1.1 Atoms and Molecules

Standard 8.1.1:

What Are Atoms?

Have you ever wondered what you and a speck of dust in outer space have in common? The answer is that you and the speck of dust are made of atoms. Atoms are the building blocks of matter. They are what makes up all solids, liquids, and gases. Atoms are extremely small, so small that they cannot be seen by the naked eye. The radius of an atom is well under 1 nanometer, which is one-billionth of a meter. If a size that small is hard to imagine, consider this: trillions of atoms would fit inside the period at the end of this sentence.

History of the Atom

The history of our understanding of the atom is a classic example of how scientific knowledge changes over time. As one thinker builds on another thinker’s ideas and as technology advances, our understanding of how the world works becomes more and more accurate. Consider the following timeline of how people have modeled the atom.

• Around 450 B.C., the Greek philosopher Democritus introduced the idea of the atom. However, the idea was essentially forgotten for more than 2000 years.
• In 1800, John Dalton re-introduced the atom. He provided evidence for atoms and developed atomic theory. His theory was essentially correct. However, he incorrectly thought that atoms are the smallest particles of matter.
• In 1897, J.J. Thomson discovered electrons. He proposed the plum pudding model of the atom. In this model, negative electrons are scattered throughout a "sea" of positive charge.
• In 1911, Ernest Rutherford discovered the nucleus. He later discovered protons as well. Rutherford thought that electrons randomly orbit the nucleus.
• Niels Bohr and Ernest Rutherford, in 1913, represented the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around it, much like planets orbit the sun in the solar system.
• A little later, building on the thoughts of Bohr, Erwin Schrodinger took the understanding of the atom in a new direction when he developed the electron cloud model. The cloud model represents a sort of history of where the electron has probably been and where it is likely to be going. Imagine that as the electron moves it leaves a trace of where it was. This collection of traces quickly begins to resemble a cloud; the electron cloud.

Parts of an Atom

Although atoms are very tiny, they consist of even smaller particles. Three main types of particles that make up all atoms are as follows.

• At the center of an atom is a nucleus made up of two types of particles called protons and neutrons.
• Protons have a positive electrical charge. The number of protons in the nucleus determines what element the atom is.
• Neutrons are about the same mass as protons but have no charge.
• Electrons, much smaller than protons or neutrons, have a negative electrical charge, move at nearly the speed of light, and orbit the nucleus at certain distances, depending on their energy. 

The model in the figure shows how these particles are arranged in an atom. At the center of the atom is a dense area called the nucleus, where all the protons and neutrons are clustered closely together. The protons and neutrons make up almost all of the mass of an atom. The electrons have almost no mass and are constantly moving around the nucleus. Because the protons and electrons have opposite electric charges they attract each other; negative electrons are attracted to the positive nucleus. This force of attraction keeps electrons constantly moving through the otherwise empty space around the nucleus. The number of protons in a neutral atom equals the number of electrons. This makes atoms neutral in charge because the positive and negative charges “cancel out.”

Using our knowledge of an atom, what is accurate and inaccurate about the model shown? The model is very useful in showing us the parts of the atom and their approximate locations. What it fails at is to show the correct scale of an atom. For example, the nucleus of an atom is one trillionth the size of the whole atom. The rest of the atom is mostly empty space. Although this model is inaccurate we will frequently see and use it because making an accurate model is impractical given the size and scale of this book. Go to the following link to understand the relative size of the atom and nucleus: https://www.ted.com/talks/just_how_small_is_an_atom (Links to an external site.)Links to an external site.  

Elements 

Think back again to you and a speck of dust in outer space. We know that we are both made of atoms but it is important to understand that not all atoms are the same. All atoms have the same structure, in that they are made of protons, neutrons, and electrons. What makes one atom different than another atom is that they have different numbers of protons. Atoms with different numbers of protons are called elements; each element has its own unique number of protons in its atoms. Elements are pure substances —such as nickel, hydrogen, and helium—that make up all kinds of matter. Examine the figures. A helium atom has two protons, whereas a lithium atom has three protons. Go to the following link to further learn about what elements are: https://youtu.be/atcrgTH_ul4 (Links to an external site.)Links to an external site. / (Links to an external site.)Links to an external site. 

Molecules

When two or more atoms combine, it makes a molecule. Molecules make up the millions of things that make up most of the world. One of the most common molecules we have on Earth is water . It is made of two atoms of hydrogen (H) and one atom of oxygen (O). (See the figure.) Other common examples of molecules are carbon dioxide which is made of two carbon atoms and one oxygen atom (CO₂), salt which is made of one atom each of sodium and chlorine (NaCl)and sugar (C₆H₁₂O₆) which is made of six carbons, twelve hydrogens, and six oxygens.