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Atomic Structure - Two Observable Facts made in the 1800’s • Law of Conservation of Mass – Matter is neither created nor destroyed in a chemical reaction • Law of Constant Composition – The elements present in a compound are present in a fixed and exact proportion by mass Dalton’s Atomic Theory • From 1803 to 1808 an English schoolmaster named John Dalton came up with a very good Atomic Theory. – All matter is composed of atoms (Dalton thought they were indestructible) • Atoms are the building blocks of matter – Atoms of one element are identical Dalton’s Atomic Theory – Atoms of different elements can be distinguished by their different masses – Compounds are combinations of atoms of different elements and possess properties different from those of their component elements – In chemical reactions, atoms are neither created nor destroyed but only exchanged between starting compounds to form new compounds Questions: How is a Hydrogen atom different from a Helium atom? • Is there something that makes them different? • We need to look at their structure to understand the difference The Structure of Atoms • Atoms aren’t the smallest particles of matter • They are made up of three types of particles – Proton - a positively charged particle – Neutron - a neutral particle – Electron - a negatively charged particle (much lighter than a Proton or Neutron) A picture of the Atom • The nuclease is at the center of the atom and contains the protons and neutrons - it is very small compared to the atom – A pea at the center of the WSU football stadium is about the same ratio • The electrons occupy the rest of the space of the atom The Charge of the Atom • The charge of a proton is exactly opposite of the charge of an electron – The proton is +1 and the electron is -1 • The number of protons in an atom is unique to each element and is called the atomic number • An atom having a net electrical charge is called an ion. – A positively charged atom is a cation – A negatively charged atom is an anion Atomic Mass • Each atom has a fixed mass • Because the carbon 12 isotope is stable and common we use it as a standard and assign it a mass of exactly 12.0000. • Everything else is compared to this carbon12 atom – Hydrogen is 1.008 on this relative scale Isotopes – Atoms of the same element have the same number of protons (atomic number) – The number of neutrons in an atom can vary • a family of elements with atoms having the same chemical properties but slightly different masses. • These atoms with differing masses are called isotopes – An isotope is identified by its mass number: the sum of the number of protons and the number of neutrons it contains checksome • Write the element (with atomic mass and atomic number) and charge for each – – – – 4 protons, 5 neutrons, 2 electrons 26 protons, 29 neutrons, 26 electrons 9 protons, 10 neutrons, 10 electrons 80 protons, 121 neutron, 78 electrons