The Development and Understanding of Atomic Theory: From Democritus to Modern Times, Study notes of Chemistry

Download The Development and Understanding of Atomic Theory: From Democritus to Modern Times and more Study notes Chemistry in PDF only on Docsity! Atomic theory can be traced back to Democritus (460-370 BC) who believed that matter could be divided into indivisible particles “atoms.” The atomist theory was squashed by Aristotle and Plato and remained unpopular for almost 2000 years! Newton, Galileo, and other luminaries embraced the idea of atoms in explaining phenomena like wind, but failed to see atoms as chemical building blocks. John Dalton devised a chemical atomic theory between 1803 and 1807. While his postulates would be proven incorrect in the coming hundred years, the groundwork for an atomic theory of matter was set and the chemical age was born! ! Atomic Theory of Matter 1. Each element is composed of extremely small particles called atoms. Atomic Theory of Matter 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses). Atomic Theory of Matter 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses). 3. Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions. Atomic Theory of Matter 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses). 3. Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions. The basis for this postulate is the conservation of mass. To Dalton, chemical reactions represented a rearrangement of atoms to give new ch mical combinations. Atomic Theory of Matter 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical; the atoms of different elements are different and have different properties (including different masses). 3. Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. Recall, we discussed the law of definite proportions when defining chemical compounds. Water, no matter where it is collected, is chemically identical. It is a compound. By contrast, sea water does not have a unique composition. It is a mixture of water, salt, potassium chloride, and many other compounds in smaller amounts. Dalton’s Law of Multiple Proportions Dalton deduced the law of multiple proportions: If two elements A and B combine to from more than one compound, the masses of B that can combine with a given mass of A are in the ratio of small whole numbers. CO, CO2 H2O, H2O2 NO, NO2, N2O5 Sometimes ratios aren’t whole numbers Crystal structure of (La1-xSrx)2CuO4, single crystals which possess high temperature superconductivity. Rutherford Experiment: Nuclear Atom 1919 proton discovered by Rutherford 1932 Chadwick discovers the neutron Mass of atoms measured in atomic mass units or Daltons, 1 amu = 1.66054 x 10-24 g. Atoms have diameters of 1 x 10-10 to 5 x 10-10 m. Typically we use Angstoms, 1 Å = 1 x 10-10 m. The nucleus has a diameter of ~ 10-4 Å. Modern View of the Atom All atoms of an element have the same number of protons in the nucleus They may have differing number of neutrons, and consequently a different mass. These are called isotopes. Carbon is present in nature as one of four nuclides: 11C, 12C, 13C, 14C Isotopes, Atomic and Mass Numbers 66 6 6 Atomic weight • Early scientist could measure the relative masses of individual elements within chemical compounds, i.e., in water there is 11% H and 89% O. Realizing that there are two hydrogens per oxygen we know that the atomic weight of O is 16 times that of H. • H was assigned a weight of 1 (no units), O a weight of 16 etc. Average Atomic Mass • It is possible to get exact weights of nuclides, with 12C being defined as having a mass of 12 amu. • In the periodic table, the mass of C is 12.011 amu because there is also naturally occurring 13C(1.07%, 13.00335 amu). • 12.011 = 0.9893*12 + 0.0107*13.00335 Mass Spectrometry • Mass spectrometers are an analytical tool used for measuring the molecular weight of a sample. • For large samples such as biomolecules, molecular weights can be measured to within an accuracy of 0.01% of the total molecular weight of the sample i.e., within 4 Daltons (Da) for a sample of 40,000 Da. This is sufficient to allow minor mass changes to be detected, e.g. the substitution of one amino acid for another. For small organic molecules the molecular weight can be measured to within an accuracy of 5 ppm, sufficient to confirm the molecular formula of a compound. • Structural information can be generated by fragmenting the sample and analysing the products generated. This procedure is useful for the structural elucidation of organic compounds, for peptide or oligonucleotide sequencing, and for monitoring the existence of previously characterised compounds in complex mixtures with a high specificity by defining both the molecular weight and a diagnostic fragment of the molecule simultaneously e.g. for the detection of specific drug metabolites in biological matrices. Why mass spec? • Mass spectrometers are used in industry and academia for both routine and research purposes. • Biotechnology: the analysis of proteins, peptides, oligonucleotides • Pharmaceutical: drug discovery, combinatorial chemistry, pharmacokinetics, drug metabolism • Clinical: neonatal screening, haemoglobin analysis, drug testing • Environmental: PAHs, PCBs, water quality, food contamination • Geological: oil composition! How does it work? • Mass spectrometers can be divided into three fundamental parts, namely the ionisation source, the analyser, and the detector • The sample under investigation has to be introduced into the ionisation source of the instrument where it is ionised, often by Electrospray Ionisation (ESI) or Matrix Assisted Laser Desorption Ionisation (MALDI) • These ions are extracted into the analyser region of the mass spectrometer where they are separated according to their mass to charge ratios (m/z) frequently by time-of-flight (TOF) analysers. • The separated ions are detected and this signal sent to a data system where the m/z ratios are stored together with their relative abundance C C C H H H Cl H H H H MW: 78.45 g/mol 1 in 4 Cl atoms is 37Cl C C C H H H Cl H H H H Peaks at 78, 80 correspond to parent ion with 35Cl (MW = 78 g/mol) and parent ion with 37Cl (MW = 80 g/mol) Peaks at 63, 65 correspond to the above with one missing methyl C C H H H Cl H C H CC H H H H H H Peak at 43 is the missing Cl