2ObjectivesCompare and contrast the atomic models of Democritus and DaltonDefine an atom.Distinguish between the sub atomic particles in terms of relative charge and mass.Describe the structure of the nuclear atom, including the locations of the subatomic particles.
3ObjectivesExplain the role of atomic number in determining the identity of an atom.Define an isotope and explain why atomic masses are not whole numbers.Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.
4Dalton’s Atomic Theory Each element consists of individual particles called atomsAtoms can neither be created nor destroyedAll atoms of a given element are identicalAtoms combined chemically in definite whole-number ratios to form compoundsAtoms of different elements have different masses
5HistoryThe electron was discovered in 1897 by Thomson. He imagined the atom as a “raisin pudding” with electrons stuck in a cake of positive chargeIn 1912, Rutherford discovered the nucleus (proton). He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun
6HistoryRutherford’s gold foil experiment was used to show that the nucleus was small, dense, and positively chargedThomson used the cathode ray to discover electronsThe neutron was then discovered in 1932 by Chadwick.
8Development of Atomic Structure • About 1900 Sir J. J. Thomson and Earnest Rutherford established the modern model of the atom.• The atom is composed of three subatomic particles.1. Electrically positive (+1) protons.2. Electrically neutral (0) neutrons.3. Electrically negative (-1) electrons.• Atoms have a net charge of zero.• The number of protons equals the number of electrons.
10Parts of an AtomEach element in the Periodic Table has a different number of protons in its nucleusChange the number of protons change elementsThe element also has the same number of electronsChange the number of electrons ionize the elementSome elements also have neutronsThey are in the nuclei of atomspen
111H The Hydrogen Atom One electron orbiting a nucleus 1 proton = Z = atomic number0 neutrons = NTotal mass = A = Z+N =1Singly ionized Hydrogen is missing one electron = 1H+Add a neutron and you have Deuterium = 2H = Dpe1H
124He The Helium Atom Two electrons orbiting a nucleus 2 protons = Z = atomic number2 neutrons = NTotal mass = A = Z+N =4Singly ionized Helium is missing one electron = 4He+epnnpe4He
133H (Tritium) 3He Isotopes and Elements If Helium loses one of its protons (and one of its electrons), it becomes a different elementpnn3H (Tritium)pneIf Helium loses one of its neutrons, it becomes an isotope3He
14Why Learn about Atomic Structure? Knowing the structure of atoms tells us about theirchemical propertieslight-emitting propertieslight-absorbing propertiesFrom this information we can learn about galaxies, stars, planets, asteroids, based on the light they emit or reflect.An example of absorption spectrafrom many different types of stars.
16Atomic StructureAn atom is composed of a dense core called a nucleus and surrounding this nucleus one or more negatively charged electrons.The nucleus is composed of positively charged protons and neutral neutrons.
17Atomic StructureAn atom is mostly empty space because the electron moves around the nucleus at such a great distance.A chemical element is determined based on how many protons the nucleus contains (Hydrogen has 1, Carbon has 6, Oxygen has 8 protons).
18An atom has the same number of electrons as protons, an atom has no net charge. If an atom loses or gains one or more electrons it has become ionized.With less electrons, the atom has a positive charge (Cation), more electrons it has a negative charge (Anion).Ions
19Parts of the atom Particle Charge Mass (g) Mass (amu) Proton +1 1.672 x 10-241Neutron1.675 x 10-24Electron-19.110x10-281/1839
20Atomic Number & Atomic Mass • The number of protons in the nucleus of an atom is called the atomic number (Z).
21Atomic Number & Atomic Mass Mass Number• The atomic mass of an atom is estimated from the total number of protons and neutrons in the nucleus.• The sum of the number of protons and neutrons in an atom is called the atomic mass number (A).Element SymbolMass NumberAtomic Number
22IsotopesIsotope - Atoms with the same atomic number but different mass numbers.• Isotopes have the same number of protons but differ in number of neutrons.
23IsotopesIsotope Abundance• Naturally occurring sample of most elements will contain a mixture of isotopes.• Percent abundance is used to quantify the amount of each isotope in an elemental sample.Example: OxygenIsotope% Abundance# in 100,00016O99.75999,75917O0.0373718O0.204204
24Atomic Mass Atomic mass unit is a relative unit 1/12 the mass of a 12C atom1 atomic mass unit (amu) = 1.66 X kg
25Atomic Mass and Isotopes The atomic mass which appears on the periodic table is the weighted average of the masses of all the isotopes.weighted fractional isotopicaverage = isotopic X massabundanceS
26Atomic Mass and Isotopes Boron has two isotopes10B abundance 19.9% mass u11B abundance 80.1% mass uAtomic mass = (0.199 x 10.01) + (0.801 x 11.01)Atomic mass = u
27Atomic Weight (Mass) – The average mass of a representative sample of atoms. Example 1: Calculate the atomic weight of oxygen given the following data.Isotope% AbundanceAtomic Weight16O99.76217O0.03818O0.200
28Atomic Weight (Mass) – The average mass of a representative sample of atoms. Example 2: Calculate the atomic weight of chlorine given the following data.Isotope% AbundanceAtomic Weight35Cl75.7736Cl0.037Cl24.23
29Atomic number and mass number +/-qZXX=symbolZ=atomic number (# of protons)A=mass number (# of protons + # neutrons)q= charge :+=electrons<protons,:-=electrons>protons
30Atomic StructureAtomic number (Z) is the number of protons in the nucleus of an atom. It is also known as the proton number.No. of protons always equals the no. of electrons in any neutral atom of an element.Mass number (A) is the sum of the number of protons and the number of neutrons in the nucleus of an atom.
31Atomic Structure - Example So how can you work out the number of neutrons in an atom?ExampleNo. of neutrons = Mass number – atomic numberNo. of neutron = Mass No. – Atomic No.= 23 – 11= 12