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1A-2 The Atom – Building Blocks of Matter Historical investigation into the composition of matter led to these classifications: Pure Substances Homogeneous composition Element A single type of atom Compound Atoms in a definite composition Physically and Physically chemically indivisible but indivisible chemically e.g. sodium (Na) divisible to the elements e.g. table salt (NaCl) Mixtures Composed of more than one type of pure substances Homogeneous Heterogeneous mixture mixture Each part Each part is represents the distinct whole e.g. salt water e.g. sand Through subsequent divisions and purifications of matter, we arrive at single types of atoms, from which all matter is composed. It became apparent that atoms are extremely small and important features of the chemistry of matter, so models were developed to understand and predict the structure and function of atoms. [See table 1.1 page 11] Key milestones along the way J. Dalton proposes the “billiard ball” model of the atom; different atoms were represented by different spheres that could be joined to make molecules; atoms are indivisible J.J. Thomson – atoms are divisible; found negatively and positively charged sub-atomic particles (electrons and protons); “plum pudding” model Rutherford’s gold foil experiments (McGill U) showed that the positive particle (proton) is infinitesimally small but very massive surrounded by a lot of negatively charged electron space; initiated the planetary model of electrons orbiting the positively-charged nucleus that contained the protons. Bohr – extended Rutherford’s model with Boltzmann’s, Planck’s and Einstein’s work on photoelectric effect; electrons exist in specific energy levels around the nucleus; electron shell model (birth of the quantum mechanical model) Schrodinger – electrons exist in regions of space around the nucleus according to specific mathematical wave functions, not in fixed orbits; electron cloud model. Representing the Atom Having a working model of the atom, especially regarding the electrons, allows us to make some hypotheses about the structure and reactivity of elements and compounds. Although our understanding of the atom is constantly being refined, earlier models like the Bohr-Rutherford models are still used to visualize these concepts. In this model, the total number of electrons in the neutral atom equals the number of protons in its nucleus (i.e. the atomic number, Z). So, hydrogen (Z = 1) has one electron in the lowest energy shell; carbon (Z = 6) has six electrons in two shells. Filling the Electron Energy Shells Each energy shell can accommodate a maximum of 2n2 electrons, where n is the shell energy level starting at the lowest energy. So, the first level (n = 1) can fit 2 electrons; the second level (n = 2) can fit 8 electrons, and so on. Also, electrons must fill a lower energy level before they begin to fill a higher energy level. (This gets a little complicated as Z increases Wave or Quantum Mechanical Model) So the electron orbital diagrams for hydrogen and carbon are: H C Electrons in an outer shell of an atom are called the valence electrons. It is usually these electrons that are responsible for the reactive properties of the element. How many valence electrons does hydrogen have? Carbon? Neon? Valence electrons take so much focus that we usually draw only these electrons for an atom – these are called Lewis Diagrams. (see Figure 1.5 page 13) Notice that there is significance to how the electrons are drawn in these diagrams. The first four are drawn as single electrons, their negative charges repelling each other. Subsequent electrons begin to pair up with the previous ones until the entire shell is filled. Unpaired valence electrons are most important as these are generally more important in chemical bond formation. The Nucleus As mentioned earlier, the total number of electrons in a neutral atom is equal to its atomic number. Since the atom is neutral, this also corresponds to the number of protons in the atom’s nucleus. The nucleus also contains almost all of the atoms mass (see Rutherford), which is comprised of protons and neutral particles called neutrons. The atomic mass (A) is the sum of the number of protons and neutrons in the atom’s nucleus, such that an atom can be symbolically expressed using its chemical symbol (X), atomic number (Z) and atomic mass (A): 𝐴 𝑍𝑋 1 𝐻. So hydrogen is symbolized as 1 Atoms of the same element can vary in the number of neutrons contained in the nucleus. These different types of the same element differ only by their atomic mass, and are called isotopes. For example, deuterium is an isotope of hydrogen that contains a neutron in addition to its proton. The symbol for deuterium is 2 3 𝐻 . Tritium 1 1𝐻 with 2 neutrons is another isotope of hydrogen. Most elements occur as more than one isotope, that is, different numbers of neutrons in their nuclei. For example, the most abundant isotope of carbon is carbon-12 (atomic mass = 12) having 6 protons and 6 neutrons: 12 6𝐶 . Carbon also exists as isotopes having 7 or 8 neutrons in the nucleus: Some isotopes are unstable; they can lose neutrons and decay into other stable isotopes nuclear chemistry. Finding the Average Atomic Mass of an Element The atomic mass of an element is given in atomic mass units, which works out to slightly less than 1 u for every proton or neutron. For example, the atomic mass of 12 6𝐶 is about 11.99 units (u). The reported atomic mass of an element must take into account the abundance of each naturally occurring isotope, and so is reported as an average of the atomic masses of each isotope. To find this average atomic mass of an element, multiply each isotopes atomic mass by its abundance, and then add these all together. Example – Average Atomic Mass of Copper Copper has two main isotopes: copper-63 (mass = 62.93 u; abundance = 69%) and copper-65 (mass = 64.93 u; abundance = 31%). The average atomic mass of copper is: http://www.youtube.com/watch?v=NSAgLvKOPLQ http://www.clickandlearn.org/gr9_sci/atoms/modelsoftheatom.html 1.1 The Atom – What have we learned? Models of the atom Electronic structure and energies The nucleus – protons and neutrons Isotopes, atomic number and atomic mass Assigned Questions – BLBMW: p. 70 #9 – 11, 13 – 15, 19 – 27, 29, 31, 35