• ### Unit 2: The Atom

The atom is the basic unit of matter and serves as our starting point for the study of chemistry. The atom is composed of the subatomic particles protons, neutrons, and electrons. Scientists have studied atoms for hundreds of years and have developed a number of different models to describe them, as experimental technology has improved and new discoveries have been made. Chemists currently use the quantum mechanical model of the atom.

In this unit, we explore the structure and properties of atoms. We also study some of the basic tenets of quantum mechanics, and how quantum mechanics describes atomic structure. Finally, we learn about the structure and organization of the periodic table of the elements.

Completing this unit should take you approximately 5 hours.

• ### 2.1: Atoms and Elements

Let's begin by discussing elements. You are probably familiar with many elements, such as sodium, oxygen, and helium, from everyday life. Elements correspond to the symbols you see listed on the periodic table. All atoms are made of the subatomic particles protons, neutrons, and electrons.

Protons and neutrons exist in the nucleus, or the high-density center of the atom. Electrons exist as an electron cloud around the nucleus.

• Protons are positively charged
• Neutrons are not charged
• Electrons are negatively charged

Different elements are defined by the number of protons in the nucleus. For example, all hydrogen atoms have one proton in their nucleus. All helium atoms have two protons in their nucleus.

• ### 2.2: Avogadro's Number and Moles

In chemistry, we need a way to connect the microscopic world of atoms to the macroscopic world we live in. Even small objects in our macroscopic everyday world consist of an enormous number of atoms. We use moles as a counting number to keep track of these enormous numbers of atoms we encounter in the lab and in our lives. The mole is a counting number in the same way a dozen is a counting number. A dozen always equals 12, no matter what the object is.

A mole (often abbreviated as mol) always equals 6.022 × 1023 objects. The number 6.022 × 1023 is known as Avogadro's Number. We can use Avogadro's number to convert between the number of atoms and the number of moles of a substance.

We can now look at the atomic masses on the periodic table in a new light. When we see the average atomic masses written on the periodic table, they are in atomic mass units (amu or u). This is the mass of a single atom of the element in a mass unit specifically for atoms. For example, one atom of hydrogen is 1.01 amu. We can also relate this atomic mass to the mass in g of a mole of the substance. So, one mole of hydrogen is 1.01 g/mol. This allows us to convert between moles and mass.

• ### 2.3: Atomic Theories

To understand atomic structure, we cannot use the classical physics used to describe the macroscopic world around us. The theory of quantum mechanics describes wave-particle duality, where waves can have particle-like properties and particles can have wave-like properties.

• ### 2.4: The Periodic Table of the Elements

We are now ready to explore how the quantum mechanical model of the atom relates to the order of the periodic table. The elements in the periodic table are listed in increasing atomic number (Z). Recall that atomic number is the number of protons. For a neutral atom, the number of protons equals the number of electrons. Therefore, the periodic table is ordered by the increasing number of electrons. We can use the periodic table as a tool to determine the electron orbitals filled in a given element.