Chapter 3 Atoms and the Periodic Table Section

Chapter 3: Atoms and the Periodic Table Section 3. 1: Structure of the Atom

Scientists and engineers use models to represent things that are difficult to see because they are too small, too large, or haven’t been created yet.

Chemists use scaled-up models to visualize how the atom is constructed. Models are often changed to include new information as it is learned.

The Greek philosopher Democritus (460 -370 BC) was the first to propose the idea that matter was made up of tiny particles (atomos) that could not be created, destroyed, or divided.

Democritus’ ideas about atoms were basically correct, but he had no way to test or prove them.

Democritus’ ideas were criticized by other philosophers, especially Aristotle (384 -322 BC) and were rejected.

Aristotle denied the existence of atoms and he was so influential that his opinion was accepted for 2000 years.

The concept of the atom was revived, studied, and revised in the 19 th century by John Dalton (1766 -1844).

English schoolteacher John Dalton conducted scientific research about atoms, which marked the beginning of the development of modern atomic theory.

Dalton’s model of the atom was a solid sphere with no smaller parts.

The next model of the atom was proposed in 1904 by English physicist Joseph John Thomson.

Thomson’s Model: Atoms contained small, negatively -charged particles called electrons that were evenly embedded in a positivelycharged sphere.

Thomson’s Model: Often called the chocolate chip cookie dough model.

Rutherford Model: The next model was proposed by British physicist Ernest Rutherford in 1911.

Rutherford Model: He said that most of the mass and all of the positive charge were located in a nucleus which was surrounded by electrons.

Bohr Model: In 1913, Danish physicist Niels Bohr proposed that electrons traveled in fixed orbits around the nucleus.

Bohr Model: James Chadwick added that the nucleus contained positive protons and neutral neutrons.

Electron Cloud Model: The currently accepted model proposed in 1926 that says electrons do not follow fixed orbits but do occur more frequently in certain areas around the nucleus.

Electron Cloud Model: *Electron cloud - the area around the nucleus of an atom where its electrons are most likely found. The electron cloud is 100, 000 times larger than the nucleus.

Electron Cloud Model: Electrons travel so quickly and have such small masses that it is impossible to know exactly where they are at any time.

Electron Cloud Model In the center of an atom there is a nucleus. *Nucleus - the small, positively-charged center of the atom

Electron Cloud Model The nucleus is made of protons and neutrons. *Proton – a positively charged subatomic particle in the nucleus of an atom *Neutron – a neutral subatomic particle in the nucleus of an atom.

Electron Cloud Model Encircling the nucleus in a cloud are the electrons. *Electron – a tiny negatively charged subatomic particle moving around outside the nucleus of an atom.

Electron Cloud Model Atoms do not have a charge because they have an equal number of protons and electrons whose charges exactly cancel.

Electron Cloud Model Electron clouds are also called orbitals. *Orbital – a region in an atom where there is a high probability of finding electrons. Each orbital can hold electrons.

Electron Cloud Model There are four different kinds of orbitals. Arranged from lowest to highest amounts of energy, they are called s, p, d, and f.

Electron Cloud Model The simplest one is an s orbital and is shaped like a sphere surrounding the nucleus.

Electron Cloud Model A p orbital is dumbbellshaped and can be oriented in three different directions.

Electron Cloud Model There are 5 different d orbitals and 7 different f orbitals. They have more complex shapes.

Electron Cloud Model *Valence electrons- electrons in the outermost energy level of an atom They are the ones farthest away from the nucleus and the ones that can take part in chemical bonds.