Discovering the Pieces of the Atom — Reading Comprehension

Matter makes up everything in our world, from the air we breathe to the furniture in our classrooms. For centuries, scientists wondered what matter is made of at its smallest level. The journey to understand matter led to the discovery of the atom's structure, forever changing how we view the universe. Today, we know that atoms are built from even smaller particles, called subatomic particles.

Revealing the Electron: Thomson’s Cathode Ray Experiments
In the late 1800s, J.J. Thomson conducted experiments using cathode ray tubes. He noticed that when electricity passed through a vacuum tube, it created a glowing beam. By applying electric and magnetic fields, Thomson observed that the beam bent toward the positive side, suggesting the particles in the beam were negatively charged. These particles were named electrons. Thomson’s experiments showed that atoms, once thought to be indivisible, actually contained smaller, negatively charged particles. He proposed a new model of the atom, known as the “plum pudding model,” where electrons were spread throughout a positively charged sphere, like raisins in a pudding.

Discovering the Nucleus: Rutherford’s Gold Foil Experiment
While Thomson’s model explained the presence of electrons, it couldn’t account for all atomic behavior. In 1909, Ernest Rutherford and his team shot positively charged alpha particles at a thin sheet of gold foil. Most particles passed straight through, but a few bounced back at sharp angles. Rutherford concluded that atoms must have a dense, positively charged center, called the nucleus. The nucleus contained most of the atom’s mass, while electrons orbited around it. This discovery led to a new atomic model and explained why atoms are mostly empty space.

Uncovering the Neutron: Chadwick’s Contribution
Even after the discovery of the nucleus, scientists noticed that atoms of the same element could have different masses, despite having the same number of protons. In 1932, James Chadwick found the answer. He discovered the neutron, a particle in the nucleus that had no electric charge but added mass. The presence of neutrons helped explain atomic stability and why some elements have several forms, called isotopes.

Evolution of Atomic Models
Each of these discoveries forced scientists to revise their atomic models. As new evidence emerged, old ideas were replaced. This process shows how science is a system for building knowledge. Today, the modern atomic model describes atoms as having a nucleus made of protons and neutrons, surrounded by a cloud of electrons. This model helps scientists explain chemical reactions, electricity, and even nuclear energy.

Real-World Impact
Understanding the structure of the atom has had enormous effects on technology, health, and society. For example, medical imaging technologies, such as MRI and PET scans, use knowledge of atomic particles to diagnose diseases. Nuclear power plants use the splitting of atomic nuclei to generate electricity. The evolution of atomic theory demonstrates how scientific progress can lead to advances that benefit humanity.

Interesting Fact: More than 99.9% of an atom’s mass is found in its nucleus, even though the nucleus is less than one-trillionth the volume of the atom!

What did J.J. Thomson discover through his cathode ray experiments?

Which part of the atom did Ernest Rutherford discover?

Who discovered the neutron?

What is the main idea of the passage?

What is an isotope?

What effect did Rutherford’s gold foil experiment have on atomic models?

Why did scientists change atomic models over time?

Which real-world technology uses knowledge of atomic particles?

True or False: The nucleus contains most of an atom's mass.

True or False: Electrons are found inside the nucleus.