Topic Overview

Chemistry is the study of matter, which is anything that has mass and volume. The desk that you are sitting at, the air around you, and your body are all made up of matter. Chemistry deals with the composition of matter and the changes that matter may undergo.

Early Studies of Atoms

The Greeks’ view of nature of matter as being composed of earth, water, fire, and air lasted until the 1600s when Robert Boyle identified gold and silver as themselves being elemental; that is, they are not themselves made of fire, air, earth or, water. As Boyle’s ideas were slowly accepted,

additional elements were discovered, and the Greek concept of what makes up of matter faded.

Dalton’s Atomic Theory

The work of Boyle led John Dalton to propose his revolutionary theory in the 1700s. He theorized that the basic unit of matter is called an atom.

Dalton’s theory of the atom can be summarized by the following points:

1. All elements are composed of indivisible atoms

2. All atoms of a given element are identical

3. Atoms of different elements are different; that is, they have different masses

4. Compounds are formed by the combination of atoms of different elements

Although we now know that some of Dalton’s theory was not correct, it laid the important groundwork for the current concept of the atom.

Structure of the Atom

Experimental studies of the atom soon showed that it was not indivisible but it was made up of smaller parts

Electrons J.J Thompson used a cathode ray tube to show one of these smaller units that make up an atom. Because the ray produced in the tube was deflected a certain way by an electrical or magnetic field, he concluded that the ray was formed by particles and that the particles were negatively charged, which he then named them electrons.

A concept of the atom developed in which these negatively charged particles were visualized as being embedded in atoms, just as we might find raisins in bread. This model was called the plum pudding model. In this model, the rest of the atom was evenly distributed and positively charged, taking up all of the space not occupied by the electrons.

The Nucleus If electrons are present in atoms, what makes up the rest of the atom? One scientist who studied this was Ernest Rutherford. A group of scientists that included Rutherford conducted an experiment that involved directing alpha particles, which are positively charged particles that are much smaller than an atom, at a thin piece of gold foil. If the plum pudding model of the atom were correct, all of the alpha particles would’ve passed through the gold foil with just a few being slightly deflected.

As the scientists expected, most of the particles passed through the gold foil, and a few were slightly deflected. But to their amazement, some of the alpha particles were greatly deflected, and some even bounced back. From this experiment Rutherford concluded that atoms have a dense central core, called the nucleus, while the remainder of an atom is essentially empty space.

Because alpha particles are positively charged and were repelled by the nucleus, the nucleus must also be positively charged because like repels like, and opposite attracts opposite.

Protons and Neutrons Since atoms are electrically neutral, scientists reasoned that there must be positive charges to offset the negatively charged electrons, and these charges must be located in the nucleus. These positively charged particles are called protons. In addition to protons it was later discovered that there are additional particles in the nucleus that do not have either a positive or negative charge. These neutral particles are called neutrons.

Modern Atomic Theory

Scientists have used the info derived from this and other experiments to further describe atomic structure.

The Bohr Model In the early 20th century the common model of the atom was the Bohr or the planetary model. The model showed a center, the nucleus, and rings of orbiting electrons.

Normally, composition of the nucleus shows the number of positively. charged protons and the number of neutral neutrons present. The outermost shell of an atom may not contain more than 8 electrons. These outermost electrons are called valence electrons.

When a valence shell is filled, the element is a noble gas. When a shell is filled, the next element begins to fill the next higher energy valence shell. The same information can be shown in a linear form. The symbol of the element is followed by showing the number of electrons in each succeeding shell. Thus Na-2-8-1 represents the sodium atom. The "Na" represents the nucleus and the 2-8-1 shows the electron arrangement in the K, L, and M shells. The electron structure of each element is shown on the Periodic Table of the Elements in the Reference Tables for Physical Setting/Chemistry.

The Wave-Mechanical Model Advances in the study of energy aided in modifying the atomic model. Energy had been viewed as being waves, and matter as particles. By the 1900s, energy and matter were both viewed as acting as both waves and particles. The wave aspect of nature was expanded, and it was also proposed that energy was made up of tiny packets called quanta. These energy packets acted like particles.

When it was later determined that the electron not only has properties of mass but also has wavelike properties, this concept of a dual nature was incorporated into the current model of the atom, the wave-mechanical model. This modern model of the atom pictures the atom as having a dense, positively charged nucleus as proposed in the planetary model. The major difference between the wave-mechanical model and the Bohr model is found in the manner in which the electrons are pictured.

Instead of moving in definite, fixed orbits around the nucleus as suggested in the Bohr model, the wave-mechanical model portrays electrons with distinct amounts of energy moving in areas called orbitals. An orbital is described as a region in which an electron of a particular amount of energy is most likely to be located. Thus, the modern model of the atom is not the invention of a single scientist, but rather one that has evolved over a long period of time.

Vocabulary

Atom - Smallest particle of an element

Gold Foil Experiment - Experiment conducted by Rutherford which determined that the nucleus is small, dense and positively charged and the atom is mostly empty space

Rutherford Model - Developed after the Gold Foil Experiment, model of the atom with a small dense, positive nucleus and is mostly empty space

Proton - Positive subatomic particle in the nucleus

Neutron - Neutral subatomic particle in the nucleus

Electron - Negative subatomic particle

Orbiital - A region of an atom in which an electron of particular energy is most likely to be located

Nucleus - The dense, positively charged central core of atom that contains protons and neutrons

Valence Shell - Outermost shell of electrons in an atom

Valence Electron - Electron in the outermost energy level

Atomic Number - Equal to the number of protons, gives the identity of the atom

Atomic Mass Unit (amu) - The mass of a proton or neutron

Mass Number - The sum of protons and neutrons

Bohr Model - Shows contents of nucleus, energy levels and number of electrons in each level of an atom of or ion

Lewis Structure - A diagram that shows valence electrons as dots and shared electrons as lines around the atomic symbol

Isotope - Atom of an element with a particular number of neutrons

Ground State - When all electrons in an atom/ion are in the lowest state of energy

Excited State - When an electron is in Ground State absorbs energy and jumps up to higher energy levels leaving vacancies in low energy levels

Spectral Lines - Often used to identity atoms and molecules