The Structure of the Atom

Section 3.2

Introduction• Atom: the smallest particle of an

element that retains the chemical properties of that element

• Nucleus: very small region located at the center; composed of proton(s) and usually one or more neutrons

• Proton: a positively charged particle in the nucleus

More

• Neutron: a neutral particle found in the nucleus

• Electron: a negatively charged particle found in a region surrounding the nucleus

• Subatomic particles: protons, neutrons, and electrons

Discovery of the Electron

• A glass tube was prepared with a gas in it with metal disks at each end

• The cathode ray tube was connected to a vacuum pump because the charges will only flow through gases at low pressure

• An electric current was passed through various gases

More

• When the current was passed through the gas, the surface of the tube opposite the cathode (negative charge) glowed

• Also, the ray of light traveled from the cathode to the anode

• The rays were deflected away from a negatively charged object

More

• In 1897, J.J. Thomson used the cathode ray tube to deduce the presence of a negatively charged particle: the electron

Charge and Mass of the Electron

• Thomson’s experiment revealed that the electron has a very large charge-to-mass ratio

• In 1909, Robert Millikan measured the charge of the electron

• More accurate experiments were conducted later to determine the mass

Mass of the Electron

The oil drop apparatus

Mass of the electron is 9.109 x 10-31kg

Robert Millikan

Thomson’s Model of the Atom

• Thomson’s model is called the plum pudding model

• His model is not valid• He believed that the negative electrons

were spread evenly throughout the positive charge of the rest of the atom

J. J. Thomson and his Model

Ernest Rutherford’s Gold Foil Experiment

• Alpha particles are helium nuclei - The alpha particles were fired at a thin sheet of gold foil

• Go

Rutherford’s Findings

• Most of the particles passed right through• A few particles were deflected• Some were greatly deflected

“Like howitzer shells bouncing off of tissue paper

Conclusions of the Experiment

–The nucleus is small–The nucleus is dense–The nucleus is positively charged–The atom is mostly empty space

Rutherford’s Model–All the positive charge, and almost all the

mass is concentrated in a small area in the center. He called this a nucleus–The nucleus is composed of protons and

neutrons (they make the nucleus!)–The electrons are distributed around the

nucleus, and occupy most of the volume–His model was called a “nuclear model”

Other Discoveries–Eugene Goldstein in 1886 observed what

is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1836 times that of an electron)–1932 – James Chadwick confirmed the

existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

Subatomic ParticlesParticle Relative

ChargeRelative

massLocation Actual mass

in kg

Electron (e-)

-1 __1__1836

electron cloud

9.109 x 10-31

Neutron (n0)

0 1 nucleus 1.675 x 10-27

Proton (p+)

+1 1 nucleus 1.673 x 10-27

Other Interesting Stuff

• Atoms are electrically neutral because they contain an equal number of protons and electrons

• The number of protons determines the atom’s identity

• There are other subatomic particles, but they have little effect on the chemical properties of matter

Forces in the Nucleus

• Protons, with their positive charge, would be expected to repel each other in the nucleus

• When they are extremely close to each other, there is a strong attraction

• A similar attraction exists when neutrons are present

More

• Nuclear forces: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus

• Nuclei are very dense

Sizes of Atoms

• Electrons occupy the outer region known as the electron cloud

• Radius of an atom is the distance from the center of the nucleus to the outer portion of the electron cloud

• Atomic radii are extremely small so they are usually measured in picometers

• 1 x 1012 pm = 1 m