Models of the Atom The Nucleus

Early Beliefs 2500 ya, early Greek philosophers believed that if you continued to divide matter eventually you would have only one particle left they called atom (means cannot be divided)

Models of the Atom In the 18 th Century man began to study why some substances could go together and how some could be taken apart Scientists called substances which couldn’t be broken down into simpler substances as elements Elements were pure and made of only one kind of atoms Silver, gold, iron, carbon and oxygen are types of elements (90 natural occuring elements)

Dalton’s Concept Early 19 th century John Dalton proposed ideas about matter 1. Matter is made up of atoms 2. Atoms cannot be divided into smaller pieces 3. All the atoms of an element are exactly alike 4. Different elements are made of different kinds of atoms Proposed that an atom looked like a tiny hard marble that was the same throughout

Crookes Cathode Experiment Late 19 th Century William Crooke took a vacumn tube, attached battery to two electrodes (anode-positive charge, and cathode-negative charge) Placed a cross in middle a of tube Connected the electric current and a shadow was shown on far end of tube (anode) What was the beam? Crooke thought it was a beam of charged particles… called cathode rays

Crookes Cathode Tube

Discovering Charged Particles JJ Thomson placed magnet next to cathode tube and bent the green glow light (light can’t be bent, so it must have been charged particles) Thomson concluded they were negative charged particles, now charged electrons

Thomson’s Cathode Tube

Thomson’s Atomic Model Atoms are neutral, but if there were negative charged electrons there must also be positive charged particles There would have to be positive charges to balance the negative charges Thomson’s model of an Atom A sphere of positive and negative charged particles evenly spaced kind of like cookie dough and chocolate chip

Rutherford’s Experiment Tested cookie dough theory Fired alpha particles through thin gold foil If positively charged particles are evenly spaced in atom, the alpha beam will pass through without deflection Found some deflection which indicated the particles would have to be concentrated in atom not randomly scattered Proposed a new shape of atom with a concentrated nucleus of positive charged particles 1920 proposed the positive charge particles as protons

Rutherford’s Experiment

Atomic Model Changed again The nucleus has two much mass for just positive protons Proposed that there is particles the same size as protons but have no charge in the nucleus Called these new particles neutrons Now the atomic model is a tightly spaced nucleus composed of protons(+) and neutrons (no charge) surrounded by a cloud of much light weighted electrons (-)

The Nucleus An element is defined by the number of protons present in nucleus. (another name for number of protons is atomic number) Neutrons however can have varying numbers of neutrons in nucleus When atoms of the same element varying in number of neutrons it is called an isotope Carbon normally has 6 proton and 6 neutrons, but carbon atoms can sometimes have 7 or 8 neutrons in the nucleus, thus those are isotopes of Carbon

Number of Neutrons Carbon normally has 6 proton and 6 neutrons, but carbon atoms can sometimes have 6, 7 or 8 neutrons in the nucleus, thus those are isotopes of Carbon Isotopes of Carbon’s mass is written as totally the numbers of protons and neutrons: Carbon 12 (6 protons and 6 neutrons) Carbon 13 (6 protons and 7 neutrons) Carbon 14 (6 protons and 8 neutrons)

Mass number Mass number of an isotope is the number of neutrons plus protons To find the number of neutrons in an isotope subtract the atomic number from the mass number Carbon 14 means it has a mass of 14 Carbon has 6 protons thus an atomic number of 6 14(mass#) – 6 (protons) = 8 (neutrons)

Strong Nuclear Force Because the nucleus is composed of positive charged protons, you would think that the like charges would repel each other Rather strong nuclear forces hold the protons together only when they are closely packed as they are in the nucleus of the atom

Radioactive Decay Nucleus are stable when the number of protons and neutrons are similar (ie: 6 protons, 6 neutrons) Some nuclei are unstable because they have too many or too few neutrons These unstable nuclei release particles to become more stable The release of nuclear particles and energy is called radioactive decay When particles like protons are emitted from nucleus, the Atomic # changes and a new atom is formed Transmutation is changing of one element into another through radioactive decay

Loss of Alpha Particle When an alpha particle is emitted from a radioactive element 2 protons and 2 neutrons are lost from nucleus (atomic mass of 4) The resulting atom has 2 less protons and atomic mass is 4 less U-238 releases alpha particle (α) and Th-234 forms U 90 Th + 2 α New element forms

Loss of Beta particles Some unstable elements undergo transmutation where a neutron becomes unstable and splits into an electron and a proton and a beta particle ( β ) is released The electron is emitted as high energy and the proton remains in the nucleus increasing the atomic number by one and changing the element Atomic number therefore is increased by one but the atomic mass stays the same because a neutron changes to a proton (same mass) Th 91 Pa + -1 β ↝

Rate of Decay Radioactive decay is random Rate of decay of a nucleus is measured by its half-life Half-life of radioactive isotope is the amount of time it takes for half of a sample of the element to decay For every half-life, ½ of the original mass is gone and you can calculate how much will be left after each half life

Half life table Number of half-lives elapsed Fraction remaining Percentage remaining 0 1/11/ /21/ /41/ /81/ / / / /

Carbon Dating Half life of Carbon is 5730 years C14 is taken in by plants just as C12 is and when organism dies radioactive decay continues and can be calculated into age We can calculate age of fossils by calculating how much C14 (radioactive) remains in a sample (accuracy to years) Uranium also is used to date rocks, however its half life is 4.5 billions years and decays to lead (Pb) scientists calculate age of earth and rocks by comparing amount uranium remaining and lead (Pb) formed

Making Synthetic Elements Synthetic elements are man made elements caused by smashing elements with alpha and beta particles These new elements are not found in nature Atomic numbers 93 to 112 and 114 have been made this way

Uses of Radioactive Isotopes Tracer elements: radioactive isotopes that are introduced into an organism where it is used to diagnose disease and study its surrounding Isotopes have short half-lifes Other radioactive elements are introduced to detect cancer, digestion problems and circulation problems Tumors and fractures can be found using isotopes Radioactive isotopes can be used to trace phosphates in plants, trace pesticides as it moves through ecosystems