Early Theories & Subatomic Particles (4.1-4.2) The Atom Early Theories & Subatomic Particles (4.1-4.2)

Objectives Describe the evolution of the atom from Dalton’s time to Rutherford’s. Define and describe the structure of an atom Identify the subatomic particles and the charge, mass and location of each.

MATTER Anything that has mass and takes up space

DEMOCRITUS (460-370 B.C.) First to propose: Matter cannot be endlessly divided An atom is a tiny particle of matter which cannot be further divided Word “atom” came from the Greek term atomos which means “indivisible” Many of his contemporaries criticized his theory

JOHN DALTON In his Atomic Theory (1808), Dalton hypothesized: Each element made up of extremely small particles called atoms Atoms of a given element are identical They have the same size, mass, chemical properties The atoms of one element are different from atoms of all other elements Different atoms combine in simple whole-number ratios to form compounds (H2O, CaCO3, NaCl) In chemical reactions it’s the atoms that are separated, combined or rearranged Atoms cannot be created, destroyed, or divided into smaller particles (conservation of matter)

Dalton’s theory was important in understanding matter but was inaccurate on certain points: Atom – is smallest particle of an element that retains the properties of that element, but atoms can be divided into smaller parts! Not all atoms of an element have identical properties--Their masses may vary slightly

diameter of a single copper atom THE ATOM An atom is the smallest particle of an element that retains the properties of the element An atom is the basic unit of an element world population 6,600,000,000 copper atoms in a penny 29,000,000,000,000,000,000,000 diameter of a single copper atom 0.000000000128 m

Rutherford’s Atom continued Atom’s diameter is 10,000 times that of the nucleus Atom: Size of the entire atom: 2 football fields Size of the nucleus: A nickel

SUBATOMIC PARTICLES Dalton believed that atoms could not be divided into smaller particles However later experiments led to the discovery that atoms were made up of even smaller particles, “subatomic particles” electrons, protons and neutrons

THE ELECTRON In 1897, through his cathode ray experiments, JJ Thomson was credited with identifying the electron When he applied high voltage to an empty glass tube a ray he called a “cathode ray” was produced he proposed that these rays are streams of particles much smaller than atoms-they are in fact pieces of atoms We now know these particles to be electrons

THOMSON CONTINUED Thomson imagined the atom as a “plum pudding” with negative charges distributed throughout a uniform positive charge

MILLIKAN’S OIL DROP EXPERIMENT (1909) Determined the charge and calculated the mass of the electron Charge -1.6022 x 10-19 C Mass 9.11 x 10-28 grams

THE PROTON AND THE NUCLEUS

DISCOVERING THE NUCLEUS: RUTHERFORD’S GOLD FOIL EXPERIMENT (1919) Carried out his gold foil experiment to test Thomson’s “plum pudding” model of the atom Directed alpha particles (+ charged) at a thin sheet of gold foil He expected that if Thomson’s model was correct and the positive charge of an atom was so diffuse, the alpha particles would mostly pass through gold foil

RUTHERFORD’S GOLD FOIL EXPERIMENT (1919)

GOLD FOIL EXPERIMENT With Thomson’s Plum Pudding Model in mind Rutherford expected the massive alpha (+ charged) particles to mostly pass through gold foil Instead, he found that although most particles went through the foil a few particles deflected back at large angles

RUTHERFORD’S CONCLUSION Most of the atom is empty space (that’s why most of the a-particles passed through foil) The atom’s positive charge is concentrated in the nucleus the nucleus is the atom’s dense central core The positively charged particles in the nucleus are called protons Nuclear model of the atom Dalton’s Atom - Thomson’s Atom + Rutherford’s Atom

RUTHERFORD CONTINUED In other experiments found the proton had the same quantity of charge as an electron (equal but opposite) and a mass of 1.67262 x10-24 g, 1840 times the mass of an electron

THE NEUTRON Rutherford & Chadwick 1932 Neutral particle in nucleus Mass nearly equal to that of a proton (slightly greater)

SUBATOMIC PARTICLES particle symbol absolute charge (coulombs) absolute mass (grams) relative charge relative mass (U) Electron e or e- - 1.6022 x 10 -19 9.11 x 10-28 -1 0.0005486 Proton p + 1.6022 x 10 -19 1.673 x 10-24 +1 1.0073 Neutron n 1.675 x 10-24 1.0087

THE ATOM IN SUMMARY The atom is a neutral particle composed of electrons, protons and neutrons: Nucleus contains: (+) charged protons & neutral neutrons (Exception: H atom has 1 proton & 0 neutrons) Most (99.7%) of the atom’s mass Most of the volume of an atom is made up of a cloud of fast moving (-) charged electrons that travel through the empty space surrounding the very small, very dense nucleus Electrostatic forces hold electrons (-) to the nucleus (+) Since atoms are neutral Number protons = number electrons