Book cover for Living by Chemistry

Living by Chemistry

Angelica M. Stacy

ISBN #9781464142314

2nd Edition

940 Questions

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129,188 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

The evolution of atomic models reflects the scientific process of hypothesis, experimentation, and refinement. Early ideas like Democritus’ indivisible atoms laid the groundwork, while experiments by Dalton, Thomson, Rutherford, Bohr, and Heisenberg revealed increasingly detailed pictures of atomic structure. Key concepts such as the nucleus, protons, neutrons, electrons, atomic number, and atomic mass help explain the behavior of matter and the differences between elements. Science continually refines models as new evidence becomes available.

Learning Objectives

1

Explain the evolution of atomic models from Dalton’s solid sphere to the electron cloud model.

2

Describe the key experiments (Dalton’s, Thomson’s, Rutherford’s, Bohr’s, Heisenberg’s) that contributed to our understanding of atomic structure.

3

Analyze how atomic number and atomic mass identify different elements.

4

Apply the concept of atomic theory to explain experimental observations in chemistry.

Key Concepts

CONCEPT

DEFINITION

Atom

The smallest unit of matter that retains the properties of an element.

Atomic Theory

A scientific theory that states all matter is composed of atoms, and these atoms combine in specific ratios to form compounds.

Model

A simplified representation of a complex structure or concept; in this context, a representation of the atom.

Nucleus

The central part of an atom containing protons and neutrons, responsible for most of its mass.

Proton

A positively charged particle located in the nucleus of an atom.

Neutron

A neutrally charged particle found in the nucleus of an atom.

Electron

A negatively charged, much smaller particle that orbits the nucleus.

Atomic Number

The number of protons in an atom’s nucleus, which defines the element.

Atomic Mass

The mass of an atom, primarily determined by the number of protons and neutrons.

Example Problems

Example 1

What evidence caused Thomson to change Dalton's solid sphere model into the plum pudding model?

Example 2

What evidence caused Rutherford to change h omson’s plum pudding model into the nuclear model?

Example 3

What evidence caused Bohr to change Rutherford’s nuclear model into the solar system model?

Example 4

Positive and negative charges are attracted to one another. Which of the following are attracted to a negative charge: an electron, a proton, a neutron, a nucleus, an atom? Explain your thinking.

Example 5

Hydrogen and helium are different elements. How can you use the plum pudding model to show how atoms of the two elements might be different from one another?
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Step-by-Step Explanations

QUESTION

How did Rutherford’s gold foil experiment support the nuclear model of the atom?

STEP-BY-STEP ANSWER:

Step 1: Alpha particles (positively charged and relatively massive) were directed at a thin sheet of gold foil.
Step 2: Most alpha particles passed through the foil with little or no deflection, indicating that atoms contain mostly empty space.
Step 3: A small fraction of the alpha particles were deflected at large angles or bounced back, implying the presence of a small, dense, positively charged region within the atom.
Step 4: Rutherford concluded that this dense region is the nucleus, containing protons (and later neutrons), around which electrons orbit.
Final Answer: Rutherford’s experiment provided evidence for a nuclear model by demonstrating that the positive charge and most of the atom’s mass is concentrated in a small nucleus.

Rutherford’s Gold Foil Experiment

QUESTION

How does the atomic number help identify an element?

STEP-BY-STEP ANSWER:

Step 1: Recognize that the atomic number is defined as the number of protons in the nucleus.
Step 2: Understand that each element has a unique number of protons.
Step 3: Connect that in a neutral atom, the number of electrons equals the number of protons, affecting the chemical properties.
Step 4: Use the atomic number to arrange elements in the periodic table in increasing order.
Final Answer: The atomic number uniquely identifies an element by its number of protons and, consequently, its chemical behavior.

Atomic Number Determination

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Common Mistakes

  • Assuming that atoms are simple, solid spheres without internal structure.
  • Confusing early models (like the plum pudding model) with the current understanding of electron probability clouds.
  • Overlooking the importance of protons and neutrons in determining atomic mass, focusing only on electrons.
  • Misinterpreting atomic number as a measure of atom size instead of the number of protons.