AtomsAtoms

• Slide 3-8 Atomic TheorySlide 3-8 Atomic Theory• Slide 9 Distinguishing Between Slide 9 Distinguishing Between

AtomsAtoms• Slide 10 IsotopesSlide 10 Isotopes

• Democritus of Abdera, was a teacher who Democritus of Abdera, was a teacher who lived in Greece before Christ.lived in Greece before Christ.

• He suggested the existence of these He suggested the existence of these particles called atoms.particles called atoms.

• He said they were indivisible and He said they were indivisible and indestructible.indestructible.

• The real nature of atoms and the The real nature of atoms and the connection between observable changes connection between observable changes and events at the atomic level were not and events at the atomic level were not established for more than 2000 years.established for more than 2000 years.

2000 years later, an 2000 years later, an English school English school teacher performed teacher performed experiments to test experiments to test and correct his and correct his atomic theory.atomic theory.

Dalton studied ratios Dalton studied ratios in which elements in which elements combine in a combine in a chemical reactions. chemical reactions. This led to the This led to the development of his development of his atomic theory.atomic theory.

All elements are composed of tiny indivisible All elements are composed of tiny indivisible particles called atoms.particles called atoms.

Atoms of the same element are identical. Atoms of the same element are identical. Atoms of any one element are different from Atoms of any one element are different from those of any other elements.those of any other elements.

Atoms of different elements can physically mix Atoms of different elements can physically mix together or can chemically combine with together or can chemically combine with another in simple or whole number ratios to another in simple or whole number ratios to form compounds.form compounds.

Chemical reactions occur when atoms are Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one separated, joined or rearranged. Atoms of one element, however, are never changed into element, however, are never changed into atoms of another element as a result of a atoms of another element as a result of a chemical reaction.chemical reaction.

• Rutherford theorized Rutherford theorized that atoms have their that atoms have their positive charge positive charge concentrated in a very concentrated in a very small small nucleus,,[5] and and thereby pioneered the thereby pioneered the Rutherford model or or planetary model planetary model of of the the atom, through his , through his discovery and discovery and interpretation of interpretation of Rutherford scattering in his in his gold foil experiment. . (wikipedia)(wikipedia)

• Niels Bohr profited by following the Niels Bohr profited by following the experimental work going on in the experimental work going on in the Cavendish Laboratory under Cavendish Laboratory under Sir J.J. Thomson's guidance, at the guidance, at the same time as he pursued own same time as he pursued own theoretical studies. In the spring of theoretical studies. In the spring of 1912 he was at work in 1912 he was at work in Professor Rutherford's laboratory in laboratory in Manchester, where just in those years Manchester, where just in those years such an intensive scientific life and such an intensive scientific life and activity prevailed as a consequence of activity prevailed as a consequence of that investigator's fundamental that investigator's fundamental inquiries into the radioactive inquiries into the radioactive phenomena. phenomena.

• Contributions to Physics and Contributions to Physics and ChemistryChemistry

• The The Bohr model of the atom, the of the atom, the theory that theory that electrons travel in travel in discrete discrete orbits around the atom's around the atom's nucleus. .

• The The shell model of the atom, where of the atom, where the chemical properties of an element the chemical properties of an element are determined by the electrons in are determined by the electrons in the outermost orbit.the outermost orbit.

• Discovery of the ElectronDiscovery of the Electron• In 1897, Thomson was the first to In 1897, Thomson was the first to

suggest that the fundamental unit suggest that the fundamental unit of the atom was over 1000 times of the atom was over 1000 times smaller than an atom, suggesting smaller than an atom, suggesting the sub-atomic particles now known the sub-atomic particles now known as electrons. as electrons.

• Thomson discovered this through Thomson discovered this through his explorations on the properties of his explorations on the properties of cathode rays. Thomson made his . Thomson made his suggestion on the 30suggestion on the 30thth of April 1897 of April 1897 following his discovery that following his discovery that Lenard rays could travel much could travel much further through air than expected further through air than expected for an atomic-sized particle.for an atomic-sized particle.[4

• Other workOther work• In 1905 Thomson discovered the In 1905 Thomson discovered the

natural radioactivity of potassium.natural radioactivity of potassium.[7][7]

• In 1906 Thomson demonstrated that In 1906 Thomson demonstrated that hydrogen had only a single electron hydrogen had only a single electron per atom. Previous theories allowed per atom. Previous theories allowed various numbers of electrons.various numbers of electrons.[8][9][8][9]

• (wikipedia)(wikipedia)

• The The atomic number atomic number of an of an element=the number of protons and element=the number of protons and the number of electrons.the number of electrons.

• Mass number Mass number is the total number of is the total number of protons and neutrons in an atom.protons and neutrons in an atom.

• To get the number of neutron do this:To get the number of neutron do this:

Mass number – Atomic number = Mass number – Atomic number = # of Neutrons.# of Neutrons.

Isotopes are atoms Isotopes are atoms that have the same that have the same number of protons but number of protons but different numbers of different numbers of neutrons. neutrons.

Because isotopes of an Because isotopes of an element have different element have different numbers of neutrons, numbers of neutrons, they also have they also have different mass different mass numbers. numbers.

Isotopes are chemically Isotopes are chemically alike because they alike because they have identical numbers have identical numbers of p+ and e-.of p+ and e-.

• Since the 1920’s, it has been Since the 1920’s, it has been possible to determine these tiny possible to determine these tiny masses (atom’s mass) by using a masses (atom’s mass) by using a mass spectrometer.mass spectrometer.

• The atomic mass of an element is a The atomic mass of an element is a weighted average mass of the atoms weighted average mass of the atoms in a naturally occurring sample of the in a naturally occurring sample of the element.element.

• IonsIons- atom or group of atoms that - atom or group of atoms that have a positive or negative charge.have a positive or negative charge.

• Charge is due to the loss or gain of Charge is due to the loss or gain of electronselectrons– CationCation – ion with a positive charge – ion with a positive charge• Ex: Ex: Al Al3+3+

--AnionAnion – ion with a negative charge – ion with a negative chargeEx:Ex: O O2-2-

• Atomic massAtomic mass- weighted average - weighted average mass of the atoms in a naturally mass of the atoms in a naturally occurring sample of the element.occurring sample of the element.

• Atomic mass unit (amu)Atomic mass unit (amu)- standard unit - standard unit used for indicating atomic mass.used for indicating atomic mass.– One -twelth the mass of a carbon-12 atom.One -twelth the mass of a carbon-12 atom.

• -Involves the relative abundance of naturally occurring -Involves the relative abundance of naturally occurring isotopes of an element.isotopes of an element.

• -Relative abundance is calculated by dividing the -Relative abundance is calculated by dividing the number of atoms of each isotope by the total # of number of atoms of each isotope by the total # of atoms in the sample.atoms in the sample.– Relative abundance can also be given as a %. If you are given Relative abundance can also be given as a %. If you are given

a percentage you must divide by 100.a percentage you must divide by 100.

• -The Relative abundance of each isotope is then -The Relative abundance of each isotope is then multiplied by the mass of that isotope. This is the multiplied by the mass of that isotope. This is the relative mass.relative mass.

• -The relative masses are added together and that -The relative masses are added together and that equals the average atomic mass.equals the average atomic mass.

• Ex:Ex: Neon has two naturally occurring isotopes. Neon has two naturally occurring isotopes. The mass and # of atoms of each isotope are The mass and # of atoms of each isotope are shown below. What is the average atomic mass shown below. What is the average atomic mass of neon?of neon?

Isotope # of atoms in sample

Mass

Neon-20 92 20.179

Neon-22 18 21.991