1. Smallest particle of an element that retains the properties of the element

2.  Democritus first suggested the existence of atoms in the 4 th century BC.  John Dalton was the first to perform scientific experiments to test and correct his idea of the atom.  He created the first “Atomic Theory”

3. 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. 3. Atoms of different elements can combine physically or chemically (in whole number ratios to form compounds). 4. Chemical reactions occur when atoms are separated, joined, or rearranged.  However, not all holds true today…

4.  Electrons – negatively charged subatomic particles  Discovered by J.J. Thomson in 1897 Conducted Cathode Ray Experiment

5.  Robert Millikan measured the quantity of charge and the ratio of charge to mass of the electron Calculated the electron’s mass Oil Drop Experiment

6.  Atoms have no net electric charge  Electric charges are carried by particles of matter  Electric charges always exist in whole number ratios  When a given number of negatives combines with the same number of positives, a neutral atom is formed. So…we know there must be something POSITIVE inside the atom.

7.  Goldstein (1886) used cathode ray tube  observed a ray in the opposite direction (canal rays)  named particle the proton  Its mass was ~ 1840 times the mass of the electron

8.  Chadwick (1932) confirmed the existence of the neutron  Approx. same mass as the proton

9.  Thomson’s “Plum Pudding” model  Electrons are interspersed throughout positive material

10.  Rutherford tested Thomson’s theory  Gold Foil Experiment

12.  Beam of Alpha particles directed at gold foil.  Particles were scattered suggesting a dense mass of positive charge  Rutherford proposed the idea of the atomic nucleus.  Most of the atom is empty space  Nucleus contains protons and neutrons  Space contains electrons

13.  In the atom, we have:  Protons (p + )  Neutrons ( n 0 )  Electrons (e - )  Protons and Neutrons are in the nucleus.  Electrons are outside the nucleus.  Identities of elements are determined by the number of protons in the nucleus.

14.  Atomic Number – number of protons (whole number)  Mass Number – Protons plus Neutrons (number of particles in the nucleus)  Atomic Mass – weighted average of the masses of all isotopes of the element (decimal number) 6 C 12.011

15.  Isotope – atoms of the same element with different numbers of neutrons  Atomic Number will be the SAME  Mass Number will be DIFFERENT  Atomic Mass will be the SAME  In an Isotope,  Protons – SAME  Neutrons - DIFFERENT  Electrons – SAME  Ex. Carbon – 12 and Carbon - 14

16. ElementAtomic Number Mass Number ProtonsNeutronsElectrons 910 147 2120 1327 5626 Carbon-13 Beryllium-10 Neon-20 Boron-11 Sulfur-33

17. ElementAtomic Number Mass Number ProtonsNeutronsElectrons Fluorine-199199109 Nitrogen-14714777 Calcium-412041202120 Aluminum- 27 1327131413 Iron-562656263026 Carbon-13613676 Beryllium-10410464 Neon-20102010 Boron-11511565 Sulfur-331633161716

18.  Atoms can also gain and lose electrons.  An atom that has lost or gained an electron is an ION.  Positive Ions (CATIONS) form when atoms lose electrons  Ex. Calcium-41 atom 20 p + 20 e - 21 n 0 Calcium-41 +2 ION 20 p + 18 e - 21 n 0  Negative Ions (ANIONS) form when atoms gain electrons.  Ex. Oxygen-16 atom 8 p + 8 e - 8 n 0 Oxygen-16 -2 ION 8 p + 10 e - 8 n 0

19.  In an ION,  Protons – Same  Neutrons – Same  Electrons – Different  More electrons means a NEGATIVE ion.  Anion (O 2- )  Less electrons means a POSITIVE ion.  Cation (Ca 2+ )