Atomic Structure

4.1 Defining the Atom

  • Atom: the smallest particle of an element that retains its identity in a chemical reaction

  • Democritus (460 B.C. - 370 B.C.) was the first to suggest the existence of atoms. Democritus reasoned that atoms were indivisible and indestructible.

  • Lacked experimental support; wasn’t based on scientific method

  • More than 2,000 years later, John Dalton used experimental methods and transformed Democritus’s ideas on atoms into a scientific theory

  • Dalton’s Atomic Theory: 
    a. All elements are composed of tiny indivisible particles called atoms 
    b. Atoms of the same element are identical. The atoms of any one element are different from those of any other element 
    c. Atoms of different elements can physically mix together or can chemically combine in simple whole number ratios to form compounds 
    d. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction 

  • Scanning electron microscopes: an instrument that allowed individual atoms to be observable 

  • Scanning electron microscopes have a higher magnification than the light microscope 

  • Scanning electron microscopes allow atoms to be moved around and arranged in patterns 

4.2 Structure of the Nuclear Atom 

  • Majority of Dalton’s theories are accepted today, but atoms are now known as being divisible

  • Subatomic particles: particles that are smaller than an atom 

  • Three kinds of subatomic particles are electrons, protons, and neutrons 

  • The English physicist J.J. Thomson (1856-1940) discovered the electron 

  • Electrons: negatively charged subatomic particles 

  • J.J. Thomson Experiment: 

    •  He determined that the beam contained negatively charged particles by 1) putting a magnet next to the beam, the beam would deflect. A magnet only deflects moving charges and it does not deflect stationary charges. Meaning, any object that is charged and is moving can be deflected by a magnet. 2) When putting a positive charged plate on the top and then a negatively charged plate on the bottom, the ray deflected towards the positively charged plate. 

    • The tube had close to 0 amounts of pressure; most of gas molecules have been removed (0.01 pascals) 

    • Cathode (negatively charged electrode) and anode (positively charged electrode) the electrons emanated from the cathode by it being heated and there’s a high voltage between the two  

    • Any type of metal can emit a cathode ray, which helped him conclude that all materials and all atoms contains negatively charged electrons

    • electrons can be accelerated by electric and magnetic fields

    • Thomson originally called the electrons corpuscles when he found out that there were negatively charged particles in the cathode

    • magnetic field can cause it to change direction

    • Cathode rays were emitted which were streams of electrons emitted from the negative cathode 

  • The four simple ideas about matter and electric charges: 1) atoms have no electric charge; they are electrically neutral 2) electric charges are carried by particles of matter 3) electric charges always exist in whole-number multiples of a single basic unit; there are no faction of charges 4) when a given number of negatively charged particles combines with an equal number of positively charged particles an electrically neutral particle is formed

  • Rays that were travelling opposite to the cathode rays were called canal rays and Eugene Goldstein concluded that they were composed of positive particles; later called protons. 

  • English physicist called James Chadwick confirmed the existence of another subatomic particle called neutrons. Neutrons have no charge but have a similar mass to protons. 

  • Theoretical physicists believe that the subatomic particles are composed of yet smaller sub nuclear particles called quarks

  • J.J. Thomson, discoverer of the electron 

  • Before, Thomson believed that the structure of the atom was that the electrons were evenly distributed throughout an atom filled uniformly with positively charged material. The plum pudding model made it seem like the electrons were in lumps of positive protons or positive particles like how the raisins are surrounded by the dough 

  • Ernest Rutherford disproved this theory. He did an experiment that had a ray of alpha particles (helium that have lost their electrons so they were double positively charged because of the remaining protons) and directed the beam at a very thin gold sheet of paper. He hypothesized that the beam would pass easily though the gold, but saw that the majority of teh time the beam did pass through the gold but at some occasions it bounced off the gold at sharp angles. He concluded that the atom is mostly made up of empty space (which is why there wasn’t that much deflection) and that all the positive charge and almost all the mass were  concentrated in a small region called the nucleus. 

  • nucleus contains protons and neutrons 

  • the Rutherford atomic model is known as the nuclear atom. In the nuclear atom, the protons and neutrons are located in the positively charged nucleus. The electrons a distributed around the nucleus and occupy almost all the volume of the atom.

4.3 Distinguishing Among Atoms 

  • Elements are different because they contain different numbers of protons 

  • atomic number: the number of protons in the nucleus of an atom of that element 

    • atomic number identifies an element 

    • # of protons = atomic number

  • Since all hydrogen atoms have one proton, their atomic NUMBER is 1

  • # of protons (+ charge) = # of electrons (- charge) which is why atoms are electrically neutral 

  • Mass number: most of the mass is concentrated in the nucleus depending on the amount of protons and neutrons are there. 

    • A helium atom has 2 protons and 2 neutrons, so the mass number is 4 

    • protons + neutrons = mass number 

    • # of neutrons = mass number - atomic number

  • Isotopes: atoms that have the same number of protons but different numbers of neutrons 

    • *Because isotopes of an element have different numbers of neutrons, they also have different mass numbers 

  • atomic mass unit (amu): defined as a unit of mass that is one twelfth of the mass of a carbon-12 atom so 1 amu = 1/12 the mass of carbon12 

  • atomic mass: a weighted average mass of the atoms in a naturally occurring sample of the element; the average mass of each isotope and the average amount of how common each isotope is in nature

    • calculate atomic mass: (mass of isotope1 * its natural abundance expressed in a percentage) + (mass of isotope2 * its natural abundance expressed in a percentage) = atomic mass expressed in amu 


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