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THINK ABOUT IT

The drawing depicts a very tiny sample of gold taken from a gold ring.

The spheres in the cube of gold are so small that they cannot be seen. What are the spheres, and what does this drawing tell you about the element gold?

How are the smallest bits of matter described?

To answer this question, you will explore

EXPLORING THE TOPIC

Atoms: Small Bits of Matter

Imagine you break a piece of matter in half, and then break it in half again and again. How many times can you do this? Can you keep going, getting ever smaller? Around 460 B.C.E., the Greek philosopher Democritus wondered the same thing. He thought that if he could just keep breaking matter in half he would eventually end up with the smallest bit of matter possible.

Democritus proposed that all matter was composed of tiny particles that could not be divided further. Today, we use the word atom to describe these bits of matter. Of course, atoms are too small to be seen. Democritus’ idea was disregarded for the next two thousand years, in part because Democritus did not have evidence to support it.

In 1803, the British scientist John Dalton suggested that the idea of atoms could help explain why elements come together in specific ratios when they form compounds. He imagined atoms of different elements combining to form compounds in the ratios specified by the chemical formulas of the compounds. For example, to form the compound titanium sulfide, TiS₂, titanium and sulfur atoms combine in a 1:2 ratio.

Dalton had more than an idea about atoms. He conducted experiments and made observations to back up his idea. His observations provided strong evidence to support his explanation of how matter behaves.

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In science, the word theory indicates that an explanation is supported by overwhelming evidence. The word theory allows room for doubt and revision, but indicates a greater degree of certainty than the word does in everyday use. The atomic theory states that all matter is made up of atoms. The atomic theory helps us make accurate predictions about the behavior of matter.

Models of the Atom

Since Dalton’s time, scientists have created many models to describe atoms and their parts. Models are simplified representations of something you want to explain. For example, a model airplane is a small representation of a larger aircraft. Models take many forms. They can be a plan, a physical structure, a drawing, a mathematical equation, or even a mental image. A model that represents the structure of an atom is called an atomic model.

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Dalton pictured the atom as a hard, solid sphere. Over the next two hundred years, scientists gathered evidence to support and expand on Dalton’s model of the atom. It became clear that the atom was more than just a solid sphere.

But how did scientists gather evidence about something too small to be seen? Scientists found they could learn more about atoms and their structure by shooting small pieces of matter at them or by heating them in a flame. Observations from these experiments provided evidence that helped scientists make changes and refine the model of the atom.

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The model of the atom was refined and changed as scientists gathered new evidence. This is what science is all about—a continual process of gathering new knowledge to improve our understanding of the world.

Simple Atomic Model

The six models shown on the timeline all have something valuable to offer in terms of visualizing matter at an atomic level. At right is a simple atomic model of an atom. In the very center of the atom is the nucleus. The nucleus consists of positively charged protons, and neutrons, which have no charge. The electrons are even tinier than the protons and neutrons, and they orbit the nucleus. In this particular atom, the electrons are located at two different distances from the nucleus.

Each electron has a charge of –1. The neutrons are neutral and so have no charge. Each proton has a charge of +1. A neutral atom has no overall charge. It has equal numbers of positive protons and negative electrons.

LESSON SUMMARY

How are the smallest bits of matter described?

Long ago, some philosophers imagined that matter was made up of tiny particles called atoms. Over time, scientists gathered evidence from experimental observations to create models of the atom. Today, we know the atom is made up of protons, neutrons, and electrons. The protons and neutrons are in the center of the atom, in the nucleus. Electrons are outside the nucleus. They are much smaller than the protons and neutrons. In a neutral atom, the positive charges on the protons are equal to the negative charges on the electrons.

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