Atoms

Atoms

Middle School

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Learning Objectives

• Describe what atoms are and their relationship to elements.

• Identify the three main subatomic particles that make up an atom.

• Explain the structure of an atom, including its nucleus and electron shells.

• Understand how ions form and what makes an atom reactive.

Key Vocabulary

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Atom

The smallest particle of an element with all the chemical properties of that element.

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Nucleus

The central part of an atom, containing protons and neutrons, and holding most of the atom's mass.

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Proton

A subatomic particle in the nucleus that has a positive electric charge and defines the element.

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Electron

A negatively charged particle that moves outside the nucleus and is much smaller than protons or neutrons.

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Isotope

Atoms of the same element that have a different number of neutrons but the same number of protons.

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What Are Atoms?

• Atoms are the basic building blocks that make up all matter.

• They are the smallest part of an element with its properties.

• An element is a pure substance made of only one type of atom.

• Atoms are so tiny that trillions can fit inside a single period.

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Multiple Choice

According to the presentation, what is the fundamental relationship between an atom and an element?

• An element is the smallest component of an atom.

• Atoms are always larger than the elements they form.

• An atom is the smallest particle of an element that has the properties of that element.

• Atoms and elements are identical and interchangeable terms.

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The Parts of an Atom

• An atom has a central nucleus with electrons moving around it.

• The nucleus contains protons with a positive charge and neutral neutrons.

• The number of protons in the nucleus determines the element.

• Electrons are tiny, negatively charged particles that orbit the nucleus.

Multiple Choice

Which of the following correctly describes the subatomic particles in an atom?

• Electrons are positive and in the nucleus, while protons are negative and outside.

• Protons are positive and in the nucleus, neutrons are neutral and in the nucleus, and electrons are negative and outside the nucleus.

• Neutrons are negative and orbit the nucleus, while protons are positive and also orbit the nucleus.

• Protons are neutral and found outside the nucleus, while electrons are in the nucleus.

Properties of Atoms

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Overall Charge

• Atoms are electrically neutral, having an overall charge of zero.

• They have equal numbers of positive protons and negative electrons.

• The positive and negative charges cancel each other out completely.

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Atomic Mass

• Atomic mass is the total mass of its protons and neutrons.

• These particles are found together inside the atom's central nucleus.

• An electron's mass is so small it is not included.

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Isotopes

• Atoms of an element can have different numbers of neutrons.

• These different versions of an element are called isotopes.

• They have the same proton count but a different atomic mass.

Multiple Choice

Why is an atom considered electrically neutral?

• Because the atomic mass is equal to zero.

• Because electrons have a neutral charge.

• Because it has more neutrons than protons, which balances the charge.

• Because it has an equal number of positive protons and negative electrons.

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Electrons and Reactivity

• Electrons are arranged in shells; the first holds 2, others can hold 8.

• Electrons in the outermost shell are called valence electrons.

• An ion is an atom that has gained or lost valence electrons.

• Atoms with 1 or 7 valence electrons are the most reactive.

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Multiple Choice

What happens to an atom when it loses one or more of its valence electrons?

• Its atomic mass significantly increases.

• It becomes a negatively charged ion.

• It becomes a new, non-reactive element.

• It becomes a positively charged ion.

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What Is a Molecule?

• A molecule is formed when two or more atoms are chemically linked together.

• For example, a water molecule is H₂O, with two hydrogen atoms and one oxygen.

• The molecular mass is the total mass of all atoms in the molecule.

• You can find it by adding the atomic masses of all atoms together.

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Multiple Choice

What is the correct definition of a molecule?

• A substance formed when two or more atoms link together.

• A particle found only in the nucleus of an atom.

• A single atom of a specific element.

• An atom that has gained or lost an electron.

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Common Misconceptions About Atoms

Misconception	Correction
Atoms are flat, like a mini solar system.	Atoms are 3D spheres, and electrons move in complex shells.
The nucleus is large because it holds most of the atom's mass.	The nucleus is tiny and dense. The atom is mostly empty space.
Protons and electrons are the same size.	Electrons are much smaller than protons and neutrons.
All atoms of the same element are identical.	Isotopes are atoms of an element with different numbers of neutrons.

Multiple Choice

How do the numbers of protons and electrons in a neutral atom compare, and what is the effect on its overall charge?

• There are more protons than electrons, resulting in a positive charge.

• The numbers are equal, resulting in a net charge of 0.

• There are more electrons than protons, resulting in a negative charge.

• The number of neutrons equals the number of protons, resulting in a neutral charge.

Multiple Choice

Why is the atomic mass of an atom determined almost entirely by the mass of its protons and neutrons?

• Because protons and neutrons have opposite charges that cancel out their mass.

• Because electrons have a mass that is negligible compared to protons and neutrons.

• Because atomic mass only measures the particles in the nucleus.

• Because electrons are not part of the atom's nucleus.

Multiple Choice

An atom has only one valence electron in its outer shell. Based on this, what can you predict about its reactivity?

• It is highly reactive because it can easily lose that one electron.

• It is non-reactive because its outer shell is stable.

• Its reactivity cannot be determined from the number of valence electrons.

• It is moderately reactive and will try to gain seven more electrons.

Multiple Choice

If a neutral atom of Lithium becomes a positive ion (Li⁺), what must have happened to its subatomic particles to cause this change?

• It gained one electron, creating a negative charge.

• It lost one electron, leaving more protons than electrons.

• It lost one neutron, changing its atomic mass.

• It gained one proton, changing it into a different element.

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Summary

• Atoms, the building blocks of matter, have a nucleus with protons and neutrons.

• An element is defined by its protons; atoms are neutral when protons equal electrons.

• Valence electrons in the outer shell determine how an atom reacts.

• Molecules are formed when two or more atoms are chemically linked.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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