# Chapter 3 Notes  Atoms- Smallest particle of an element that retains the chemical identity of that element.  Principles of chemical behavior: Lavoisier:

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Chapter 3 Notes

 Atoms- Smallest particle of an element that retains the chemical identity of that element.  Principles of chemical behavior: Lavoisier: Law of Conservation of Matter Lavoisier: Law of Conservation of Matter Proust: Law of Constant Composition Proust: Law of Constant Composition a compound always contains the same elements in the same proportions by mass.a compound always contains the same elements in the same proportions by mass.

John Dalton’s: Atomic Theory of Matter  elements are composed of atoms  all atoms of an element are identical, but different from atoms in other elements  atoms are neither created nor destroyed  a given compound always has the same relative number and kind of atoms.

Michael Faraday  atoms contain particles that have an electrical charge

Ben Franklin  studied electricity  he determined the following: there are 2 kinds of charge positive and negative there are 2 kinds of charge positive and negative 2 like charges repel each other 2 like charges repel each other opposites charges attract each other opposites charges attract each other excess negative charge can be discharged as static electricity excess negative charge can be discharged as static electricity

Ben Franklin  Do you remember Ben Franklin’s famous kite experiment?

J.J. Thomson  called the negative particles electrons  determined the charge to mass ratio of an electron  The Plum Pudding Model is Thomson’s name for his model of the atom

JJ Thomson’s Plum Pudding Model of the Atom  www.tamucc.edu

Cathode Ray Tube (CRT)  negative end is the cathode  positive end is the anode A cathode ray is radiation streaming from a cathode to an anode in a CRT  it is a stream of particles  a magnet can deflect the ray  cathode ray particles have a negative charge

 http://www.chem.uiuc.edu/clcwebsite/cathode.html

Robert Millikan  measured the charge of an electron using the oil drop experiment.  x-rays gave the oil a negative electron  1.60x10 -19 coulomb is the charge of an electron  using Thomson’s charge to mass ratio, he determined the mass of the electron is 9.11x10 -28 g

Robert Millikan’s Oil Drop Experiment  Robert Millikan received the Nobel Prize for his work  www.68pair.com

Henri Becquerel  discovered that uranium exhibits radioactivity  the chemical properties of an element change as it gives off radiation

Ernest Rutherford aaaalpha particles have a +2 charge bbbbeta particles are high speed electrons ggggamma rays are not composed of particles

Rutherford  Gold Foil Experiment (alpha scattering)  he determined that an atom’s positive charge, and most of its mass, was concentrated in the core (most of the atom is empty space)  he named the core of the atom “the nucleus”

Rutherford’s Gold Foil Experiment  Over 98% of the particles went straight through  Approx. 2% of the particles were deflected  Approx. 0.01% of the particles bounced off the gold foil www.sci.tamucc.edu

Rutherford’s Nuclear Model  The atom contains a tiny dense center called the nucleus  The nucleus is essentially the entire mass of the atom  The nucleus is positively charged  The amount of positive charge of the nucleus balances the negative charge of the electrons  The electrons move around in the empty space of the atom surrounding the nucleus www.sci.tamucc.edu

Modern Atomic Theory *atoms are composed of protons, neutrons and electrons *Recently, scientists discovered these particles contain particles (i.e. quarks, gluons) *A nucleus is a positively charged central core containing protons and neutrons *An individual proton has a charge that is equal to the charge of an electron, but the charge is positive *In a neutral atom: the number of protons equals the number of electrons *a proton is much more massive than an electron

Charge and Mass aaaa proton’s charge is +1 aaaan electron’s charge is -1 mmmmass is expressed in atomic mass units (amu) tttthe mass of a proton is approx. 1 amu tttthe mass of a neutron is approx. 1 amu tttthe mass of an electron is approx. 0 amu

Henry Mosely  an atom’s identity comes from the number of protons in its nucleus  the number of protons is called the atomic number  every element has a unique atomic number  the atomic number is written above the chemical symbol on the periodic table  since atoms are electrically neutral, the number of positive charges equals the number of negative charges  the number of neutrons equals the mass number minus the atomic number  # of no = mass # - atomic #

