Models of The Atom Chapter 4
Dalton Model of the Atom Dalton proposes that all matter is made up of tiny particles, which are molecules or atoms. Molecules can be broken down into atoms by chemical processes. Atoms cannot be broken down by chemical or physical processes. 2
Dalton s s Model of Atom (1808) Based on law of definite composition,, the mass ratio of carbon to oxygen in carbon dioxide was always the same. Carbon dioxide was composed of 1 carbon atom and 2 oxygen atoms. 3
Dalton s s Atomic Theory 1) An element is composed of tiny, indivisible, indestructible particles called atoms. 2) All atoms of an element are identical and have the same properties. 3) Atoms of different elements combine to form compounds. 4) Compounds contain atoms in small whole number ratios. 5) Atoms can combine in more than one ratio to form different compounds. 4
Cont d Dalton Dalton s s Atomic Theory It was found that the first two parts of Dalton s s atomic theory were incorrect. Today proposals of 3, 4, and 5 are still accepted. Dalton s s theory will stay as an important step in the further development of atomic theory. 5
Subatomic Particles 50 years later after Dalton s s proposal, it was found that atoms were divisible. Two subatomic particles were discovered: Negatively charged electrons,, e. Positively charge protons,, p +. An electron has a relative charge of -1 and a proton has a relative charge of +1. 6
Thomson Model of the Atom subatomic model of the atom (1903) Plum Pudding Model of the atom: Electrons were distributed evenly throughout a homogeneous sphere of positive charge. 7
Mass of Subatomic Particles Thomson was able to calculate only the mass-to to- charge ratio of a proton and an electron. Millikan determined the charge of an electron (1911). Using the charge of electron found by Millikan, Thomson calculated the masses of a proton and electron: An electron has a mass of 9.11 10-28 g A proton has a mass of 1.67 10-24 g 8
Types of Radiation There are three types of radiation: Alpha (α),( Beta (β),( & Gamma (γ)( Alpha rays are composed of helium atoms stripped of their electrons (helium nuclei). Beta rays are composed of electrons. Gamma rays are high energy electromagnetic radiation. 9
Rutherford s s Gold Foil Experiment Rutherford fired alpha particles at thin gold foils. If the plum pudding model of the atom was correct, most α-particles should pass through undeflected. However, some of the alpha particles were deflected backwards. 10
Explanation of Scattering Since atom is largely empty space, most of the alpha particles passed through the foil. Atomic nucleus contains the atom s s protons and it is located at the center of an atom. The α-particles that bounced backwards did so after striking the dense nucleus. 11
Rutherford s s Model of the Atom a new model of the atom Negatively charged electrons are distributed around a positively charged nucleus. An atom has a diameter of about 1 10-8 cm and the nucleus has a diameter of about1 10-13 cm. 12
Subatomic Particles Revisited Rutherford predicted that because of the heaviness of the nucleus, nucleus must contain neutral particles in addition to protons. Neutrons, n 0, were discovered about 30 years later. A neutron has no charge and it is about the size of a proton. 13
Atomic Notation Atomic notation represents the number of protons and neutrons in the nucleus of an atom. Atomic number, Z, represents the characteristic number of protons in the nucleus. Atomic number is element specific. Mass number, A, represents the total number of protons and neutrons in the nucleus of an atom. 14
Using Atomic Notation Example: The element name: Sodium (symbol Na). The atomic number is 11 sodium has 11 protons. The mass number is 23 = the atom of sodium has 23 protons + neutrons. The number is neutrons is: A Z = 23 11 = 12 neutrons. 15
Isotopes Atoms of the same element that have a different number of neutrons in the nucleus are called isotopes. Isotopes have the same atomic number but different mass numbers. All atoms of the same element have the same number of protons. Most elements occur naturally with varying numbers of neutrons. 16
Cont d Isotopes To refer to an isotope state the name of the element followed by the mass number. Examples: Question: Co Cobalt-60, Carbon-14 60 14 37 6 C How many protons and neutrons does an atom of mercury-202 have? The atomic number of Hg is 80, so it has 80 protons Hg-202 has 202 80 = 122 neutrons 17
Average Atomic Mass Not all isotopes of an atom are present in equal proportions, to find the average atomic mass: Use isotopes of Copper: 63 Cu with a mass of 62.930 amu and 69.09% abundance 65 Cu with a mass of 64.928 amu and 30.91% abundance Average atomic mass of copper: (62.930 amu)(0.6909) + (64.928 amu)(0.3091) = 63.55 amu 18
Review Atoms are composed of protons, neutrons, and electrons. The protons and neutrons are located in the nucleus and the electrons are outside the nucleus. Atoms are mostly empty space. The number of protons is referred to as the atomic number for the atom. 19
Cont d Review All atoms of the same element have the same number of protons. Isotopes are atoms with the same number of protons but different numbers of neutrons. The mass number for an isotope is the total number of protons plus neutrons. The atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes. 20