Presentation on theme: "Atoms The smallest particle of an element that retains its identity in a chemical reaction."— Presentation transcript:
1 AtomsThe smallest particle of an element that retains its identity in a chemical reaction.
2 Atomic TheoryDemocritus believed that atoms were indivisible and indestructible.He did not have experimental support nor did he explain chemical behavior.It took 2000 years after Democritus for the real nature of atoms and events at the atomic level to be established
3 Dalton’s Atomic Theory Using experimental methods, Dalton transformed Democritus’s ideas on atoms into a scientific theory1. All elements are composed of tiny indivisible particles called atoms2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.3. Atoms from different elements can physically mix together or can chemically combine in simple whole- number ratios to form compounds.4. Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.
4 Structure of the Nuclear Atom Three kinds of subatomic particles: electrons, protons, and neutronsElectrons are negatively charged subatomic particles, mass=9.11 X 10-28Protons are positively charged subatomic particles, mass = 1.67 X 10-24Neutrons are subatomic particles with no charge, mass 1.67 X 10-24
5 ExperimentsJJ Thomson discovered the electron by passing an electric current through gases at low pressure.The gases were sealed in a glass tube fitted at both ends with metal disks called electrodes.One electrode became positively charged (anode) and the other negatively charged (cathode)This created what was called a cathode ray which was attracted to a positive plate and repelled by a negative plate
6 ExperimentsEugen Goldstein observed a cathode-ray tube and found rays traveling in the opposite direction of those in the cathode rays.He referred to these as canal rays and concluded they were positively chargedLater they became known as protons
8 Rutherford’s Gold-Foil Experiment Used alpha particles (He atoms that have lost their two electrons and have a double positive charge) as a narrow beam directed at a very thin sheet of gold foil.Hypothesis-beam would easily pass straight through with small deflectionsResults-most passed through with no deflection, some bounced backResulted in the atomic model containing a nucleus or central core of an atom that is composed of protons and neutrons.
10 Atomic NumberElements are different because they contain different numbers of protons Atomic Number: The number of protons in the nucleus of an atom of that element. This is used to identify an element. Example: Carbon’s atomic number is 6 because there are 6 neutrons in each Carbon atom’s nucleus For each element the number of protons equals the number of electrons. Atoms are electronically neutral, so the negative charge must equal the positive charge.
11 Mass NumberThe total number of protons and neutrons in an atom If you know the atomic number and mass number of an atom of any element, you can determine the atom’s composition. The number of neutrons in an atom is the difference between the mass number and atomic number. Number of neutrons= mass # - atomic # The composition of any atom can be represented in shorthand notation using atomic number and mass number
12 IsotopesAtoms that have the same number of protons but different numbers of neutrons
13 IsotopesBecause isotopes of an element have different number of neutrons, they also have different mass numbers.Despite differences, isotopes are chemically alike because they have identical numbers of protons and electrons, which are the subatomic particles responsible for chemical behavior.To the right is an example of carbon isotopes
14 Atomic MassSince the 1920’s the atomic mass has been able to be determined by using a mass spectrometerBecause the actual masses of individual atoms are so small the atomic mass unit was developedAtomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom.It is more useful to compare the relative masses of atoms using a reference isotope (carbon-12) as a standard.This isotope was assigned a mass of exactly 12 amuExample: He-4 with a mass of amu has about one-third the mass of a carbon-12 atom whereas nickel-60 has about 5 times the mass of a carbon-12 atom.
15 Atomic Mass Continued… A carbon-12 atom has 6 protons and 6 neutrons in its nucleus, and a mass set at 12 amuSince the protons and neutrons account for nearly all of this mass a single proton or neutron is about 1 amuIn nature most elements occur in two or more isotopesThe atomic mass of an element is a weighted average of the atoms in a naturally occurring sample of the element.A weighted average reflects both the mass and the relative abundance of the isotopes as they occur in nature.
16 Calculating Atomic Mass for an Element Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. Example: Element X has two natural isotopes. The isotope with a mass of amu has relative abundance 19.91%. The isotope with a mass of amu has a relative abundance of 80.09% Calculate the atomic mass of this element.
17 Calculating Atomic Mass For An Element Solution: Knowns: Isotope 10X has mass= amu abundance: 19.91%= Isotope 11X has mass= abundance: 80.09%= For 10X amu X = For 11X X = For element X = amu The calculated mass value is closer to the mass of the more abundant isotope, which is what you would expect
18 The Periodic TableA periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements)Period- Each horizontal row of the periodic table. The properties of the elements vary as you move across it from element to element.Group (or family)- Each vertical column of the periodic table. Elements within a group have similar chemical and physical properties.