Atoms and Elements

Atoms and Elements

Middle School

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Learning Objectives

• Tell the difference between atoms, elements, molecules, and compounds.

• Describe the parts of an atom: protons, neutrons, and electrons.

• Explain how the periodic table organizes elements by their atomic number.

• Compare ionic and covalent bonds and how they form new substances.

Key Vocabulary

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Atom

The smallest unit of matter that retains the chemical properties of an element and makes up everything.

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Element

A pure substance that is made up of only one type of atom and is defined by it.

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Atomic Number

The number of protons in an atom's nucleus, which determines the identity of the element itself.

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Compound

A pure substance formed when two or more different elements are chemically combined in fixed ratios.

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Ionic Bond

A chemical bond formed when one atom gives an electron to another, typically between metals and nonmetals.

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Covalent Bond

A chemical bond formed when two or more atoms share electrons, which occurs between nonmetal atoms.

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The Structure of an Atom

• All matter is made of tiny particles called atoms.

• The nucleus contains positive protons and neutral neutrons.

• Negative electrons orbit the nucleus in a large cloud.

• An atom’s mass comes from its protons and neutrons.

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Multiple Choice

Which part of the atom contains most of its mass and has a positive charge?

• The electrons, which are the heaviest part of the atom.

• The electron cloud, which has a negative charge.

• The nucleus, which contains protons and neutrons.

• The neutrons alone, which have a positive charge.

Categories of Elements

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Metals

• ​Metals are typically shiny, solid, and are excellent conductors of electricity.

• ​​Most known elements are classified in the periodic table as metals.

• ​They are essential materials for construction, electronics, and many other industries.

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Nonmetals

• ​Nonmetals are often gases and they generally act as good insulators.

• ​​They are located on the right side of the periodic table.

• ​Living organisms are mostly composed of nonmetals like carbon and oxygen.

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Metalloids

• ​Metalloids have a mix of properties from both metals and nonmetals.

• ​​They are found between the metals and nonmetals on the periodic table.

• ​These elements are widely used as semiconductors in electronic devices.

Multiple Choice

How is an element's identity determined, and how are elements organized on the periodic table?

• By its number of electrons; organized alphabetically.

• By its number of protons (atomic number); organized by atomic number.

• By its state of matter; organized into metals and nonmetals only.

• By its number of neutrons; organized by mass.

Forming New Substances

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Ionic Bonds

• An ionic bond forms when a metal atom gives electrons to a nonmetal.

• This creates charged ions that have a strong attraction for each other.

• Compounds formed this way are pure substances, like sodium chloride or table salt.

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Covalent Bonds

• A covalent bond is formed when two or more nonmetal atoms share electrons.

• Atoms in a covalent bond are locked together to form stable new molecules.

• Water (H₂O) is a common example of a molecule with covalent bonds.

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Mixtures

• A mixture contains different substances that are not chemically bonded to each other.

• The different parts of a mixture can be easily separated by physical means.

• Salt dissolved in water is a good example of a common liquid mixture.

Multiple Choice

What is the key difference between a compound and a mixture?

• A mixture is a pure substance, while a compound is not.

• A compound involves sharing electrons, while a mixture involves transferring electrons.

• A mixture has a fixed chemical formula, while a compound does not.

• A compound is made of chemically bonded elements in a fixed ratio, while a mixture is a physical combination that can be separated.

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Common Misconceptions

Misconception	Correction
Atoms are tiny, solid balls.	Atoms are mostly empty space with a dense nucleus and an electron cloud.
Atomic mass is determined by all subatomic particles.	Only protons and neutrons contribute to the atomic mass.
Compounds and mixtures are the same thing.	Compounds have chemical bonds and fixed ratios. Mixtures are physical combinations.
The number of electrons always defines the element.	The number of protons (the atomic number) defines the element.

Multiple Choice

A substance is shiny, conducts electricity well, and is solid at room temperature. How would you classify this substance based on its properties?

• As a metal

• As a compound

• As a metalloid

• As a nonmetal

Multiple Choice

An atom has an atomic number of 11 and an atomic mass of 23. How many protons, neutrons, and electrons does a neutral atom of this element have?

• 23 protons, 11 neutrons, 23 electrons

• 11 protons, 12 neutrons, 11 electrons

• 12 protons, 11 neutrons, 11 electrons

• 11 protons, 11 neutrons, 12 electrons

Multiple Choice

Sodium (a metal) and Chlorine (a nonmetal) combine to form table salt (NaCl). Analyze this interaction to determine what type of chemical bond is formed and why.

• No bond is formed; it creates a mixture because one is a metal and one is a nonmetal.

• An ionic bond is formed because the metal atom transfers an electron to the nonmetal atom.

• A metallic bond is formed because both elements are metals.

• A covalent bond is formed because the atoms share electrons to become stable.

Multiple Choice

Imagine you discover a new element with 120 protons. What would this discovery imply about its placement on the periodic table and its likely properties?

• It is impossible to have an element with that many protons.

• It would be a metalloid with mixed properties, placed in the middle of the table.

• It would be a lightweight nonmetal at the top of the table.

• It would be placed at the very end of the current periodic table and would likely be a heavy, radioactive metal.

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Summary

• All matter is made of atoms, containing a nucleus and an electron cloud.

• An element is a pure substance defined by its unique number of protons.

• The periodic table organizes elements by atomic number and shared properties.

• Compounds are new substances formed by chemical bonds, unlike physically blended mixtures.

Poll

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

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• 3

• 4