2 Democritus (400 BC) Greek philosopher. All things are composed of tiny,indivisible particles: called:Atomos means“something thatcannot be cut”.
3 Lavoisier (1789) French scientist. Came up with the law of conservation ofMass.“Matter is neithercreated nor destroyed.”
4 Dalton (1803) Came up with the first “official” atomic Theory: Elements are made of tiny particles called atoms.All atoms of a given element are the same.
5 Dalton Atoms of different elements are different. Atoms of different elements combine to form compounds.Atoms cannot be subdivided, created, or destroyed.
6 Thomson (1906) Experimented with cathode rays Discovered that cathode rays arenegatively chargedDeveloped the“plum pudding”model of the atom
7 Cathode Ray Experiment Thomson passed negatively charged Cathode rays through a tube of Hydrogen: the rays bent toward the positively charged plate.He tried it again with different gases and always got the same result.He realized that all matter must contain these negatively charged particles (electrons).
8 “Plum Pudding”Thomson said atoms were probably composed of negatively charged particles mixed with a positively charged substance.
9 History Summary: Democritus – Atomos Lavoisier – Law of Conservation of MassDalton – First official Atomic TheoryThomson – Plum Pudding Model, theory of electrons
10 Subatomic ParticlesSubatomic ParticlesChargeSymbolMass (kg)Electron1-e-9.02x10-31Proton1+p+1.673x10-27Neutronno1.675x10-27Electrons are about 1837 times smaller than protons, so they do not significantly contribute to an atom’s mass.1 neutron has about the same mass as 1 proton, so we calculate the mass of an atom to be protons + neutrons.Ex. Hydrogen – AMUHydrogen’s mass is essentially that of 1 protonH has 1 proton, 0 neutrons and 1 electron
11 Atomic Number An element is defined by its number of protons Ex. All examples of Nitrogen (number 7 on the periodic table) have 7 protons. If you change this number, you change the element!Ex. If we change to 8 protons, what do we now have?Oxygen
12 Calculating Number of Neutrons All the atoms of one element have the same number of protons in their nucleus, but electrons & neutrons differFrom the Periodic Table:Atomic Number (Z) = # of protonsMass Number (A) = # of protons + # of neutronsAtomic Symbol = X
13 Calculating Number of Neutron Example: For Ca (Calcium)Z = 20 = (20 protons)A = 40 = (20 protons + neutrons)Number of Neutrons = A - ZA – Z = 40 – 20 = 20 neutrons Ca has 20 protons and neutrons
14 The Nucleus & Standard Atomic Notation Z = # of protonsA = # of protons + # of neutronsX = CaAZ4020
15 Inferring the Number of Electrons You can infer the number of electrons from ZProtons have a positive charge = p+ But, the atom is neutral on the periodic table Therefore, there must be enough electrons to BALANCE the number of protonsIf the atom has p+ = 5 and NO CHARGE e- = 5 BALANCE
17 Isotopes Mass # (P + N) Atomic # (P) Nuclear symbol: Isotopes are atoms of the same element with different mass numbers/different number of neutrons.Nuclear symbol:Mass # (P + N)Atomic # (P)Hyphen notation: carbon-12 (C-12)
21 Average Atomic Mass Weighted average of all isotopes. This is the crazy number on the Periodic Table.You should round to the nearest tenth.Avg.AtomicMassORAvg.AtomicMass= (mass)(% in nature) + (mass)(% in nature)
22 Average Atomic MassEX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.Avg.AtomicMass16.0g/molThis is the same number you get when you round the atomic mass to the nearest tenth on the Periodic Table!ORAvg.AtomicMass= (16)(.9976) + (17)(.0004) + (18)(.020) = 16.0g/mol
23 Average Atomic MassEX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.Avg.AtomicMass35.4 g/molORAvg.AtomicMass= (35)(.8) + (37)(.2) = 35.4 g/mol
24 Electrostatic forcesThese are the forces of attraction between electrons and protons
25 Nuclear forceDifferent from electrostatic forces, this is the force that keeps nucleons (particles found in the nucleus of an atom) together.Fun fact: the total mass of the nucleus is actually slightly greater than the mass of the protons and the neutrons together- when you break a nucleus down, some of that mass is released as energy, which we use in nuclear power plants and nuclear weapons