Science 9
Atoms - The Inside Story
We used the Particle Theory of Matter to help us explain our observations of
matter and identify a substance. This theory explained physical and chemical
properties as well as physical and chemical changes. However, it cannot explain
everything about pure substances. Because the Particle Theory does not distinguish one particle
from another, a more powerful theory is needed.
John Dalton (1766-1844) introduced a new way of talking about and explaining chemical facts and
laws in his ______________________________________.
Dalton’s Atomic Theory:
 All matter is made up of small particles called ____________.
 Atoms cannot be _________________, destroyed or divided into smaller particles
 All atoms of the same element are _________________ in mass and size, but they are
different in mass and size from the atoms of other elements
 Compounds are created when atoms of different elements _____________ together in
definite proportions
Can matter be continually _________________ into smaller and
smaller pieces?
This question had puzzled scientists for hundreds and hundreds of years. As they tried to find an
answer to that question, models of the atom slowly developed. Many of these models were strange
and wrong but they all helped scientists develop the model of the atom that we use today.
An ATOM is the smallest particle into which an element can be divided and still maintain the
properties of that element. Because all matter is made up of elements, and all elements are made
up of atoms, atoms are considered the building blocks of matter
Atoms are made up of even smaller __________________________________
called: protons, neutrons and electrons. Located in the center of the atom are the
protons and neutrons, which stick together to form an atom's __________________.
The ________________________ are found in regions surrounding the nucleus.
Protons
 The ______________________ charged particles located in the nucleus of an atom.
 All protons are identical.
 An atom is identified by the ______________________ of protons in its nucleus. For
example, an atom with one proton is called Hydrogen; an atom with eight protons is called
oxygen.
 All atoms of the same element have the ___________________ number of protons.
Neutrons
 Also located in the nucleus.
 As their name suggests, neutrons are electrically ___________________. That is, they have no
electric charge.
 All neutrons are identical.
 Sometimes, atoms of the same element can have different numbers of neutrons, these atoms
are called _______________________. Because neutrons have no charge, the overall charge
of the atom is not changed by the extra neutrons.
Electrons
 The ________________________ charged particles found outside the nucleus of an atom.
 All electrons are identical.
 The number and arrangement of the electrons in an atom determine its chemical properties.
 Scientists used to think that electrons orbited the nucleus in fixed paths, the way the planets
orbit the sun. Now they theorize that electrons travel in __________________ paths in areas
around the nucleus called electron _____________________.
 An electron's energy level determines its average ____________________ from the nucleus.
Information on Charges:
The charge of an electron (-) is equal in size to the charge of a proton
(+), but opposite in sign. Since unlike charges attract each other,
electrons and protons exert an attractive electrical force on each other.
That's what holds electrons close to the nucleus.
Science 9
Atoms - The Inside Story
We used the Particle Theory of Matter to help us explain our observations of matter and identify
a substance. This theory explained physical and chemical properties as well as physical and
chemical changes. However, it cannot explain everything about pure substance. Because the
Particle Theory does not distinguish one particle from another, a more powerful theory is needed.
John Dalton (1766-1844) introduced a new way of talking about and explaining chemical facts and
laws in his Atomic Theory.
Dalton’s Atomic Theory:
 All matter is made up of small particles called atoms
 Atoms cannot be created, destroyed or divided into smaller particles
 All atoms of the same element are identical in mass and size, but they are different in mass
and size from the atoms of other elements
 Compounds are created when atoms of different elements link together in definite
proportions
Dalton’s model uses the idea that elements are different because their “particles” (atoms) are different.
The model in the particle theory does not use this idea. This leads to a more accurate definition of the
word “element” - a pure substance made up of one type of particle, or atom. Each element has its own
distinct properties and cannot be broken down into simpler substances by means of a chemical change.
Can matter be continually divided into smaller and smaller pieces? This question had puzzled scientists for
hundreds and hundreds of years. As they tried to find an answer to that question, models of the atom
slowly developed. Many of these models were strange and wrong but they all helped scientists develop the
model of the atom that we use today.
An ATOM is the smallest particle into which an element can be divided and still maintain the properties of
that element. Because all matter is made up of elements, and all elements are made up of atoms, atoms are
considered the building blocks of matter
Atoms are made up of even smaller subatomic particles called protons, neutrons and electrons.
Located in the center of the atom are the protons and neutrons which stick together to form an atom's
nucleus. The electrons are found in regions surrounding the nucleus.
Protons
 The positively charged particles located in the nucleus of an atom.
 All protons are identical.
 An atom is identified by the number of protons in its nucleus. For example, an atom with one proton
is called Hydrogen; an atom with eight protons is called oxygen.
 All atoms of the same element have the same number of protons.
Neutrons
Also located in the nucleus.
 As their name suggests, neutrons are electrically neutral. That is, they have no electric charge.
 All neutrons are identical.
 Sometimes, atoms of the same element can have different numbers of neutrons, these atoms are called
isotopes. Because neutrons have no charge, the overall charge of the atom is not changed by the
extra neutrons.

Electrons
 The negatively charged particles found outside the nucleus of an atom.
 All electrons are identical.
 The number and arrangement of the electrons in an atom determine its chemical properties.
 Scientists used to think that electrons orbited the nucleus in fixed paths, the way the planets orbit the
sun. Now they theorize that electrons travel in random paths in areas around the nucleus called
electron clouds.
 An electron's energy level determines its average distance from the nucleus.
The charge of an electron (-) is equal in size to the charge of a proton (+), but opposite in sign. Since
unlike charges attract each other, electrons and protons exert an attractive electrical force on each other.
That's what holds electrons close to the nucleus.