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Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter 2:1 Foundations of Atomic Theory In the 1790s the study of matter was revolutionized by the quantitative analysis of chemical reactions, which had been greatly improved by more accurate balances. This led to several laws that are very important: 1. THE LAW OF CONSERVATION OF MASS states mass is neither created nor destroyed during ordinary chemical reactions or physical changes. (This will be the basis for balancing equations.) Ex.- Page 1 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter 2. THE LAW OF DEFINITE PROPORTIONS states that a chemical compound contains the same number of elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound. Ex.- Sodium chloride [NaCl] (table salt) always consists of 39.34% by mass of the element sodium [Na] and 60.66% by mass of the element chlorine [Cl]. 3. THE LAW OF MULTIPLE PROPORTIONS states if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first Page 2 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter element is always a ratio of small whole numbers. Ex.- The elements carbon and oxygen form two compounds, carbon dioxide [CO2] and carbon monoxide [CO]—the ratio of the masses of oxygen in these two compounds is always 2 to 1. 2:2 Structure of the Atom In the late 1800s, experiments showed that atoms are actually composed of several basic types of smaller particles and that the number and arrangement of these particles within an atom determine that atom’s chemical properties. ATOM the smallest particle of an element that retains the chemical properties of that element Page 3 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter The atom consists of two regions: the nucleus and the electron cloud. Three subatomic particles are protons, neutrons and electrons. (Note: protons and neutrons can be further divided into QUARKS). NUCLEUS contains the positively charged PROTONS and neutral NEUTRONS. ELECTRONS are negatively charged. Atoms are neutral (not charged), so an atom must have the same number of electrons as protons, though neutrons can differ. Super important concept: Page 4 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter The table above shows that a proton and neutron are approximately the same size and are 2000 times larger than an electron. If a large football stadium were an atom, this would make the nucleus (including the protons and neutrons) about the size of a marble. The electrons would be similar to dust particles floating about the stadium. This means that in our current model of the atom, the atom is mostly empty space. 2:3 Counting Atoms As atoms are generally too small to be measured individually in experiments, it is important to know how to analyze atoms quantitatively by knowing fundamental properties of the atoms of each element. You can use these properties to count the number Page 5 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter of atoms in an element in a sample with a known mass. Atoms of different elements have different numbers of protons; Atoms of the same element have the same number of protons. The ATOMIC NUMBER (Z) of an element is the number of protons of each atom of that element, so it identifies an element. The atomic number can be found above an element’s symbol on the periodic table. Elements are arranged in the Periodic Table in increasing atomic number. Ex.- Silver [Ag] has an atomic number of 47. This means that a silver atom has 47 protons, and since all atoms are neutral, 47 electrons. Page 6 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter HOWEVER, many naturally occurring elements have different numbers of neutrons and are called ISOTOPES. Most elements consist of mixtures of isotopes. ISOTOPES are atoms of the same element that have different masses. Ex.- Three types of hydrogen atoms are known. All only have one proton. The most common type accounts for 99.9885% of the hydrogen atoms found on Earth and is called protium, which has no neutrons and one electron. Deuterium with one proton, one neutron, one electron and accounts for 0.0115% of hydrogen atoms. The third is tritium with two neutrons, and it is radioactive. It exists in very small amounts in nature but can be artificially prepared. Page 7 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter The identification of an isotope requires knowing both the name and atomic number of the element and the mass of the isotope. The MASS NUMBER is the total number of protons and neutrons that make up the nucleus of an isotope. The three isotopes of hydrogen mentioned above have mass numbers of 1, 2 and 3. Isotopes are usually identified by specifying their mass number as shown with helium below. For example, the uranium isotope that is used as fuel for nuclear power plants has a mass number of 235 and is known as uranium235 in hyphen notation. Isotopes can also be written with nuclear symbols in which the superscript indicates the mass number (protons+neutrons) and the subscript indicates the atomic number (number of protons). Page 8 of 9 Intro to Chemistry and Physics/Pre-AP Chem UNIT 2 Atoms: The Building Blocks of Matter Ex.So, to find the number of neutrons: Mass number-atomic number=number of neutrons 235-92 (can be found/matched on periodic table)=143 Isotopes of Hydrogen and Helium ISOTOPE ATOMIC Number (hypen NUMBE of notation) R (# of electrons protons) Hydrogen-1 1 1 (protium) Hydrogen-2 1 1 (deuterium) Hydrogen-3 1 1 (tritium) Helium-3 2 2 Helium-4 2 2 Page 9 of 9 Number MASS of NUMBER neutrons (protons + neutrons) 0 1 1 2 2 3 1 2 3 4