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Atomic Structure What is an atom? • Atom: the smallest unit of matter that retains the identity of the substance • First proposed by Democritus Atomic Structure • Atoms are composed of 2 regions: • Nucleus: the center of the atom that contains the mass of the atom (majority) • Electron cloud: region that surrounds the nucleus that contains most of the space in the atom Electron Cloud Nucleus What’s in the Nucleus? • The nucleus contains 2 of the 3 subatomic particles: • Protons: positively charged subatomic particles • Neutrons: neutrally charged subatomic particles What’s in the Electron Cloud? • The 3rd subatomic particle resides outside of the nucleus in the electron cloud • Electron: the subatomic particle with a negative charge and relatively no mass How do these particles interact? • Protons and neutrons live compacted in the tiny positively charged nucleus accounting for most of the mass of the atom How do these particles interact? The negatively charged electrons are small and have a relatively small mass but occupy a large volume of space outside the nucleus Relative Weights of Sub-Atomic Particles How do the subatomic particles balance each other? • In a neutral atom: • The protons equal the electrons • If 20 protons are present in an atom then 20 electrons are there to balance the overall charge of the atom—atoms are neutral • The neutrons have no charge; therefore they do not have to equal the number of protons or electrons How do we know the number of subatomic particles in an atom? • On the Periodic Table Atomic Number Symbol Name Atomic Mass (Mass Number) How do we know the number of subatomic particles in an atom? • Atomic number: this number indicates the number of protons in an atom • Ex: Hydrogen’s atomic number is 1 • So hydrogen has 1 proton • Ex: Oxygen’s atomic number is 8 • So oxygen has 8 protons **The number of protons identifies the atom. Ex. 29 protons = which element??? How do we know the number of subatomic particles in an atom? • Atomic #- 8 (protons) • Since it has 8 protons • Mass #: the number of it must have 8 neutrons protons and neutrons in (16-8) the nucleus • Ex: oxygen has a mass of 16. • # of neutrons = mass # - atomic # Determining the number of protons and neutrons • Li has a mass number of 7 and an atomic number of 3 • Protons = 3 (same as atomic #) • Neutrons= 7-3 = 4 (mass # - atomic #) • Ne has a mass number of 20 and an atomic number of 10 • Protons = 10 • Neutrons = 20 - 10= 10 What about the electrons? • The electrons are equal to the number of protons • So e- = p = atomic # • Ex: O has a mass # of 16 and an atomic # of 8 • p+ = 8 • no = 8 • e- = 8 Determine the number of subatomic particles in the following: Cl has a mass # of 35 and an atomic # of 17 • p+ = 17, • no = 18, • e- = 17 • P+ = 19, • no = 20 • e- = 19 How exactly are the particles arranged? • Bohr Model of the atom: Electron levels All of the protons and the neutrons The 3rd ring can hold up to 18 eThe 4th ring and any after can hold up to 32 e- The 1st ring can hold up to 2 eThe 2nd ring can hold up to 8 e- What does carbon look like? Mass # = 12 The second electron shell can hold how many electrons? 8 6 p and 6 n live in the nucleus Carbon has four of the 8 . p+ = 6 atomic # = 6 no = 6 Therefore, carbon has 4 VALENCE ELECTRONS (the # of electrons found in the outermost shell) e- = 6 Your Turn • Bohr Models • Hydrogen • 1 valence Beryllium 2 valence Nitrogen 5 valence A quick word about ions • Ions- a charged particle that forms when an atom (or group of atoms) gains or loses one or more electrons. • + Ions: have lost one or more electrons • - Ions: have gained one or more electrons Some examples… …and some jokes