its outer shell, it will usually
give that electron away and use the next lower shell as a "full" outer shell. When it
gives a negative electron away, it becomes a positively charged ion.
Positive and negative ions are attracted to one another and bond together in
A salt is a dry solid composed of atoms connected by ionic bonds Ex- table
A covalent bond results when two atoms share electrons, thereby completing
their valence shells
When molecules or compounds are chemically changed it is called a chemical
Photosynthesis is an example of a chemical reaction. In photosynthesis (the
chemical equation is shown below), the atoms in water and carbon dioxide are
rearranged to form sugar and oxygen gas.
Molecules that participate in a reaction are reactants.
Molecules formed by a reaction are products.
Water is the single most important molecule of earth
All organisms are 70-90% water.
Water has unique properties that make it a life-supporting substance.
Atoms differ in their electronegativity, or their attraction for electrons in a
Oxygen has a very strong attraction for electrons, so when oxygen is sharing
electrons with two hydrogen atoms, it gets the negative electrons slightly more than
its fair share of the time. Since the negative electrons are near the oxygen end, more
of the time, the oxygen is slightly negative. The hydrogen ends of water are slightly
positive because the hydrogen atoms each have a positively charged proton that is left
by itself when oxygen is sharing the electrons unfairly.
The unequal sharing of electrons in a molecule such as water makes the
Polar water molecules are attracted to one another and can form hydrogen
Properties of Water
Water is a solvent that can dissolve many substances.
NaCl is the chemical compound that we call table salt.
Molecules that are non polar have no charges cannot attract water. These are
called hydrophobic (ex. oil, grease, fat).
Water dissolves polar substances and ions.
Water molecules stick to each other and to other substances
Water also has a high surface tension.
Frozen water (ice) is less dense than liquid water, so ice floats.
Unlike other substances, water expands as it freezes. This is because the
hydrogen bonds of liquid water are continuously breaking and reforming. When
water freezes, all of the water molecules are perfectly hydrogen bonded together. In
order for them to be hydrogen bonded together, they must all be perfectly aligned
with each other. the water molecules must spread out a little in order for them to all
line up perfectly as water freezes. This makes them spread out as the water freezes.
This spreading during freezing can burst water pipes and automobile radiators.
So far, we’ve studied atoms and compounds and how they react with each
hold atoms and molecules of substances together. There are several different kinds of
bonds; the type of bond seen in elements and compounds depends on the chemical
properties as well as the attractive forces governing the atoms and molecules. The
three types of chemical bonds are Ionic bonds, Covalent bonds, and Polar covalent
bonds. Chemists also recognize hydrogen bonds as a fourth form of chemical bond,
though their properties align closely with the other types of bonds.
In order to understand bonds, you must first be familiar with electron
properties, including valence shell electrons. The valence shell of an atom is the
outermost layer (shell) of an electron. Though today scientists generally agree that
electrons do not rotate around the nucleus, it was thought throughout history that each
electron orbited the nucleus of an atom in a separate layer (shell). Today, scientists
have concluded that electrons hover in specific areas of the atom and do not form
orbits; however, the valence shell is still used to describe electron availability.
One can determine how many electrons an atom will have by looking at its
periodic properties. In order to determine an element’s periodic properties, you will
need to locate a periodic table. After you’ve found your periodic table, look at the
roman numerals above each column of the table. You should see that above
Hydrogen, there’s a IA, above Beryllium there’s a IIA, above Boron there’s a IIIA,
and so on all the way to Fluorine, which is VIIA. Also, note that the metals are all in
group B—their roman numerals have the letter B afterwards instead of the letter A.
For now, we are going to ignore the columns with a B, and focus on the columns with
an A (the non-metals, generally speaking). Once you have located the group-A
elements, we are going to count across, giving each column a number, like this:
The first A-column is I (1), then counting across, 2-8 (skipping the B group,
however when counting electrons, we’ll count it as 8. Now, we can determine how
many valence electrons each element has in its outermost shell. The elements in the
IA column have 1 valence electron. The elements in the IIA column have 2 bonding
electrons, and so on. By the time we get to the noble gases (the column labeled 0), we
are up to 8 bonding electrons. This means that these gases can stand on their own, or
donate electrons to another element, but they cannot accept any more electrons. This
is because the electrons they have satisfy the octet rule.
The Octet and Duet Rules
When it comes to bonding, everything is based on how many electrons an
element has or shares with its compound partner or partners. The octet rule is
followed by most elements, and it says that to be stable, an atom needs to have eight
electrons in its outermost shell. Elements that do not follow the octet rule are H, He,
B, Li and Be (sometimes). Lithium gives up an electron whereas the other elements
listed here gain one. These elements instead follow the duet rule which says that the
atoms only need two valence electrons to be stable. When bonding, stability is always
considered and preferred. Therefore, atoms bond in order to become more stable than
they already are.
Not all atoms bond the same way, so we need to learn the different types of
bonds that atoms can form. There are three (sometimes four) recognized chemical
bonds; they are ionic, covalent, polar covalent, and (sometimes) hydrogen bonds.