The Structure of the Atom Chapter 4 The Structure of the Atom

Early Ideas about Matter and the Atom Very early in history, Greek philosophers thought that matter was composed of four different elements: earth, water, air, and fire.

Democritus (460-370 B.C.) First person to propose the idea that matter was not infinitely divisible. He came up with the thought that matter was made of tiny particles called atomos. His atom could not be further divided.

Aristotle (384-322 B.C.) Aristotle was a big supporter of the idea that matter was made of earth, wind, fire, and air.

John Dalton (1766-1844) Atomic theory didn’t come back until John Dalton proposed that matter was made of extremely small particles called atoms.

Dalton’s Atomic Theory 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms.

J.J. Thomson JJ Thomson introduces the concept of the electron using an experiment involving a “cathode ray”. From his experiment, he developed the “Plum Pudding Model”

http://www.youtube.com/watch?v=7YHwMWcxeX8

JJ Thomson’s Plum Pudding Model

Ernest Rutherford (1911) Rutherford came up with the idea of the nucleus, where most of the mass of an atom lies, as well as all of the positively charges particles. He later called the particles “protons”.

Rutherford’s Gold Foil Experiment

http://www.youtube.com/watch?v=wzALbzTdnc8

Thomson vs. Rutherford

Niels Bohr (1913) Bohr concluded that the electrons in an atom travel in a fixed path, called “orbits” around the nucleus.

James Chadwick (1932) Chadwick proved the existence of neutrons in the nucleus, which improved upon Bohr’s Model.

The current Model It is now accepted that electrons do not travel in fixed orbits, rather they travel in what is known as an “electron cloud”. These are areas where the probability of finding an electron are high.

Modern Atomic Model

Anatomy of an atom Atoms are made of 3 basic subatomic particles Protons (+) Neutrons (0) Electrons(-)

Masses of Subatomic Particles Electron Proton Neutron 9.11 X 10 -28 g 1.673 X 10-24g 1.675 X 10-24g

Size of an Atom

Size of an Atom If a nucleus was increased to the size of an orange, then comparatively an orange would be the size of the earth

Atomic Number The atomic number is equal to the number of protons and the number of electrons in an atom.

Atomic Mass Atomic Mass is the equal to the number of protons plus the number of neutrons. AM= p + n

Number of Neutrons What’s the formula for the number of neutrons?

Warm Up SYMBOL Protons Electrons Neutrons Rb Fe Al Cs

Something interesting How we can see atoms: Scanning Tunneling Microscope

HOW STM WORKS http://www.youtube.com/watch?v=47UgMpXFVj4&feature=related http://www.youtube.com/watch?v=YcqvJI8J6Lc&NR=1&feature=fvwp

Image from a STM

STM With a STM, scientists can actually move atoms around to form shapes and even simple machines. This promoted the birth of a branch of science known as “nanotechnology”. It is even thought that we may some day be able to make machines the size a single molecule.

Isotope Isotopes are atoms that have the same number of protons, but have a different number of neutrons. -If two atoms have a different number of protons, can they be the same element?

Examples of Isotopes Carbon 12 Carbon 13 Carbon 14 Protons Electrons Neutrons

Behavior of Isotopes Despite having different numbers of neutrons, isotopes behave in generally the same way as one another. -Chemical behavior depends mainly on the number of electrons, not the neutrons.

Atomic Mass unit Scientists needed to come up with a standard mass to measure the masses of atoms. The decided to say that carbon weighs 12 amu and that 1 amu would be the mass of 1/12 a carbon atom. The mass of 1 amu is very nearly the mass of 1 proton or 1 neutron.

Practice Today Page 105: 5 Page 114: 7, 8, 10 Page 121: 21, 22 If you do not finish it in class, it will be due for homework tomorrow.

Atomic Mass Atomic Mass is the weighted average mass of the isotopes of an element.

Computing Average Atomic Mass Look at the Atomic Masses on the periodic table. Are there any whole numbers? The reason is because of isotopes.

Average atomic mass The average atomic mass is the averaged mass of all the isotopes of a certain element. It depends largely on the abundance of each isotope.

Three isotopes of Silicon Isotope name Isotope mass (amu) Relative Abundance Silicon-28 27.98 92.21 Silicon-29 28.98 4.70 Silicon-30 29.97 3.09

Calculating A.A.M To calculate the A.A.M, you need to first calculate the contribution of each isotope. (AM of Isotope A)(% of Isotope A) + (AM of Isotope B)(% of Isotope B)… AM = atomic mass of the isotope % = the percent abundance of the isotope.

CALC. AAM for silicon