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The Bohr-Rutherford Model of the Atom Bohr and Lewis diagram drawing review; HW, read Russian story. The Bohr model is like a ‘planetary’ model: a core nucleus of protons and neutrons is surrounded by electrons. The electrons are in distinct energy levels (shells). The protons and neutrons make up most of the mass of an atom. If the atom is neutral, the protons = electrons. An ion gains or loses electrons to get a full valence (outer) shell. - protons = 1+ charge neutrons = 0 charge electrons = 1- charge The shells hold the same number of electrons as there are elements in each period (row): 2 – 8 – 8 – 18 electrons Atomic Notation and Drawing Bohr and Lewis Diagrams: Atomic Notation: The mass number of an element tells us the protons and neutrons – these particles both weigh 1 atomic mass unit (1 amu). We round the mass number to a whole number (it’s a decimal due to isotopes – tomorrow…) The masses are scaled from a standard – C-12 which has a mass of 12 amu (more later) The atomic number tells us the number of protons in the nucleus. This is what defines an element Bohr Diagram: Lewis Dot Diagram: _____________________________________________________________________ Going down a column (group), what happens to the size of the atom? (eg. Li compared to Na compared to K) Going across a period, what happens to the strength of the nucleus’s positive charge?