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

I. Early Philosophers A. Democritus (460 – 370 B.C.) Democritus was the first person to propose the idea that matter was made up of tiny individual particles called atomos He believed matter is composed of empty space through which atoms move - we now know that matter is composed of atoms, not empty space

B. John Dalton (1766-1844) Dalton revised Democritus theory in the 19th Century He believed atoms are separated, combined, or rearranged in chemical reactions - this follows the conservation of mass that states atoms cannot be created, destroyed, or divided

II. Defining the Atom At atom is the smallest particle of an element that retains the properties of the element A typical copper penny contains 29,000,000,000,000,000,000,000 (2.9x1022) atoms of copper The scanning tunneling microscope allows individual atoms to be seen

III. Discovering the Electron Scientists passed electricity through a vacuum pump in which most air had been removed with metal electrodes located at opposite ends of the tube

The electrode connected to the negative terminal of the battery is called the cathode The electrode connected to the positive terminal of the battery is called the anode Cathode Anode

As the electricity passed through the vacuum it lit up as it hit a light-producing coating that head been applied to the end of the tube Because the radiation originated from the cathode end of the vacuum tube, it became known as a cathode ray tube

Altering the gas in the tube and the material used for the cathode had no effect on the cathode ray - it was then concluded that the particles in the ray must be part of all matter

- the particles were called electrons and held a 1- charge The particles in the ray were deflected towards a positively charged plate when two opposing charged plates were placed around the tube - it was then concluded that the particles in the ray must be negatively charge - the particles were called electrons and held a 1- charge http://www.youtube.com/watch?v=GzMh4q-2HjM

J.J. Thomson determined the mass of the electron by determining its charge to mass ratio - he compared the ratio to other known ratios and found the mass of the electron was much smaller than the mass of a hydrogen atom, the lightest known atom - it was thus found that atoms were divisible into smaller subatomic particles

Because most matter is neutral, it was determined that neutral atoms contained an equal number of positive and negative charges

IV. The Nuclear Atom Ernest Rutherford studied positive alpha particles and their interactions with solid matter by aiming a small beam of alpha particles at a thin foil of gold A zinc sulfide coated screen surrounded the gold foil that produced a flash of light whenever it was struck by a deflected alpha particle

Instead the alpha particles were sometimes deflected Because it was thought that the positive charge was evenly distributed in an atom, Rutherford thought the alpha particle beam would pass directly through the foil Instead the alpha particles were sometimes deflected http://www.youtube.com/watch?v=5pZj0u_XMbc

Rutherford concluded that the atom consisted mostly of empty space He also concluded that there was a tiny, dense region, which he called the nucleus, centrally located within the atom that contained all of an atom’s positive charge and virtually all of its mass

Rutherford also determined that electrons are free to move through the available space surrounding the nucleus and are held within the atom by their attraction to the positively charged nucleus - the space electrons move in, the atom’s diameter, is 10,000 times larger than the nucleus - if the nucleus were a pea, the atom’s diameter would be a football field wide

V. Completing the Atom – The Discovery of Protons and Neutrons Rutherford concluded that the nucleus contained positively charged particles called protons - a proton is a subatomic particle carrying a charge equal to but opposite that of an electron, 1+ - for an atom to be neutral, there are equal numbers of electrons and protons

Rutherford’s coworker, James Chadwick, showed that the nucleus also contains another subatomic particle, a neutral particle called the neutron that has a mass nearly equal to that of a proton http://www.youtube.com/watch?v=oQZBBQmw-a0

VI. Atomic Number Henry Moseley discovered the atoms of each element contain their own unique positive charge in the nuclei The number of protons in an atom is referred to as the element’s atomic number

Atomic number = # protons = # electrons Atomic number determines the element’s position in the periodic table and are organized left-to-right and top-to-bottom by increasing atomic number Atomic number = # protons = # electrons

VII. Isotopes and Mass Number Not all atoms of a particular element are identical - it is true that all atoms of a particular element have the same number of protons and electrons - however, the number of neutrons on their nuclei may differ - atoms with the same number of protons but different number of neutrons are called isotopes

In nature most elements are found as a mixture of isotopes, usually in consistent ratios

To make it easy to identify each of the various isotopes of an element, chemists add a number after the element’s name, called the mass number, that represents the sum of protons and neutrons in the nucleus Example: Potassium isotope with 19 p and 20 n = potassium-39 Potassium isotope with 19 p and 21 n = potassium-40

# of neutrons = mass number - atomic number The number of neutrons in an isotope can be calculated from the atomic number and mass number # of neutrons = mass number - atomic number #neutrons = (# protons + # neutrons) - # protons

VIII. Mass of Individual Atoms Together, electrons, protons, and neutrons account for all of the mass of an atom, or atomic mass - the mass of an electron is 9.11 x 10-28 g - the mass of a proton is 1.673x10-24 g - the mass of a proton is 1.675x10-24 g

Because masses expressed in scientific notation are difficult to work with, chemists have developed a method of measuring the mass of an atom using atomic mass units (amu) - chemists chose the carbon-12 atom as a standard of measurement, having exactly 12 atomic mass units - thus one atomic mass unit is defined as 1/12 the mass of a carbon-12 atom

Using atomic mass units: - electrons are 0.000549 amu - protons are 1.007276 amu - neutrons are 1.008665 amu Because elements are found in consistent combinations of isotopes, the atomic mass of an element is the weighted average mass of the isotopes of that element

IX. Radioactivity The number of protons in the nucleus determines the identity of an atom When there is a change in the nucleus, or a reaction that involves an atom of one element changing into an atom of another element, it is called a nuclear reaction

Some elements have been observed to spontaneously emit radiation in a process called radioactivity Rays and particles emitted are called radiation Unstable radioactive atoms undergo radioactive decay by losing energy until they form stable nonradioactive atoms

A. Alpha Radiation Alpha particles that deflect towards a negatively charged plate Each alpha particle contains two protons and two neutrons, thus having a 2+ charge An alpha particle is equivalent to a helium-4 nucleus and is represented by 42 He or α

B. Beta Radiation Beta particles, fast moving electrons, deflect towards a positively charged plate Each beta particle has a 1- charge represented by the symbol 0-1 β

Usually accompanied by alpha and beta radiation C. Gamma Radiation Gamma rays are high-energy radiation that possess no mass or electric charge and thus are not deflected by electric or magnetic fields Represented by 00 γ Usually accompanied by alpha and beta radiation Account for most of the energy lost during radioactive decay http://www.youtube.com/watch?v=ZlFz3AAwwBQ