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Arsenic was known to the ancient Egyptian, and is mentioned in one papyrus as a ways of gilding metals. The Greek philosopher Theophrastus knew of two arsenic sulfide minerals: orpiment (As2S3) and realgar (As4S4). Pen Ts’ao Kan-Mu compiled his great work on the natural world in the 1500s, during the Ming dynasty. He noted the toxicity associated with arsenic compounds and mentioned their use as pesticides in rice fields. A more dangerous form of arsenic, called white arsenic, has also been long known. This was the trioxide, As2O3, and was a by-product of copper refining. When this was mixed with olive oil and heated it yielded arsenic metal itself. The discovery of the element arsenic is attributed to Albertus Magnus in ~ AD 1251. http://www.rsc.org/periodic-table/history Atomic Structure Three particles: Proton - particles with a positive charge - located in nucleus. Neutrons - particles with a neutral charge - located in nucleus. Electrons - particles with a negative charge. Located in a roughly spherical zone around the nucleus. Atomic number and symbol Atomic number is the number of protons in the nucleus. All atoms of same element will have same number of protons. Atomic symbol is the letter designation for a given element: C - Carbon (6 protons) O - Oxygen (8 protons) Na - Sodium (natrium) (11 protons) Cl - Chlorine (17 protons) Fe - Iron (ferrum) (26 protons) Atomic Weight Atomic Weight is sum of weights of all atomic particles. proton - 1.6 x 10-27 kg neutron - 1.6 x 10-27 kg electron - 1 x 10-30 kg Atomic Mass Number is based on each proton and neutron having a mass of 1. Oxygen has 8 protons and 8 neutrons - atomic mass number of 16. Atomic mass number is the total number of nucleons in the atom’s nucleus. Isotopes Atomic mass number for Oxygen on the Periodic Table of Elements? Every oxygen atom has 8 protons. The number of neutrons in the nucleus can change. O16 - 8 protons, 8 neutrons O17 - 8 protons, 9 neutrons O18 - 8 protons, 10 neutrons O17 and O18 are isotopes of common oxygen O16. Ionic bonding involves transfer of an electron from one atom (which becomes a positively charged cation) to another (which becomes a negatively charged anion). The two ions attract strongly to form a crystal lattice. Covalent bonding occurs between atoms of non-metals. When the atoms form a bond they become lower in energy and the system is more stable. The energy saved by moving to a more stable situation is released as heat. For this reason bond formation is always exothermic, i.e. heat energy is released. Conversely, in order to break a chemical bond energy must be used - it is an endothermic process. Metallic Bonding Bonds are formed between metal atoms that have low electronegativities and so do not attract their valence electrons strongly. This allows the outermost electrons to be shared by all the surrounding atoms, resulting in positive ions (cations) surrounded by a sea of electrons . A solid (not molten) metal is crystalline with its metal cations packed together as closely as possible. Several packing arrangements are possible: FCC (face centered cubic) structures are more likely to be ductile than BCC, (body centered cubic) or HCP (hexagonal close packed). Limitations: This Drude (1900) – Lorence (1923) electrons-sea theory explains many properties of metals such as malleability and thermal conductivity. But some questions of quantitative aspect are still unsolved such as why does the resistance to electrical conductance of some metals increase with rise of temperature while in some cases it decreases exponentially; why do some metals exhibit almost no electrical conductance in the solid state and must be classified as insulators. Electronegativity is defined as the tendency of an atom in a molecule to attract electrons to itself. Linus Pauling Van Der Waals bonds are intermolecular forces of attraction or repulsion which act between neighboring particles (atoms, molecules or ions). They are weak compared to the intramolecular forces (e.g., covalent and ionic bonding) that keep a molecule together. Some dipoles are permanent, and other temporary. Van der Waals bonds arises due to temporary dipoles on atoms. The positively charged region of an atom attracts electrons (which have a negative charge) from the neighboring atom, setting up a dipole in that atom. This sets off a domino-like cascade of dipoles throughout the atoms of a substance. Beryl (unit cell outlined) The crystal structure of Quartz (SiO2).