2 Development of the Atomic Theory Atom - the smallest particle into which an element can be divided and still be the same substanceIt still retains its characteristic propertiesAtoms make up elementsElements combine to form compoundsBecause all matter is made up of elements or compounds, atoms are considered the building blocks of matter.
3 Democritus Proposes the Atom In 440 B.C. Democritus believed that there was a point where a particle could not be divided any further.This was against popular belief held by AristotleHe called this particle an atomos (Greek for atom), which means indivisibleHe believed these particles were constantly moving and they form different materials by joining together
4 Dalton Creates an Atomic Theory Based on Experiments John Dalton, a British chemist, wanted to know why elements combine in specific proportions to form compoundsi.e. water (H2O) always has two hydrogen and one oxygen atoms bound togetherHe performed experiments on compounds and concluded that they combined in specific proportions because of atoms
6 He stated the following: After many experiments and observations Dalton published his atomic theory in 1803.He stated the following:1) All substances are made of atoms. ATOMS Are small particles that cannot be created, divided, or destroyed2) Atoms of the same element are exactly alike, and atoms of different elements are different3) Atoms join with other atoms to make new substancesThis was the first step in our understanding of the atom and our current atomic theory
7 Thomson Finds Electrons in the Atom In 1897, a British scientist named J. J. Thomson discovered that atoms are made of small particles.This was contrary to Daltons atomic theoryThomson experimented with a cathod rayHe noticed that the ray was affected (bent) by a positive chargeHe concluded that the ray contained negatively charged particles
8 These negatively charged particles are now called electrons Thomson revised Dalton's atomic theory to include the presence of electrons even though he did not know how they were arranged in the atomHe came up with the plum pudding modelModel - is a representation of an object or systemThomson’s model illustrated positively charged material with negatively charged particles located throughout.
11 Rutherford Opens an Atomic “Shooting Gallery” In 1909, a former student of Thomson’s named Ernest Rutherford tested Thomson’s theoryHe aimed positively charged particles, larger than protons, at gold foilMost particles passed straight throughBut to Rutherford’s amazement;Some were deflected at various anglesAnd a few were bounced straight backIt became obvious to Rutherford that Thomson’s model was wrong
13 Rutherford Presents a New Atomic Model In 1911 Rutherford revised the atomic theoryHe concluded that atoms are mostly empty space with lightweight negative electrons moving aroundAnd in the center of the atom is a tiny, extremely dense, positively charged region - NucleusHe calculated that the diameter of the nucleus was 100,000 times smaller thanthe atom itself
14 Bohr States That Electrons Can Jump Between Levels In 1913 a Danish scientist who worked with Rutherford, Niels Bohr, suggested that electrons travel around the nucleus in definite pathsThese paths are located at levels at certain distances from the nucleusHe also believe that electrons could jump from one level to the nextBohr’s model was valuable in predicting some atomic behavior, but was too simple to explain all atomic behavior
16 The Modern Theory: Electron Clouds Surround the Nucleus Many scientists have contributed to our current understanding of the atomThis led to the following change to the atomic theory;Electrons do not travel in definite paths around the nucleusThe exact paths cannot be predictedThe regions around the nucleus where electrons are likely to be found are called - Electron Clouds
18 Chapter 12 Quiz 11) __________is the smallest particle into which an element can be divided and still be that element2) __________ is a unifying explanation for a broad range of hypothesis and observations that have been supported by testing3) __________ are the negatively charged particles found in all atoms4) ___________ discovered the nucleus5) ___________ are regions where electrons are likely to be foundBonus) __________ are atoms that have the same number of protons, but different numbers of neutrons
21 How Small Is an Atom?An average sized atom (i.e. aluminum) has a diameter of cmThat’s three hundred millionths of a centimeterIt would take a stack of 50,000 aluminum atoms to equal the thickness of a sheet of aluminum foilThere are 2 x 1022 atoms in one penny!20,000,000,000,000,000,000,000 atomsThat’s twenty thousand billion billion atomsThat is 4,000,000,000,000 times more atoms than people on Earth!
22 What’s Inside an Atom?As tiny as an atom is, it consists of even smaller particlesProtons, Neutrons, and ElectronsProtons and Neutrons make up the nucleus, which is at the center of the atom
23 Protons - the positively charged particles of the nucleus All protons are identicalEach has an approximate mass of 1.7 x grams or 1 amuNeutrons - particles of the nucleus that have no chargeNeutrons have a slightly greater mass than protonsThe nucleus of the atom is small, but very dense
24 Outside the Nucleus Electrons - negatively charged particles in atoms Electrons are found moving around the nucleus within electron cloudsElectrons are very small in mass compared to protons and neutrons ( amu)It takes more than 1,800 electrons to equal the mass of one proton
25 Positively charged atoms are Cations The charges of protons and electrons are opposite, but equal in strengthTherefore if there are equal amounts of protons and electrons, the overall charge of the atom is zero - neutrali.e. 10 protons - 10 electrons = 0If the number of protons and electrons differ, the atom becomes a charged particle - IonPositively charged atoms are CationsNegatively charged atoms are Anions
27 How Do Atoms of Different Elements Differ? There are 117 different elements, each made of different atomsThe number of Protons determines the elementThis is called the Atomic numberEach element is composed of atoms with the same atomic number
28 Are All Atoms of an Element the Same? Isotopes - are atoms that have the same number of protons, but have different number of neutronsAtoms that are isotopes of each other are the same element because they have the same number of protonsSome isotopes have unstable nuclei and become radioactive
29 How Can You Tell One Isotope from Another? You can identify one isotope from another by its mass numberMass number - is the sum of the protons and neutrons in an atomElectrons are not included in this calculation because their mass too small to affect itTo identify a specific isotope write the name of the element followed by a hyphen and the mass numberi.e. carbon - 12 (C-12) has contains 6 protons and 6 neutron, while carbon - 13 has 6 protons and 7 neutrons
30 How Do You Calculate the Mass of an Element Most elements found in nature contain a mixture of different isotopesi.e. All copper is composed of copper-63 and copper-65 atomsAtomic Mass Unit (amu) - is the weighted average of the masses of of all naturally occurring isotopesTo calculate the amu of an element, multiple the mass of each element by its percentage of abundance (in decimal form)
31 Copper consists of copper-63 at 69% abundance, and copper-65 at 31% abundance. What is the amu of copper?(63 x .69) =(65 x .31) =63.62 amu* So the amu of copper is amu
32 Now You Try OneChlorine consists of chlorine-35 at 76%, and chlorine-37 at 24%. What is the amu of chlorine(35 x .76)= 26.6(37 x .24)=35.5 amu
33 What Forces Are at Work in Atoms? There are four basic forces at work everywhere, including in atoms.They include:1) Gravity - the attraction between two objects based on their massesBecause the masses of the particles in atoms is so small, the gravity between them is small2) Electromagnetic Force - particles of the same charge repel, while opposite charges attract.The electromagnetic force holds electrons around the nucleus
34 3) Strong Force - At the close distances between protons in the nucleus, the strong force is greater than the electromagnetic force between the protonsThis keeps the nucleus together4) Weak Force - In certain unstable, a neutron can change into a proton and a electron.It is an important force in radioactive atoms
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