Science 10
Chemistry Unit- Chapter 4, Lesson 1
Atomic Theory
John Dalton (1766-1844) hypothesized that all mater could be broken up into
smaller pieces and that the smallest possible piece was the atom. He was right, all
matter can be broken down into smaller pieces and one of those is the atom, but
atoms can also be broken down into smaller pieces. These smaller pieces are
called subatomic particles.
Subatomic particles are the particles that make up an atom.
A subatomic particle is a part of an atom much the same as a wheel is part of a
car. The three main subatomic particles are protons, neutrons, and electrons.
Today, scientists know subatomic particles in turn can be broken down into still
smaller particles.
The modern atomic theory states that an atom consists of:

Protons are subatomic particles that have an electrical charge of 1+
(positive) and a mass of 1u (atomic mass unit). They are at the center of an
atom.


Neutrons are subatomic particles that have no electrical charge. They are
neutral, and have a mass of 1u (atomic mass unit). They are also at the
center of an atom.


Electrons are subatomic particles that have an electrical charge of 1(negative) and effective mass of 0u (atomic mass unit). They are outside the
center of the atom.
An atom consists of a dense nucleus, made up of protons and neutrons,
surrounded by electrons moving in space. Neils Bohr was a scientist who
suggested that electrons travel in a circular orbit or energy level in well-defined
path around the center of an atom.
Nucleus
At the center of an atom is a nucleus. The nucleus is tiny in comparison to the
whole atom. A nucleus is made of protons and neutrons. Protons and neutrons
both have a mass of 1u.
If an atom were the size of a football stadium, the nucleus would be the size of a
football.
Different elements are made up of different atoms. Each atom is made of the
same three subatomic particles, but different because of the number of each
particle present.
The simplest nucleus is just one proton. This is in the atom Hydrogen. Hydrogen
can also have neutrons present in the nucleus but it is possible for it to have just
one proton. Atoms with nuclei that have less than 30 protons usually have the
same number of protons and neutrons. As the protons in the nucleus increase
you need extra neutrons to help make it stable.
The nucleus always has a positive charge because it contains positively charged
protons and neutral neutrons.
Electrons
Electrons are so light they are considered to have zero mass and they have a
negative electric charge (-1).
Electrons move around the nucleus in specific paths called energy levels.
Energy levels exist whether there is an electron in them or not.
Electrons occupy certain energy levels depending on the atom. For example,
hydrogen has a single electron in the first energy level. Sodium has two electrons
in the first energy level, eight electrons in the second energy level and one
electron in the third energy level.
An atom can have a maximum of two electrons occupying the first energy level,
eight in the second energy level and eight in the third energy level. Each energy
level must be filled before electrons occupy the next one.
Exercise 1
Charge
Protons
Location
Mass
symbol
Electrons
Neutrons
Atomic Number
The atomic number is equal to the number of protons in the nucleus.
The atomic number is special because it can be used to identify any known
element.
It is easy to identify copper because it has an atomic number of 29. Carbon has
an atomic number of 6, oxygen has an atomic number of 8, and so on. The atomic
number of any element is found in the periodic table.
Mass Number
All atoms have a mass number.
The mass number is equal to the number of protons plus the number of neutrons
(e.g., carbon has six protons and six neutrons).
The mass number for carbon would be 6 (protons) + 6 (neutrons) = 12.
Mass number = atomic number (number of protons) + number of neutrons
The atomic mass of an atom is expressed in atomic mass units (u). This means the
atomic mass of carbon is expressed as 12 u.
You can solve for atomic number and number of neutrons by modifying the
formula shown above.
Atomic number = mass number - number neutrons
Number of neutrons = mass number - atomic number
Exercise 2
Use your understanding of mass number to calculate the values for the blank
spaces in the table below. Place the proper symbol for the elements in the
brackets.
Refer to the periodic table. Place the atomic symbol in the brackets following the
names in the table below. Fill in the blanks with the values for the atomic number
and number of protons.
Iron
(_____)
Krypton
(_____)
Uranium
(_____)
Sodium
(_____)
Atomic Number
Number of
Protons
Lithium
(_____)
Chlorine
(_____)
Number of
Protons
3
17
Number of
Neutrons
4
Mass number
Copper
(_____)
47
34
35
Silver
(_____)
63
61