Structure of the Atom Chapter 4

Early Ideas About Matter Ancient Scholars used to believe that matter was composed of: earth, fire, water, and air. Many also believed that matter could be endlessly divided into smaller pieces.

Greek Philosophers Democritus (460-370 B.C.) – was the first person to propose the idea that matter was not infinitely divisible, but that it is made up of individual particles called atomos (indivisible). Aristotle (484-322 B.C.) – disagreed with Democritus, because he did not believe that empty space could exist. Aristotle’s views were not rejected for over 2000 years, until science developed new ways to test the validity of the ideas.

Democritus – Ideas on Matter Matter is composed of atoms, which move through empty space. Atoms are solid, homogenous, indestructible, and indivisible. Different kinds of atoms have different sizes and shape. Size, shape, and movement of atoms determine the properties of matter.

Aristotle – Ideas on Matter Empty space cannot exist. Matter is made of earth, fire, air and water.

John Dalton – Ideas on Matter John Dalton revived the idea of the atom in the early 1800’s based on numerous chemical reactions. Matter is composed of extremely small particles called atoms. Atoms are indivisible and indestructible. Atoms of a given element are identical in size, mass, and chemical properties. Atoms of a specific element are different from those of another element. Different atoms combine in simple whole number ratios to form compounds. In a chemical reactions, atoms are separated, combined and rearranged.

Who was the first person to propose the idea that matter is not infinitely divisible? A) Aristotle B) Plato C) Dalton D) Democritus

Dalton’s theory also help explain which of the following? A) the electron B) the neutron C) law of conservation of mass D) law of Democritus

Defining the Atom The atom is the smallest particle of an element that retains the properties of the element. Individual atoms can be seen with a scanning tunneling microscope (STM). Uses electron particle beam vs. light waves – Why?

The Electron J. J. Thomson measured the effects of both magnetic and electric fields on the cathode ray to determine the charge-to-mass ratio of a charged particle. The mass of the charged particle was over 2000x smaller than the mass of one hydrogen atom (which was the smallest known atom). Thomson received a Noble Prize in 1906 for identifying the first subatomic particle (the electron).

The Electron (continued) Robert Millikan used an oil-drop apparatus (shown below) to determine the charge of an electron. (early 1910s)

The Electron (continued) With the electron’s charge and a change-to-mass ratio known, Millikan was able to calculate the mass of a single electron: the mass of a hydrogen atom

The Electron (continued) Matter is neutral. If it was not you would receive an electric shock when touching it. How did this create a dilemma with the discovery of electrons? J. J. Thompson proposed a model of the atom to help solve this dilemma. His plum pudding model states that that atom is a uniform, positively charged sphere containing electrons.

The Nucleus Rutherford studied how positively charged alpha particles interacted with solid matter. He designed a gold foil experiment: By aiming the particles at a thin sheet of gold foil, Rutherford expected the paths of the alpha particles to be only slightly altered by a collision with an electron. Although most of the alpha particles went through the gold foil, a few of them bounced back, some at large angles. The repulsive force between the positively charged nucleus and positive alpha particles caused the deflections.

The Nucleus (continued) Rutherford concluded that atoms are mostly empty space. Almost all of the atom's positive charge and almost all of its mass is contained in a dense region in the center of the atom called the nucleus. He would then call the positive particles in the nucleus – protons. Electrons are held within the atom by their attraction to the positively charged nucleus. Later James Chadwick (1935) would discover the neutral particles in the nucleus - neutrons. The neutrons would account for the remainder of the atoms atomic mass.

Defining the Atom All atoms are made of three subatomic particles: Protons (positive) – mass of 1 Neutrons (neutral/no charge) – mass of 1 Electrons (negative) – insignificant mass (1/1840) Atoms are spherically shaped. Atoms are mostly empty space, and electrons travel around the nucleus held by an attraction to the positively charged nucleus. Chemical behavior of elements can only be explained by considering an atom’s electrons.

Subatomic Particles of an Atom

Atoms are mostly ________. A) positive B) negative C) solid spheres D) empty space

What are the two subatomic particles found in the nucleus? A) proton and electron B) proton and neutron C) neutron and electron D) neutron and positron