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We are made up of elements Properties determined by the atoms that make them up Each atom made up of three particles Protons (mass=1; charge=+1) Neutrons (mass=1; no charge) Electrons (mass =0; charge=-1) Protons & neutrons are in the nucleus A helium atom (fig. 2.3) (fig. 2.3) # of protons gives identity of the element (atomic number) So what is helium’s atomic number? (fig. 2.3) Protons + Neutrons gives atomic mass So what is the atomic mass of helium? (fig. 2.3) For uncharged (electrically neutral) atoms: # of protons = # of electrons Re-cap Protons and neutrons are in the nucleus Their combined mass gives the mass of the atom Electrons “orbit” the nucleus Organized in energy shells Electron shells figure 2.5 e- are negatively charged -> attracted to nucleus Takes E to “push” them to higher shells Release E when they “fall” back down Shells fill from lowest to highest Electron shells figure 2.6 1st shell Holds 2 end 2 shell Holds 8 rd 3 shell Holds 8 Valence shell Outermost shell with e- present Electron shells figure 2.6 Look at carbon How many e- in its valence shell? How many more would it take to fill? (pause the video if necessary) Electron shells figure 2.6 Look at oxygen How many e- in its valence shell? How many more would it take to fill? (pause the video if necessary) Electron shells figure 2.6 Look at neon How many e- in its valence shell? How many more would it take to fill? (pause the video if necessary) Electron shells figure 2.6 Difference b/w neon and other atoms shown? Valence shell Full Not full If not full Atoms will react to fill Form bonds Next lecture ;)