2. 1 Mr. ShieldsRegents Chemistry U01 L04

3. 2 Dalton’s Atomic Theory Recall that one of Dalton’s postulates said: All atoms of the same element are the same This is not actually correct. Let’s see why.

4. 3 Nucleons All nuclei of the same element MUST have the same number of Protons. Why? The proton is the nucleon that determines atomic number and thus the identity of the element The electrons balance the positive charge of the nucleus and it’s the electron that determines the elements chemical properties --++ = 0= 0

5. 4 Mass Number Only protons determine an elements chemical properties Because it alone causes a change in the # of electrons. Therefore – Changing the number of neutrons WILL NOT change the chemical properties of an atom. - It WILL CHANGE however the atoms Mass Number

6. 5 Mass Number Atoms of the same element can have different nos. of neutrons. SO THEY ARE NOT THE SAME! – –Example: 12 C 13 C 14 C or 234 U 235 U 238 U – –No. of Neutrons = 6, 7, 8 and 142, 143, 146 S So What’s not correct about Dalton postulate that All atoms of the same element are the same)?

7. 6 Isotopes Atoms of the same element that have different nos. of Neutrons are said to be ISOTOPES of that elementAtoms of the same element that have different nos. of Neutrons are said to be ISOTOPES of that element The Chemical properties of all isotopes of an element are the same i.e. they react exactly the sameThe Chemical properties of all isotopes of an element are the same i.e. they react exactly the same However, Physical properties of isotopes may not be the same. For example mass, andHowever, Physical properties of isotopes may not be the same. For example mass, and Rate of diffusion. Rate of diffusion.

8. 7 Isotopes All elements found in nature are MIXTURES of isotopesAll elements found in nature are MIXTURES of isotopes Isotopes of an element are designated by mass number in either of two waysIsotopes of an element are designated by mass number in either of two ways –1: 235 92 U –2: U-235 or Uranium-235 Using the above how would we designate the isotope of carbon that has 8 neutrons?Using the above how would we designate the isotope of carbon that has 8 neutrons? 14 6 C or C-14 (Carbon-14)

9. 8 Isotopes Only HYDROGEN has special names for its isotopesOnly HYDROGEN has special names for its isotopes 1 1 H 2 1 H 3 1 H 1 1 H 2 1 H 3 1 H 1 proton1 proton1 proton1 proton1 proton1 proton 0 neutrons1 neutron2 neutrons0 neutrons1 neutron2 neutrons HydrogenDeuteriumTritiumHydrogenDeuteriumTritium This isotope is Radioactive