Ch. 2 Atoms, Molecules, and Ions

1. Ch. 2 Atoms, Molecules, and Ions

2. 2.1 The Atomic Theory of Matter • Early Greek philosophers thought that the material world must be made up of tiny indivisible particles they called atomos, meaning "indivisible or uncuttable" • Later, Plato and Aristotle formulated the notion that there can be no ultimately indivisible particles • As chemists learned to measure the amts of elements that reacted with one another to form new substances, the ground was laid for NA atomic theory that linked the idea of elements with the idea of atoms

3. Dalton's Atomic Theory • 1803-1807 John Dalton formulated the following postulates: • Each element is composed of extremely small particles called atoms • All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements • Atoms of an element are not changed into atoms of a different by chemical rxns; atoms are neither created nor destroyed in chemical rxns • Cmpds are formed when atoms of more than one element combine; a given cmpd always has the same relative # and kind of atoms • According to Dalton's atomic theory atoms are the smallest particles of an element that retain the chemical identity of the element

4. 2.1 cont • An element is composed of only one kind of atom, whereas a cmpd contains atoms of two or more elements • The law of constant composition: In a given cmpd, the relative #s and kinds of atoms are constant • Based on Dalton's Law Postulate #4 • Law of conservation of mass (Law of conservation of matter) – Postulate #3 • The total mass of materials present after a chemical rxn is the same as the total mass present before the rxn • Atoms always retain their identities and that atoms taking part in a chemical rxn rearrange to give new chemical combinations

5. 2.1 cont • A good theory should not only explain the facts known but should also predict new ones • Dalton used his theory to deduce the law of multiple proportions • If two elements A and B combine to form more than one cmpd, the masses of B that can combine with a given mass of A are in the ratio of small whole #s

6. 2.2 The Discovery of Atomic Structure • We now know that the atom is composed of still smaller subatomic particles • The atom is composed in part of electrically charged particles, some with a positive (+) charge and some with a negative (-) charge • Particles with the same charge repel one another, whereas particles with unlike charges are attracted to one another • In the mid-1800s scientists began to study electrical discharge through partially evacuated tubes (tubes that had been pumped almost empty of air) • A high voltage produces radiation within the tube

7. Cathode Rays and Electrons • This radiation became known as cathode rays bc it originated from the negative electrode, or cathode • It was not initially clear clear whether the rays were an invisible stream of particles or a new form of radiation • Experiments showed that cathode rays are deflected by electric or magnetic fields in a way consistent with their being a stream of (–) electrical charge • JJ Thomson concluded that cathode rays are streams of (-) charged particles • Thomson's paper is generally accepted as the "discovery" of what later became known as the electron

8. 2.2 cont • Earnest Rutherford revealed three types of radiation: alpha, beta, and gamma radiation • Each type differs in its response to an electric field • The paths of both alpha and beta radiation are bent by the electric field, although in opposite directions, whereas gamma radiation is unaffected • Alpha particles have a 2+ charge • Beta particles have a 1- charge • Gamma radiation does not consist of particles and carries no charge

9. The Nuclear Atom • Thomson proposed that the atom consisted of a uniform (+) sphere of matter in which the e- were imbedded • This so-called "plum pudding" model was very short-lived • In 1910-1911, Rutherford disproved this model with the gold foil experiment • Subsequent experimental studies led to the discovery of both (+) particles (protons) and neutral particles (neutrons) in the nucleus • Protons discovered by Rutherford in 1919 • Neutrons discovered by James Chadwick in 1932

10. 2.3 The Modern View of Atomic Structure • Only three subatomic particles (proton, neutron, and electron) have bearing on chemical behavior • Proton charge 1+ • Electron charge 1- • Neutrons carry no charge and are therefor electrically neutral • Every atom has an equal # of electrons and protons, and so atoms have no net electrical charge • Protons and neutrons reside in the nucleus of the atom, which, as Rutherford proposed, is extremely small

11. 2.3 cont • The e- are attracted to the protons in the nucleus by the force that exists btwn particles of opposite electrical charge • Bc it would be cumbersome to express such small masses of atoms in grams, we use instead the atomic mass unit, or amu • 1 amu = 1.66054 * 10-24g • The masses of the proton and neutron are very nearly equal, and both are much greater than that of the e- (5.486 * 10-4) • It would take 1836 e- to equal the mass of one proton • The nucleus contains most of the mass of an atom

12. 2.3 cont • A convenient, although non-SI, unit of length used to express atomic dimensions is the angstrom (Å) • One Å = 10-10m • EX The diameter of a penny is 19mm. The diameter of a silver atom is 2.88Å. How many atoms could arranged side by side in a straight line across the diameter of a penny? Ag atoms = (19mm)(10-3m/1mm)(1Å/10-10m)(1 Ag atom/2.88Å) = 6.6*107 Ag atoms

13. Atomic Numbers, Mass # and Isotopes • The # of protons in the nucleus of an atom of any particular element is called the element's atomic # • Bc an atom has no net electrical charge, the # of e- it contains must equal the # of protons • i.e. C has 6 protons and 6 e- and O has 8 protons and 8 e- • Atoms of a given element can differ in the # of neutrons they contain and consequently in mass • The atomic # is shown by the subscript,, and the superscript, called the mass #, is the total # of protons plus neutrons in the atom