IONS  when an atom loses or gains one or more electrons it acquires a net charge and is called an ion  if there are more electrons than protons, the ion has a negative charge  charge of ion = (# of p+) - (# of e-)  charge can be written as +2 or 2+  place the charge to the upper right corner of the chemical symbol

ISOTOPES  atoms that have the same number of protons, but different numbers of neutrons  the chemical properties of an element depend primarily on its electrons and protons  isotopes of the same element, with different numbers of neutrons have essentially the same chemical properties  the major difference between 2 isotopes is their mass  isotopes with more neutrons have higher mass than those with fewer neutrons  mass # = (isotope’s # of p+) + (isotope’s # of n 0 )  to identify an isotope you add the mass number after the element’s name (i.e. chlorine -37)  number of neutrons = mass # - atomic #

MASS OF AN ATOM *amu can be defined as the mass of a proton or neutron *Generally: mass of an atom = number of protons + number of neutrons *an amu is defined in terms of an arbitrary standard: a carbon-12 atom *scientists set the mass of a carbon-12 atom to be exactly equal to 12 amu *therefore, one amu is 1/12 of the mass of a carbon-12 atom *1 amu = 1/12 (mass of 126 C atom) = 1.66 x 10-24 g

MASS OF AN ATOM  when determining the mass of a large number of atoms:  remember that elements have different isotopes  determine the mass of an “average” element’s atom  the isotopes of some elements are found in constant ratios  these rations are called fractional abundances  atomic mass is the average mass of an element’s atoms  the atomic mass for each element is written on the periodic table below the element symbol

RADIOACTIVITY  nuclear reactions change the composition of an atom’s nucleus.  alpha, beta and gamma radiation are produced by nuclear reactions  alpha and beta radiation consist of particles that are emitted from the nucleus  Radioactivity is the spontaneous emission of radiation from an atom  atoms with stable nuclei are not radioactive

PURPOSE OF NEUTRONS  not all combinations of protons and neutrons create a stable nucleus  protons in the nucleus should repel each other, but there is a strong nuclear force (that is an attractive force) that holds the nucleus together  the nuclear force is only strong between subatomic particles that are extremely close  the presence of neutrons in the nucleus adds a net attractive force to the inside of the nucleus  think of neutrons as the glue that holds the nucleus together

STABLE OR NOT?  elements with:  atomic number 1-20: stable nuclei have almost equal numbers of protons and neutrons  atomic number 21-83: nuclei need more neutrons than protons to be stable  atomic number >83: no number of neutrons is sufficient to hold the nucleus together indefinitely.  All nuclei with atomic numbers greater than 83 are radioactive.

UNSTABLE NUCLEI  nuclei are unstable if they contain too few or too many neutrons  nuclei that have excess neutrons are likely to emit beta radiation  Generally, isotopes that are much heavier (contain more neutrons) or much lighter (contain fewer neutrons) than the most common isotope are likely to be radioactive.

TYPES OF RADIOACTIVE DECAY *radioactive elements emit different kinds of radiation (three types are alpha, beta and gamma) *these types of radiation can be distinguished by their: -charge -mass -penetrating power

ALPHA PARTICLES *consist of a stream of high-energy alpha particles *alpha particle consists of 2 protons and 2 neutrons (it is identical to the helium-4 nucleus) *these particles do not have much penetrating power *these particles travel only a few centimeters in air and can be stopped by paper or clothing *not normally a health hazard

BETA RADIATION *consists of a stream of high-speed electrons *the electrons come from changes in the nucleus *in the process that produces beta radiation, a neutron changes into a proton and an electron *the proton remains in the nucleus and the electron (now the beta particle) is propelled out of the nucleus at high speed *the mass number for a beta particle is zero because an electron has a very small mass compared with a proton or neutron *beta radiation is approx. 100 times more penetrating than alpha *it can penetrate clothing and harm skin

GAMMA RAYS  gamma rays are not particles  they are a very energetic form of light that we cannot see  it can penetrate deeply into solid material (it can only be stopped by concrete or lead)

RADIOACTIVE DECAY  Radioactive Decay occurs when an atom emits alpha or beta particles or gamma rays.  The term decay is used because the original nucleus decomposes (decays) to form a new nucleus, releasing radiation in the process.  Use a nuclear equation to show the radioactive decay.

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