Section Assessment

Section Summary

I Atoms consist of protons, neutrons, and electrons.

I An element consists of atoms that have a specific number of protons in their nuclei.

I Isotopes of an element differ by the number of neutrons in their nuclei.

I Elements with full outermost energy levels are highly unreactive.

I Ions are electrically charged atoms or groups of atoms.

Understand Main Ideas

1. iman<TTfla Differentiate among the three parts of an atom in terms of their location, charge, and mass.

2. Explain why the elements magnesium and calcium have similar properties.

3. Illustrate how a neutral atom becomes an ion.

4. Compare and contrast these isotopes: uranium-239, uranium-238, and uranium-235.

Think Critically

5. Design an illustration using the concepts of valence electrons and energy levels to explain why oxygen might combine with magnesium.

6. Interpret the representation of magnesium in the periodic table. Explain why the atomic mass of magnesium is not a whole number.

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7. As the radioactive isotope radium-226 decays, it emits two protons and two neutrons. How many protons and neutrons are now left in the nucleus? What is the atom's new atomic number? What is the name of this element?

Self-Check Quiz

Section 3.2


I Describe the chemical bonds that unite atoms to form compounds.

I Relate the nature of chemical bonds that hold compounds together to the physical structures of compounds.

I Distinguish among different types of mixtures and solutions.

Review Vocabulary ion: an electrically charged atom

New Vocabulary compound chemical bond covalent bond molecule ionic bond metallic bond chemical reaction solution acid base

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