Chapter 3: Atomic Structure

Scientists Democritus- Matter composed of atoms (indivisible) (~450B.C.) Democritus- Matter composed of atoms (indivisible) (~450B.C.) Lavoisier – conservation of mass Lavoisier – conservation of mass Proust – law of constant composition Proust – law of constant composition Dalton – modern atomic theory (KNOW the 4 postulates) Dalton – modern atomic theory (KNOW the 4 postulates)

Law of Constant Composition Faraday – Atoms contain charged particles Faraday – Atoms contain charged particles Thomson – atoms are divisible, he discovered electrons Thomson – atoms are divisible, he discovered electrons Millikan – found the charge and mass of electrons Millikan – found the charge and mass of electrons

Scientists Becquerel –discovered radioactivity Becquerel –discovered radioactivity Marie Curie – isolated radioactive elements. Marie Curie – isolated radioactive elements. Rutherford – demonstrated existence of neutrons and the nucleus Rutherford – demonstrated existence of neutrons and the nucleus

Rutherford Modern atom Modern atom Nucleus - central charge concentrated into a very small volume in comparison to the rest of the atom Nucleus - central charge concentrated into a very small volume in comparison to the rest of the atom tiny electrons circling around the nucleus like planets around the sun. tiny electrons circling around the nucleus like planets around the sun.electrons

Modern Atomic Theory Atoms are composed of three fundamental particles Atoms are composed of three fundamental particles Protons p + Neutrons n o Electrons e - Protons p + Neutrons n o Electrons e - The nucleus is made of protons and neutrons, (positively charged) The nucleus is made of protons and neutrons, (positively charged) Electrons orbit the nucleus in and electron cloud (negatively charged) Electrons orbit the nucleus in and electron cloud (negatively charged) An atom is neutral, the # p+ = # e- An atom is neutral, the # p+ = # e-

Chart ParticleLocationCharge Mass (g) Mass AMU Proton Inside Nucleus x ~ 1 Neutron Inside Nucleus x ~1 Electron Outside nucleus x ~0

Atomic Number Atoms identity comes from the number of protons in the nucleus Atoms identity comes from the number of protons in the nucleus In a chemical reaction, atoms gain/lose electrons and become an ion. In a chemical reaction, atoms gain/lose electrons and become an ion. Ion is a charged particle. This can be + or – depending on whether an electron is gained or lost. Ion is a charged particle. This can be + or – depending on whether an electron is gained or lost.

Calculating charges and writing ions. If an electron is gained, the charge becomes negative. If an electron is gained, the charge becomes negative. If an electron is lost, the charge becomes positive. If an electron is lost, the charge becomes positive. Ex. Magnesium Ex. Magnesium Number of protons = 12 Number of protons = 12 Number of electrons = 10 Number of electrons = 10 Charge of ion = 2+ or Mg +2 Charge of ion = 2+ or Mg +2

Isotopes Isotopes - atoms of the same element (same #p+) but with different number of neutrons. Isotopes - atoms of the same element (same #p+) but with different number of neutrons. All elements have isotopes. All elements have isotopes. Isotopes of elements are almost indistinguishable (they exhibit the same properties) Isotopes of elements are almost indistinguishable (they exhibit the same properties)

The mass number is used to identify isotopes. The mass number is used to identify isotopes. Mass number = the sum of the p + and n 0 Mass number = the sum of the p + and n 0 Mass number → 37 Mass number → 37 Cl Cl Atomic number → 17 Atomic number → 17

More Examples Other examples Other examples Cl-35C-12 C-14 Cl-35C-12 C Cl C C Cl C C

Even MORE examples Ions Ions Fe +2 O 2- Al +3 Fe +2 O 2- Al

Mass of atoms- Masses measured in amu’s Masses measured in amu’s AMU = atomic mass unit = 1/12 the weight of a carbon-12 atom AMU = atomic mass unit = 1/12 the weight of a carbon-12 atom Atomic Mass = atomic weight = average atomic mass Atomic Mass = atomic weight = average atomic mass