14. 2.3 cont • Bc all atoms of a given element have the same atomic #, the subscript is redundant and is often omitted. Thus, the symbol for carbon-12 can be represented simply as 12C • Atoms w/ identical atomic #s but different mass #s (same # of protons but different # of neutrons) are called isotopes

15. 2.4 Atomic Weights • Most elements occur in nature as mixtures of isotopes. We can determine the average atomic mass of an element by using the masses of various isotopes and their relative abundance • i.e. C is composed of 98.93% 12C and 1.07% 13C. The masses of these isotopes are 12 amu and 13.00355 amu, respectively. (0.9893)(12 amu) + (0.0107)(12.0035 amu) = 12.01 amu • The average atomic mass of each element (in amu) is also known as its atomic weight

16. 2.5 The Periodic Table • Periodic table was developed in 1869 • If the elements are arranged in order of inc atomic #, their chemical and physical properties are found to show a repeating, or periodic, pattern • i.e. Each of the oft reactive metals comes immediately after one of the nonreactive gases • The arrangement of elements in order of inc atomic #, with elements having similar properties placed in vertical columns, is known as the periodic table

18. 2.5 cont • The horizontal rows are called periods • The vertical columns are called groups • Elements that belong to the same group often exhibit similarities in physical and chemical properties • Except for H, all the elements on the left side and in the middle of the periodic table are metallic elements, or metals • The majority of elements are metallic, and they all share many characteristic properties, such as luster and high electrical and heat conductivity • All metals, except Hg, are solids at room temp

19. 2.5 cont • The metals are separated from the nonmetallic elements, or nonmetals, by a diagonal steplike line that runs from B to At • H, although on the left side, is a nonmetal • Many of the elements that lie along the line that separates metals from nonmetals have properties that fall between those of metals and nonmetals • These elements are often referred to as metalloids

20. 2.6 Molecules and Molecular Compounds • Only the noble gases (Group 18) are normally found in nature as isolated atoms • Most matter is composed of mcules or ions, both of which are formed from atoms • A mcule is an assembly of two or more atoms tightly bound together • The resultant "package" of atoms behaves in many ways as a single, distinct object • Oxygen normally found in air consists of mcules that contain 2 oxygen atoms • We represent this molecular form by the chemical formula 02

21. 2.6 cont • The subscript in the formula tells us that two oxygen atoms are present in each mcule • A mcule that is made up of two atoms is called a diatomic mcule • A mcule that is made up of more than two atoms is a polyatomic mcule • O also exists as 03, ozone • Even though "normal" oxygen and ozone are composed of only O atoms, they exhibit very different chemical and physical properties • 02 is essential to life, but 03 is toxic; 02 is odorless my whereas 03 has a sharp, pungent smell

22. 2.6 cont • The elements that normally occur as diatomic mcules are H, O, N, and the halogens (Group 17/7A) • Cmpds that are composed of mcules are called molecular compounds and contain more than one type of atom • i.e. A mcule of water consists of two H atoms and one O atoms, and is represented by the chemical formula H20 • Lack of a subscript on the O atom represents that there is one O atom in water • Chemical formulas that indicate the actual #s and types of atoms in a mcule are called molecular formulas • Chemical formulas that give only the relative # of atoms of each type in a mcule are called empirical formulas

23. 2.6 cont • The subscripts in an empirical formula are always the smallest possible whole # ratios • i.e. The molecular formula for hydrogen peroxide is H2O2, whereas its empirical formula is HO • For most substances the molecular formula and the empirical formula are identical • Molecular formulas provide more information about mcules than do empirical formulas • You can determine the empirical formula from the molecular formula, but not conversely

24. Picturing Mcules • The structural formula of a substance shows which atoms are attached to which within the mcule • The atoms are represented by their chemical symbols, and the lines are used to represent the bonds that hold the atoms together • A structural formula usually does not depict the actual geometry of the mcule, that is, the actual angles at which atoms are joined together • A structural formula can be written as a perspective drawing to give some sense of the 3D shape

25. 2.6 cont • Scientists rely on various models to help visualize mcules • Ball-and-stick models show atoms as spheres and bonds as sticks • A space-filling model depicts what the mcule would look like if the atom • The molecular formula of a substance summarizes the composition of the substance, but does not show how the atom comes together to form the mcule • The structural formula of a substance shows which atoms are attached to which within the mcule

26. 2.7 Ions and Ionic Compounds • If e- are removed from or added to a neutral atom, a charged particle called an ion is formed • An ion with a + charge is called a cation • A – charged ion is called an anion • The net charge on an ion is represented by a superscript • Ex Na2+ Cl2- • In general, metal atoms tend to lose e- to form cations, whereas nonmetal atoms tend to gain an e- to form anions

27. 2.7 cont • There are polyatomic ions which consist of atoms joined as in a mcule

28. 2.8 Naming Inorganic Compounds

29. 2.9 Some Simple Organic Compounds