Except for relativity, everything you have studied until now was known by 1900. But within the span of just a few years, around 1900, investigations into the structure of matter and the properties of light led to new discoveries at odds with classical physics.

Our ultimate goal is to understand the new theories of matter and light that arose in the 20th century. But there’s a problem: We can’t see atoms. Thus the goal of this chapter is to establish the experimental evidence for how we know about atoms and their structure.

Scientists use spectroscopy, with diffraction gratings, to study how light is emitted and absorbed.

• Solids emit a continuous spectrum known as blackbody radiation.

• Atoms emit and absorb many distinct wavelengths, called a discrete spectrum.

Experiments to study electricity in gases found that unknown “rays” travel outward from the cathode—the negative electrode. J. J. Thomson discovered that these cathode rays are subatomic particles—electrons—that exist within atoms. The electrons of all atoms were found to be identical.

Ernest Rutherford discovered that the positive charge within an atom is concentrated in a small, dense nucleus. This led Rutherford to propose a solar-system model of the atom with light, negative electrons orbiting a tiny, positive nucleus.

《 LOOKING BACK Chapter 25 Electric potential and potential energy

The nucleus consists of positive protons and neutral neutrons.

• The number of protons is the atomic number. Every element has a different atomic number.

• Neutrons provide additional “glue” to hold the nucleus together.

《 LOOKING BACK Section 18.2 Atomic masses

Modern technology, from lasers to semiconductors, is based on the properties of atoms. The worldwide system of timekeeping utilizes atomic clocks. Much of what we know about the cosmos comes from studying the atomic spectra of stars and galaxies. And quantum computers, when they become available, will require the careful manipulation of atomic states. It’s an atomic world!