1 Atoms, Molecules, and Ions Chapter 2

2 John Dalton ( early 1800s): –Each element is composed of atoms –All atoms of an element are identical. –In chemical reactions, the atoms are not changed. –Compounds are formed when atoms of more than one element combine. Dalton’s law of multiple proportions: When two elements form different compounds, the mass ratio of the elements in one compound is related to the mass ratio in the other by a small whole number. The Atomic Theory of Matter Hydrogen to oxygen mass ratio of water (H 2 O) is twice that of hydrogen peroxide (H 2 O 2 )

3 Dalton – atoms are basic building blocks Thomson – cathode ray experiments Millikan – oil drop experiment Rutherford’s gold foil experiment

4 Cathode Rays and Electrons The Discovery of Atomic Structure

5 A cathode ray tube (CRT) is a hollow vessel with an electrode at either end. A high voltage is applied across the electrodes. The voltage causes negative particles to move from the negative electrode to the positive electrode. The path of the electrons can be altered by the presence of a magnetic field. Cathode Rays and Electrons

6 Consider cathode rays leaving the positive electrode through a small hole. If they interact with a magnetic field perpendicular to an applied electric field, the cathode rays can be deflected by different amounts. The amount of deflection of the cathode rays depends on the applied magnetic and electric fields. In turn, the amount of deflection also depends on the charge to mass ratio of the electron.

7 Cathode Rays and Electrons In 1897, Thomson determined the charge to mass ratio of an electron to be 1.76  10 8 C/g. Goal then became: find the charge on the electron to determine its mass. The Discovery of Atomic Structure

8 Millikan Oil Drop Experiment

9 Consider the following experiment: Oil drops are sprayed above a positively charged plate containing a small hole. As the oil drops fall through the hole, they are given a negative charge. Gravity forces the drops downward. The applied electric field forces the drops upward. When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate.

10 With Millikan oil drop experiment, able to measure charge of electron; therefore can calculate mass

11 Cathode Rays and Electrons Using this experiment, Millikan determined the charge on the electron to be 1.60  C. Knowing the charge to mass ratio, 1.76  10 8 C/g, Millikan calculated the mass of the electron: 9.10  g. With more accurate numbers, we get the mass of the electron to be  g. The Discovery of Atomic Structure

12 Radioactivity The Discovery of Atomic Structure

13 Radioactivity Consider the following experiment: A radioactive substance is placed in a shield containing a small hole so that a beam of radiation is emitted from the hole. The radiation is passed between two electrically charged plates and detected. Three spots are noted on the detector: a spot in the direction of the positive plate, a spot which is not affected by the electric field, a spot in the direction of the negative plate.

14 Radioactivity A high deflection towards the positive plate corresponds to radiation which is negatively charged and of low mass. This is called  -radiation (consists of electrons). No deflection corresponds to neutral radiation. This is called  -radiation. Small deflection towards the negatively charged plate corresponds to high mass, positively charged radiation. This is called  -radiation. The Discovery of Atomic Structure

15 The Nuclear Atom From the separation of radiation we conclude that the atom consists of neutral, positively, and negatively charged entities. Thomson erroneously assumed all these charged species were found in a sphere. The Discovery of Atomic Structure

16 The Nuclear Atom Rutherford carried out the following experiment: A source of  -particles was placed at the mouth of a circular detector. The  -particles were shot through a piece of gold foil. Most of the  -particles went straight through the foil without deflection. Some  -particles were deflected at high angles. If the Thomson model of the atom was correct, then Rutherford’s result was impossible. The Discovery of Atomic Structure

17 The Nuclear Atom Rutherford’s  -particle experiment: The Discovery of Atomic Structure

18 In order to get the majority of  -particles through a piece of foil to be undeflected, the majority of the atom must consist of a low mass, diffuse negative charge  the electron. To account for the small number of high deflections of the  -particles, the center or nucleus of the atom must consist of a dense positive charge.

19 The Nuclear Atom Rutherford modified Thomson’s model as follows: assume the atom is spherical but the positive charge must be located at the center, with a diffuse negative charge surrounding it. The Discovery of Atomic Structure

20 Dalton – atoms are basic building blocks Thomson – cathode ray experiments to determine ratio of charge per mass Millikan – oil drop experiment to determine charge of an electron; could then calculate mass from Thompson’s ratio Rutherford’s gold foil experiment - most of the mass of an atom and all of its positive charge in nucleus

21 The atom consists of positive, negative, and neutral entities (protons, electrons, and neutrons). Protons and neutrons are located in the nucleus of the atom, which is small. Most of the mass of the atom is due to the nucleus. There can be a variable number of neutrons for the same number of protons. Isotopes have the same number of protons but different numbers of neutrons. Electrons are located outside of the nucleus. Most of the volume of the atom is due to electrons. The Modern View of Atomic Structure

22 The Modern View of Atomic Structure

23 The Modern View of Atomic Structure Errors in table: Proton charge = +1Electron charge = -1 Electron Mass = x 10 -4

24 Isotopes, Atomic Numbers, and Mass Numbers Atomic number (Z) = number of protons in the nucleus. Mass number (A) = total number of nucleons in the nucleus (i.e., protons and neutrons). By convention, for element X, we write Isotopes have the same Z but different A. The Modern View of Atomic Structure

25 Isotopes, Atomic Numbers, and Mass Numbers The Modern View of Atomic Structure

26 The Periodic Table

27 Some of the groups in the periodic table are given special names. These names indicate the similarities between group members: Group 1A: Alkali metals. Group 2A: Alkaline earth metals. Group 6A: Chalcogens. Group 7A: Halogens. Group 8A: Noble gases. The Periodic Table

