Skip to Content Go to sign in Skip to Primary Navigation Skip to Page Navigation Skip to Header Navigation Skip to Footer Navigation Toggle Accessibility Options or learn more about accessability

Physics Classroom is making strides to make our site accessible to everyone, and features many accessibility features.

Our site contains 6 navigation areas. The Primary, Secondary, and Page Level navigations have a screen reader version of their nav structure that allows using the left and right keys to navigate sibling navigation items, and up or down keys to navigate parent or child navigation items. The others can be navigated using tabs.

Within the main content, we leverage headers to provide in page or in tool navigation.

Although we are still rebuilding our content to leverage these tools, our images should have both short and verbose descriptions, the later describing in great detail the image for those who cannot see. Any formulas found within the images are often in the image figure below the image.

Equations and formulas are rendered using MathJax, which has both verbal, braille (including nemath braille), and keyboard navigation within them. Learn how to configure and leverage this for various screen readers on our Equation Navigation Page.

While not every area of Physics Classroom is usable purely from keyboard and screen reader, we are committed to continue work on making this possible. If you have questions or need additional help, please use this link to contact us .

Return to screen reader navigation

Chapter 3: Elements, Atoms, and Ions - Lesson 1: In Search of the Atom

Subatomic Particles

  1. Democritus to Dalton
  2. The Inside Story of the Atom
  3. Subatomic Particles
Getting your Trinity Audio player ready...

Hold down the T key for 3 seconds to activate the audio accessibility mode, at which point you can click the K key to pause and resume audio. Useful for the Check Your Understanding and See Answers.

The Big Idea

This lesson describes the world of subatomic particles (from electrons to protons and neutrons), explaining how their charges, masses, and roles affect the structure and behavior of atoms in modern chemistry.

 

Is the Atom Really Indivisible?

Is the atom an indivisible billiard ball or is there a structure to it?We started Lesson 1 by discussing the effort of natural philosophers like Democritus and scientists like John Dalton to establish a reasonable belief in the atom as the indivisible building block of matter. Dalton was indeed successful in convincing the scientific world of the value and validity of thinking in terms of atoms. But the concept of the atom as indivisible was quickly dispelled by the work of J.J. Thomson (1899) and Ernest Rutherford (1911). The atom is not a solid, miniaturized billiard ball with no internal structure or contents. By 1911, it was well understood that the atom contained electrons and protons. And there was plenty of evidence suggesting that there may be a third particle contained in an atom.
 
 
 

The atom contains electrons, protons, and neutronsDiscovery of the Neutron

In 1932, English physicist James Chadwick, a student of Ernest Rutherford, conducted studies of the element beryllium. When bombarding beryllium with alpha particles, Chadwick observed the emission of neutral particles. Chadwick confirmed that the particles had no charge and had a mass similar to the mass of a proton. Chadwick named these particles neutrons.
 
 
 

A Comparison of Three Subatomic Particles

The proton, the neutron, and the electron are the three so-called subatomic particles. The proton and neutron are each approximately 1000 times more massive than the electron. And both are located in the nucleus. This means that nearly all the mass of an atom is located in its very dense center. The positive charge (possessed by protons) is in the nucleus and the negative charge (possessed by electrons) is outside the nucleus. For an electrically neutral atom, the number of electrons must equal the number of protons.
 

Table comparing the location, relative mass, and relative charge of electron, proton, and neutron.



 
 

 

The Empty Space Model

The Rutherford atomic model and the Bohr atomic model are both considered nuclear models. That is, there is a dense, positively charged and massive nucleus at the core of the atom. The electrons are located outside the nucleus. And for both Rutherford and Bohr, those electrons can be thought of as being in orbit. The size of the nucleus is miniscule when compared to the size of the atom. Consider this comparison:
 

If the nucleus were the size of a golf ball, then the atom would be roughly the size of a professional sports stadium.

 
Put another way, the atom is about 100 000 times the size of its nucleus. When given these relative dimensions, one can imagine the atom as being primarily empty space.
 

 

Atomic structure diagram emphasizing the expanse of


 

 

 
 

 

The Rest of the Story

The story of the atom's structure will continue in Chapter 5. For now, we need to learn the story of how the elements were organized into the Periodic Table. Continue to the Lesson 2 to learn the story of how a Russian chemist by the name of Dimitri Mendeleev organized the known elements into rows and columns.
 
 
 

Before You Leave - Practice and Reinforcement

Now that you've done the reading, take some time to strengthen your understanding and to put the ideas into practice. Here's some suggestions.

  • The Check Your Understanding section below includes questions and problems with answers and explanations and solutions. It provides a great chance to self-assess your understanding.
  • Download our Study Card on Subatomic Particles. Save it to a safe location and use it as a review tool.
 
 
 
 

Check Your Understanding of Subatomic Particles

Use the following questions to assess your understanding of subatomic particles. Tap the Check Answer buttons when ready.
 
1. Identify the following statements as being TRUE or FALSE. If false, correct the statement or explain what is incorrect about it.
  1. Most of the charge of the atom is located in the nucleus.
    Check Answer

     
  2. The nucleus is mostly empty space.
    Check Answer

     
  3. The proton and the electron have the opposite types of charge.
    Check Answer

     
  4. The mass of a proton or a neutron is approximately 1000 times the mass of an electron.
    Check Answer

     
  5. Most of the mass of the atom is located in the nucleus.
    Check Answer

     
  6. The expanse of an atom consists primarily of empty space.
    Check Answer

     
  7. Most of the volume of the atom is attributed to the size of the nucleus.
    Check Answer

     
  8. Mass is uniformly distributed throughout the mass of the atom.
    Check Answer

     
  9. The proton and the neutron have the opposite type of charge.
    Check Answer

     
  10. The positive charge of an atom is in the nucleus and the negative charge is outside the nucleus.
    Check Answer

     
  11. The neutrons are located in the nucleus. The protons and electrons are located outside the nucleus.
    Check Answer

     

 
 
 
 
 


Return to Screen Reader Navigation
Tired of Ads? Go Ad Free ($5/year))