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What are Valence Electrons — Reading Comprehension

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This science reading passage for middle school students delves into the concept of valence electrons, the outermost electrons of an atom. The text, written in a clear and engaging teacher's voice, explains how the number of valence electrons dictates an element's chemical properties and reactivity. It introduces core concepts like the octet rule, which explains why atoms seek stability by gaining, losing, or sharing these electrons to form chemical bonds. The passage also makes real-world connections, explaining how valence electrons are responsible for a material's electrical conductivity, using examples like copper and plastic. This content is aligned with NGSS standards related to the structure and properties of matter (MS-PS1-1) and aims to build a foundational understanding of atomic behavior for students. The accompanying multiple-choice questions assess students' comprehension of key vocabulary and their ability to apply these concepts to new scenarios.

CONTENT PREVIEW

An atom's valence electrons, located in the outermost shell, are the key players in forming chemical bonds and determining the element's properties.

Valence electrons are the electrons located in an atom's outermost energy level or shell. These specific electrons are critical in determining an atom's chemical behavior. Unlike electrons in inner shells, which are tightly bound to the nucleus, valence electrons are the primary participants in chemical reactions and the formation of bonds between atoms. The number of valence electrons an atom possesses is directly responsible for its chemical properties and its reactivity.

The stability of an atom is largely dependent on its valence electron configuration. Atoms are most stable when their outermost shell is completely filled with electrons, a state governed by the octet rule for many elements. This rule states that atoms tend to react in ways that give them a full outer shell, typically containing eight electrons. To achieve this stable configuration, atoms will either lose, gain, or share their valence electrons with other atoms, a process that forms a chemical bond. For example, a sodium atom (with one valence electron) can easily transfer this electron to a chlorine atom (with seven valence electrons), resulting in the formation of an ionic bond and the compound sodium chloride, or table salt.

The mobility of valence electrons also explains the differences between electrical conductors and insulators. In materials classified as metals, the valence electrons are not bound to a single atom; instead, they are free to move throughout the material. This "sea of electrons" allows metals like copper to efficiently conduct electricity. Conversely, in non-metals and insulators such as plastic, the valence electrons are tightly held by individual atoms. This restriction on electron movement prevents the flow of electric current, making them effective insulators.

The periodic table is a useful tool for identifying the number of valence electrons an element has. For the main group elements (Groups 1, 2, and 13–18), the group number indicates the count of valence electrons. For instance, all elements in Group 1 have one valence electron, while all elements in Group 15 have five valence electrons.

Fun Fact: The inert gases in Group 18, such as neon and argon, are known for their lack of reactivity because their valence shells are already full, making them chemically stable.

What are valence electrons?

Electrons closest to the nucleus.Electrons in the innermost shell.Electrons in the outermost energy level.Protons in the atom's nucleus.

What determines an atom's chemical properties and reactivity?

The number of neutrons.The number of protons.The number of valence electrons.The atom's overall size.

According to the passage, what principle states that atoms are most stable when their outermost shell is full, often with eight electrons?

The Law of Conservation of MassThe Octet RuleThe Law of GravityThe Periodic Law

How do atoms form chemical bonds?

By changing the number of protons.By gaining, losing, or sharing valence electrons.By absorbing neutrons from other atoms.By moving to a new position.

Based on the passage, why is copper a good electrical conductor?

Its valence electrons are tightly held.Its valence electrons are free to move.It has a higher number of protons.It is a very dense metal.

The passage states that elements in Group 16 have six valence electrons. Which element is in Group 16?

Sodium (Na)Carbon (C)Oxygen (O)Neon (Ne)

Which of the following best summarizes the main idea of the passage?

The periodic table helps to find protons.The nucleus is the most important part of an atom.Valence electrons determine an atom's behavior and properties.All elements are highly reactive.

An unknown element is found to be very unreactive and does not easily form compounds. Based on the passage, what can you infer about this element?

It has only one valence electron.Its outermost shell is likely full.It has a very large atomic mass.It is a transition metal.