Calculation of average atomic mass = Average mass of an element’s atoms = Average mass of an element’s atoms Lithium – % = 6 x = Lithium – % = 6 x = Lithium – % =7 x.9258 = Lithium – % =7 x.9258 = amu amu

You try it! Neon – % = Neon – % = Neon – % = Neon – % = Neon – % = Neon – % = amu amu

Changes in the nucleus Nuclear Reactions – Change the composition of the nucleus. Nuclear Reactions – Change the composition of the nucleus. Atoms undergo nuclear decay and produce new elements! Atoms undergo nuclear decay and produce new elements! Why are some atoms radioactive? Why are some atoms radioactive?

Changes in the Nucleus What governs nuclear stability? What governs nuclear stability? part of reason is the # of p + and # n o part of reason is the # of p + and # n o strong nuclear force strong nuclear force –force which holds the nucleus together

Pattern of stable nuclei “belt of stability” – as atomic number increases, you need more neutrons to keep the atom stable All atoms with an atomic number greater than 83 are radioactive All atoms with an atomic number greater than 83 are radioactive

Radioactive Decay Radioactive Decay – emission of radiation Radioactive Decay – emission of radiation 3 types: 3 types: Alpha: Alpha: High-energy alpha particles High-energy alpha particles 2p + and 2 n 0. 2p + and 2 n 0. 4   2 Weak, stopped by paper or clothing Weak, stopped by paper or clothing Mass number 4 Mass number 4

Radioactive Decay Beta: Beta: High speed electrons High speed electrons  e  e Mass number = 0 Mass number = 0 Can pass through clothing, some damage to skin Can pass through clothing, some damage to skin

Radioactive Decay Gamma: Gamma: Most dangerous Most dangerous Consists of radiation waves Consists of radiation waves Only stopped by heavy dense material like lead/concrete Only stopped by heavy dense material like lead/concrete 0   0

Writing nuclear equations: Atomic mass Atomic mass Chemical symbol Chemical symbol Atomic Number Atomic Number

Alpha particles When a nucleus emits an alpha particle, the mass decreases by 4 amu’s and the number decreases by 2 amu’s  Ra → Rn+   Ra → Rn+ 

Beta particles When a nucleus emits a beta particle, the mass of the atom is practically unchanged, but the atomic number increases by one unit. When a nucleus emits a beta particle, the mass of the atom is practically unchanged, but the atomic number increases by one unit  I → Xe +   I → Xe + 

Gamma Rays When a nucleus emits a gamma ray, both the atomic number and atomic mass remain the same  In → In +   In → In + 

Application of Nuclear Chemistry Use of half life + Radioactive Dating Use of half life + Radioactive Dating Nuclear Bombardment – Reactions Nuclear Bombardment – Reactions Create radioactive isotopes used in medicine Create radioactive isotopes used in medicine Power Generation Power Generation Fission – Limerick Generating Plant Fission – Limerick Generating Plant Fusion – “research” Fusion – “research”

Radioisotope – an isotope that is radioactive. Radioisotope – an isotope that is radioactive. Half-life – The amount of time it takes for ½ of a sample of a radioactive isotope to decay. (1/2 of the radioactive atoms) Half-life – The amount of time it takes for ½ of a sample of a radioactive isotope to decay. (1/2 of the radioactive atoms) Ex. 90Sr = 28.8 yrs Ex. 90Sr = 28.8 yrs

Radiocarbon Dating uses carbon-14 uses carbon-14 carbon-14 is radioactive carbon-14 is radioactive half-life is 5370 yrs half-life is 5370 yrs Produced naturally from reaction between N-14 and cosmic rays Produced naturally from reaction between N-14 and cosmic rays Rate of production carbon-14 = rate of decay of carbon-14 Rate of production carbon-14 = rate of decay of carbon-14