28 Molecules and Chemical Formulas Molecules are assemblies of two or more atoms bonded together. Each molecule has a chemical formula. The chemical formula indicates which atoms are found in the molecule, and in what proportion they are found. Compounds formed from molecules are molecular compounds. Molecules and Molecular Compounds

29 Molecular and Empirical Formulas Molecular formulas give the actual numbers and types of atoms in a molecule. Examples: H 2 O, CO 2, CO, CH 4, H 2 O 2, O 2, O 3, and C 2 H 4. Empirical formulas give the relative numbers and types of atoms in a molecule. That is, they give the lowest whole number ratio of atoms in a molecule. Examples: H 2 O, CO 2, CO, CH 4, HO, CH 2. Molecules and Molecular Compounds

30 Picturing Molecules Molecules occupy three dimensional space. However, we often represent them in two dimensions. The structural formula gives the connectivity between individual atoms in the molecule. The structural formula may or may not be used to show the three dimensional shape of the molecule. Molecules and Molecular Compounds

31 Picturing Molecules Molecules and Molecular Compounds

32 Molecules and Molecular Compounds Picturing Molecules If the structural formula does show the shape of the molecule, then either a perspective drawing, ball-and-stick model, or space-filling model is used.

33 When an atom or molecule loses electrons, it becomes positively charged. For example, when Na loses an electron it becomes Na +. Positively charged ions are called cations. Ions and Ionic Compounds

34 When an atom or molecule gains electrons, it becomes negatively charged. For example when Cl gains an electron it becomes Cl . Negatively charged ions are called anions. An atom or molecule can lose more than one electron. Ions and Ionic Compounds

35 Predicting Ionic Charge The number of electrons an atom loses is related to its position on the periodic table. Metals tend to form cations whereas non-metals tend to form anions. Ions and Ionic Compounds

36 Ionic Compounds The majority of chemistry involves the transfer of electrons between species. Example: To form NaCl, the neutral sodium atom, Na, must lose an electron to become a cation: Na +. The electron cannot be lost entirely, so it is transferred to a chlorine atom, Cl, which then becomes an anion: Cl -. The Na + and Cl - ions are attracted to form an ionic NaCl lattice which crystallizes. Ions and Ionic Compounds

37 Important: note that there are no easily identified NaCl molecules in the ionic lattice. Therefore, we cannot use molecular formulas to describe ionic substances. Ions and Ionic Compounds

38 Ionic Compounds Consider the formation of Mg 3 N 2 : Mg loses two electrons to become Mg 2+ Nitrogen gains three electrons to become N 3-. For a neutral species, the number of electrons lost and gained must be equal. However, Mg can only lose electrons in twos and N can only accept electrons in threes. Therefore, Mg needs to lose 6 electrons (2  3) and N gain those 6 electrons (3  2). Ions and Ionic Compounds

39 Ionic Compounds I.e., 3Mg atoms need to form 3Mg 2+ ions (total 3x2+ charges) and 2 N atoms need to form 2N 3- ions (total 2x3- charges). Therefore, the formula is Mg 3 N 2. Ions and Ionic Compounds

40 Names and Formulas of Ionic Compounds Naming of compounds, nomenclature, is divided into organic compounds (those containing C) and inorganic compounds (the rest of the periodic table). Cations formed from a metal have the same name as the metal. Example: Na + = sodium ion. If the metal can form more than one cation, then the charge is indicated in parentheses in the name. Examples: Cu + = copper(I); Cu 2+ = copper(II). Cations formed from non-metals end in -ium. Example: NH 4 + ammonium ion. Naming Inorganic Compounds

41 Names and Formulas of Ionic Compounds Naming Inorganic Compounds

42 Names and Formulas of Ionic Compounds Monatomic anions (with only one atom) are called  ide. Example: Cl  is chloride ion. Exceptions: hydroxide (OH  ), cyanide (CN  ), peroxide (O 2 2  ). Polyatomic anions (with many atoms) containing oxygen end in -ate or -ite. (The one with more oxygen is called -ate.) Examples: NO 3 - is nitrate, NO 2 - is nitrite. Naming Inorganic Compounds

43 Names and Formulas of Ionic Compounds Polyatomic anions containing oxygen with more than two members in the series are named as follows (in order of decreasing oxygen): per-….-ate -ate -ite hypo-….-ite Naming Inorganic Compounds

44 Names and Formulas of Ionic Compounds Naming Inorganic Compounds

45 Names and Formulas of Ionic Compounds Polyatomic anions containing oxygen with additional hydrogens are named by adding hydrogen or bi- (one H), dihydrogen (two H), etc., to the name as follows: CO 3 2- is the carbonate anion HCO 3 - is the hydrogen carbonate (or bicarbonate) anion. H 2 PO 4 - is the dihydrogen phosphate anion. Naming Inorganic Compounds

46 Names and Formulas of Ionic Compounds Name the anion then cation for the ionic compound. Example: BaBr 2 = barium bromide. Naming Inorganic Compounds

47 Names and Formulas of Binary Molecular Compounds Binary molecular compounds have two elements. The most metallic element is usually written first (i.e., the one to the farthest left on the periodic table). Exception: NH 3. If both elements are in the same group, the lower one is written first. Greek prefixes are used to indicate the number of atoms. Naming Inorganic Compounds

48 Names and Formulas of Binary Molecular Compounds Naming Inorganic Compounds

49 Atoms, Molecules, and Ions End of Chapter 2