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The Mighty Electron Charges • Remember, – protons always have a positive charge – electrons always have a negative charge Neutral Atoms vs. Ions • When an atom is neutral it means that it has an equal number of protons and electrons. . • A neutral atom of boron would have 5 protons, so it would have 5 electrons. (Neutrons don’t effect the charge.) • Ions have an uneven number of protons and electrons. This gives them a charge. • If it has more protons, it’s a positive charge, more electrons is a negative charge. This atom of oxygen has 8 protons and 10 electrons, giving it a -2 charge! • To find the number of electrons and protons: – Look at the symbol. If there is a little plus or minus sign, its an ion! If there is no sign, its neutral. • P = neutral atom of phosphorous • O-2 = ion of oxygen w/a -2 charge • Li+ = ion of lithium w/a +1 charge • Find the atomic number. This is ALWAYS the number of protons – All that can change is the number of electrons. – So F- has 1 extra electron. • Fluorine’s atomic number is 9, so this ion has 9 protons and 10 electrons. 9 proton s – Ca+2 is missing two electrons (more protons gives it a positive charge) • Calcium’s atomic number is 20, so it has 20 protons and 18 electrons. – Ar is neutral (no + or -), so it has an equal number of protons and electrons • Argon’s atomic number is 18, so it has 18 protons and 18 electrons. To Find Neutrons • The atomic mass is the average number of protons plus neutrons, so we need to round it. – So atomic mass (rounded) minus atomic number is the average number of neutrons. • Ex. Chlorine – Mass = 35.453 » Round it to 35 – Atomic number = 17 35 – 17 = 18 Chlorine has 18 neutrons on average. Element Atomic # Atomic mass # protons # electrons # neutrons F F- Na Na+ S-2 As-3 Kr K+ • Find protons electrons, and neutrons for each of these: Element S-2 Na+ As -3 Kr Fe+2 Protons Electrons Neutrons Bohr Models • Named after Neils Bohr - discovered much of the structure of the atom. • First, find out the number of protons and electrons. • Then fill in the shells of the model from the center outward. – The first shell can hold 2, the next 8, and the third 8 as well. Holds 8 Write number of protons and neutrons in the nucleus Holds 2 P+ = N0= Holds 8 • Ex. – C has an atomic number of 6, so it has 6 protons and 6 electrons (if its neutral!) as well as an average of 6 neutrons. 1. First, put the number of protons (6) and neutrons (12-6=6) in the nucleus 2. Then, add electrons, starting with 2 in the inner shell. P+ = 6 N0 =6 3. Finally, add electrons to the next shell (maximum of 8) until you have added them all! e- = (16+2) = 18 …2 in the inner shell, S-2 Protons = 16 P+ =16 N0=16 Neutrons = 32-16=16 8 in next shell (it can only hold 8!) …and finally, 8 in the last shell! Valence Electrons • The electrons in the outermost shell of an atom are called valence electrons. – These are VERY important! They determine how (and if) the atom will bond to other atoms. Valence Electrons P+ = N0=