The AtomEarly Theories & Subatomic

Particles (4.1-4.2)

Objectives

1. Describe the evolution of the atom from Dalton’s time to Rutherford’s.

2. Define and describe the structure of an atom

3. 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:1. Each element made up of extremely small

particles called atoms

2. 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

3. Different atoms combine in simple whole-number ratios to form compounds (H2O, CaCO3, NaCl)

4. In chemical reactions it’s the atoms that are separated, combined or rearranged

5. 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:

1. Atom – is smallest particle of an element that retains the properties of that element, but• atoms can be divided into smaller

parts!

2. Not all atoms of an element have identical properties--Their masses may vary slightly

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 penny29,000,000,000,000,000,0

00,000

diameter of a single copper atom

0.000000000128 m

• 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

Rutherford’s Atom continued

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 electronCharge -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 -10.00054

86

Proton p + 1.6022 x 10 -19 1.673 x 10-24 +1 1.0073

Neutron

n 0 1.675 x 10-24 0 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