Lower Secondary Checkpoint

Atoms & Elements

Mr. Mark

K1 Chemistry

• Matter is anything that takes up space and has mass.
• All matter is made of atoms

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• Over 2000 years ago, a Greek Philosopher, Democritus suggested that everything was made of of tiny particles.
• He suggested that if we keep on cutting up a substance into smaller and smaller pieces, we would end up with very small piece that would not be cut up anymore.
• Atom = “atomos”; indivisible

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• Atoms – smallest part of an element that still retains all properties of that element.
• Atoms can be broken down into subatomic particles.

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An atom has three parts:

Proton = positive

Neutron = no charge

Electron = negative

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The proton & neutron are found in the center of the atom, a place called the nucleus.

The electrons orbit the nucleus.

What are elements?

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• Elements are made of one kind of atoms.
• Elements: He (Helium gas), Ar (Argon gas) , O₂ (oxygen gas), H₂ (hydrogen gas), N₂ (nitogen gas), O₃ (ozone)

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More about Elements..

• Elements are the building blocks of all matter.
• The periodic table is a list of all of the elements that can build matter. It’s a little like the alphabet of chemistry.
• The periodic table tells us several things…

8

O

Oxygen

16

Atomic Number:

Number of protons and it is also the number of electrons in an atom of an element.

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Element’s Symbol:�An abbreviation for the element.

Elements Name

Atomic Mass/Weight:

Number of protons + neutrons.

Atom Models

• There are two models of the atoms we will be using in class.
• Bohr Model
• Lewis Dot Structure

Bohr Model

• The Bohr Model shows all of the particles in the atom.
• In the center is circles. Each circle represents a single neutron or proton. Protons should have a plus or P written on them. Neutrons should be blank or have an N.
• In a circle around the nucleus are the electrons. Electrons should have a minus sign or an e.

+

+

-

-

Electrons have special rules….

• You can’t just shove all of the electrons into the first orbit of an electron.
• Electrons live in something called shells or energy levels.
• Only so many can be in any certain shell.

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Electrons have special rules….

• You can’t just shove all of the electrons into the first orbit of an electron.
• Electrons live in something called shells or energy levels.�
• Only so many can be in any certain shell.
• The electrons in the outer most shell of any element are called valence electrons.

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So let’s try it….

• How to draw a Lithium atom
• First, look at the Periodic Table
• Second, determine the number of protons (Look @ the atomic number)
• Then determine the number of neutrons (Atomic mass – atomic number)
• Then determine the number of electrons (Look @ the atomic number)

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3

Li

Lithium

7

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3

Li

Lithium

7

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+

+

+

Protons = 3

Neutrons = 4

(7-3=4)

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Electrons = 3

2 in the 1^st shell, 1 in the 2^nd shell

Lewis Dot Structure

O

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• •

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• The Lewis Dot Structure is a bit different from the Bohr model.
• It only shows the element symbol and it’s outer most electron shell.

Periodic table