Atoms, Molecules, and Ions Unit 2 Atoms, Molecules, and Ions

The Atomic Theory of Matter Democritus is generally credited with putting forth the first ideas about atomic structure Stated that all of matter consisted of indivisible particles called “atoms”

Dalton’s Atomic Theory Dalton proposed the first serious theories on the structure of matter Most of his postulates still hold true today and are known as Dalton’s atomic theory

Dalton’s Atomic Theory Matter is composed of small indivisible particles called atoms. The atom is the smallest unit of an element that has all the properties of that element. An element is composed entirely of one type of atom. The chemical properties of all atoms of any element are the same. A compound contains atoms of two or more different elements. The relative number of atoms of each element in a particular compound is always the same. Atoms do not change their identities in chemical reactions. Chemical reactions rearrange only how atoms are joined together. Essentially a restatement of the Law of Conservation of Mass

Chemical Laws Associated with Dalton’s Atomic Theory Law of Constant Proportions The relative numbers and kinds of atoms remain constant for a given compound Ex: Water (H2O) always has two hydrogens for every one oxygen Law of Multiple Proportions If two elements are capable of combining to form more than one type of product, they do so in a ratio of small whole numbers i.e. There is H2O and H2O2, but no H2O1.5

Atomic Composition and Structure Dalton also said that the atom was indestructable, which is untrue Actually made up of subatomic particles Protons Neutrons Electrons The electron was the first subatomic particle to be discovered as a result of experiments performed with cathode ray tubes

Discovery of the Electron J.J. Thomson is credited with discovery of the electron through his work with cathode ray tubes Referred to as “cathode rays” because they originated from the positively charged cathode Quantified a value of 1.76 x 108 C of charge per gram of electrons

Millikan’s Oil Drop Experiment Determined that each electron possessed -1.60 x 10-19 C of charge.

Mass of an Electron With Thomson’s mass to charge ratio and Millikan’s charge per electron, the mass of a single electron could now be calculated:

Plum Pudding Model Positively charged blob with individual electrons randomly distributed throughout

Rutherford’s Gold Foil Experiment Proved the existence of a positively charged nucleus According to Thomson’s model, a scattering angle of 1 was expected; however scattering at much larger angles was also observed “It was almost as if you fired a fifteen inch shell at a piece of tissue paper and it bounced back and hit you.”

Experimental Design

Explanation of Scattering Data Extremely dense, positively charged center of mass at the core of each atom Later dubbed the nucleus

The Neutron Summary of subatomic particles: Discovered by James Chadwick in 1932 Held together with protons by the strong nuclear force inside the nucleus of atoms Summary of subatomic particles:

Describing Atoms and Ions

Isotopes Isotopes differ from other atoms only by the number of neutrons contained in the nucleus For example: Symbol # of Protons # of Electrons # of Neutrons 24Mg 12 25Mg 13 26Mg 14

Example Write the symbol that describes each of the following isotopes: An atom that contains 7 protons and 8 neutrons An atom that contains 31 protons and 39 neutrons An atom that contains 14 protons and 14 neutrons

Ions Formation of ions involves the gain or loss of electrons from outside the nucleus Never involves the gain or loss of protons A cation is created from the loss of an electron (positively charged) An anion is created from the gain of an electron (negatively charged)

Symbols of Ions Give the chemical symbol, including mass number, for each of the following ions: The ion with 22 protons, 26 neutrons, and 19 electrons The ion of sulfur that has 16 neutrons and 18 electrons How many protons, neutrons, and electrons does the 79Se2- ion possess? 48Ti3+ 32S2- 34 protons, 45 neutrons, 36 electrons

Atomic Masses Atoms have extremely small masses (largest atom has m = 4 x 10-22 g) Atomic Mass Unit (amu) was implemented to avoid expression of such cumbersome numbers Defined as 1/12 the mass of a 12C atom Particle Charge Mass(amu) Proton Positive (+1) 1.0073 Neutron Neutral 1.0087 Electron Negative(-1) 5.486 x 10-4

Average Atomic Mass Because most elements exist in nature as a mixture of isotopes, we must express a large sample of these elements in the form of an average atomic mass (sometimes referred to as atomic weight Schematic of a mass spectrometer which is designed to measure atomic mass distributions

Example Data obtained with a mass spectrometer show that, in a sample of an element, 60.11% of the atoms have masses of 68.926 u, whereas the remaining 39.89% of the atoms have masses of 70.926 u. Calculate the atomic mass of this element and give its name and symbol. 69.724 u Gallium (Ga)