Atomic Structure & Compounds (loosely based on Chapter 3 Sec 1 thru 4 of Jespersen 6th ed) Dr. C. Yau Spring 2014 1

Dalton's Atomic Theory (p.10) Matter consists of tiny particles called atoms. In any sample of a pure element, all the atoms are identical in mass and other properties. The atoms of different elements differ in mass and other properties. When atoms of different elements combine they form compounds with the elements in a fixed ratio by mass. Reactions are merely rearrangement of atoms to form different compounds. Atoms are indestructible . In chemical reactions, the atoms rearrange but they do not themselves break apart. (?) Chemical (?) 2

The 3 major subatomic particles Know this well ! Name & Symbol Mass Charge Location proton (p) ~ 1 u +1 inside nucleus neutron (n) electron (e-) ~ 0 u -1 outside nucleus u (amu) = atomic mass unit (a very small unit of mass) 1 gram = 602,000,000,000,000,000,000, 000 u 3 3

Atomic Mass Dalton’s atomic theory states that atoms of an element have a constant, characteristic atomic mass or atomic weight measured in u. Atomic masses are based on a standard mass, that of an atom of C. 1 atom of carbon-12 weighs 12 u exactly. Thus 1 u = 1/12 the mass of a carbon-12 atom. This is the definition of u. Chem FAQ: What is an atomic mass unit?

Isotopes Most elements in nature are uniform mixtures of two or more kinds of atoms with slightly different masses. For example: There are 3 isotopes of hydrogen, 4 isotopes of iron and 10 isotopes of tin. Isotopes are atoms of the same element with different masses. are atoms with the same #p but with different #n. Chemically, isotopes have virtually identical properties. (There are slight differences that we will note be concerned with at this level of chemistry.) KNOW WELL The relative proportions of the different isotopes are essentially constant

Atomic Notation An element is a substance whose atoms all contain the identical number of protons, called the atomic number (Z) Isotopes are distinguished by the mass number (A): Atomic number, Z = number of protons Mass number, A = (# of protons) + (# of neutrons) Note that for atoms, A is greater than Z: the symbol is "top-heavy." For neutral atoms, the number of electrons and protons must be equal.

Example: uranium-235 This indicates the mass number. 235 U 92 Mass number, (protons + neutrons)  Chemical Symbol  Atomic number, (number of protons)  From the nuclear symbol we can determine the # protons, electrons and neutrons: # protons = 92 # electrons = 92 (since no charge is shown) #neutrons = 235 – 92 = 143 Note that the number that follows the dash is the mass number. Chem FAQ: How do I write symbols for isotopes?

Fill in the blanks: symbol neutrons protons electrons 60Co 81Br 36 29 29 33 27 27 46 35 35

Trick question! You can’t tell! How many neutrons are there in 53Fe? This is how Fe appears in the periodic table. How many neutrons are there in Fe? Trick question! You can’t tell! How many neutrons are there in 53Fe? A) 26 B) 55 C) 27 D) none of these Ans. 53 – 26 = 27 mass number - # protons = # neutrons 26 Fe 55.85 This is NOT the atomic mass number.

protium deuterium tritium Hydrogen has 3 isotopes: 99.985% abundance 0.000 % abundnace 1 proton no neutron 1 proton 1 neutron 1 proton 2 neutrons approx. mass of each atom = 1 u 2 u 3 u 10 10

What we call "heavy water" is D2O. (Regular water being H2O, with practically all the hydrogen atoms being protium.) One molecule of H2O weighs 18 u. One molecule of D2O weighs 20 u. Hence D2O is known as “heavy water.” Tritium is the only radioactive isotope of hydrogen. Such isotopes are referred to as a radioisotope. NOTE: The term "isotope" does NOT mean that it is radioactive. 11 11

atomic mass = 35.4518 u (2 decimal places) Naturally occurring chlorine is a mixture of two isotopes. In every sample of this element, 75.77% of the atoms are chlorine-35 and 24.23% are chlorine-37. The measured mass of chlorine-35 is 34.9689 u and that of chlorine-37 is 36.9659 u. Calculate the average atomic mass of chlorine. 0.7577 x 34.9689 u = 26.495 u (4 sig. fig.) 0.2423 x 36.9659 u = 8.9568 u (4 sig. fig.) atomic mass = 35.4518 u (2 decimal places) Chem FAQs: How do I calculate an average atomic mass from isotopic masses and abundances? How do I calculate isotopic masses or abundances using average atomic mass? 17 Cl 35.45 35.45 u This is the atomic mass or atomic weight.

Hint: Let x be the mass of the other isotope. There are 2 isotopes of element Z. The first is 56.5% in abundance and has a mass of 152.3 u. If the atomic mass is 155.5 u, what is the mass of the other isotope? Hint: Let x be the mass of the other isotope. What % in abundance is this other isotope? 159.6 u = 160. u (3 sig.fig.)

Some Important Classifications: A groups = representative elements or main group elements I A = alkali metals II A = alkaline earth metals VII A = halogens VIII = noble (inert) gases B groups = transition elements Inner transition elements = elements 58 – 71 and 90 – 103 58 – 71 = lanthanide elements 90 – 103 = actinide elements See figure 1.15 for graphical distinction of these groups.

Metals, Nonmetals, And Metalloids FIG. 2.7 Distribution of metals, nonmetals, metalloids among the in the periodic table.

Properties Of Metals Metals... reflect light (have metallic luster) can be hammered or rolled into thin sheets (are malleable) and can be drawn into a wire (are ductile) are solids at room temperature (except Hg) conduct electricity and heat

Nonmetals And Metalloids lack the properties of metals tend to pulverize when struck with a hammer Are non-conductors of electricity and heat Many are gases, a few solids, and one liquid (Br2) react with metals to form (ionic) compounds Metalloids have properties between metals and nonmetals Chem FAQs: How do I classify elements as metals, nonmetals, or metalloids? How do I classify an element as a metal or nonmetal based on its properties?

EXTREMELY IMPORTANT! nonmetal + nonmetal nonmetal + metal covalent molecules ionic compounds alloys Note: Alloys are not compounds. They are homogeneous mixtures of metals, sometimes with a small amount of nonmetals, such as C in steel.

Learning Check: Count The Atoms In A Chemical Formula 3 2 1 Na2CO3 (NH4)2SO4 Mg3(PO4)2 CuSO4•5H2O ___Na, ___ C, ___ O ___N, ___H, ___S, ____O ___Mg, ___P, ____O ___Cu, ___S, ___O, ___H 2 8 1 4 3 2 8 1 9 10 What exactly does this dot tell you? Chem FAQ: How do I count the atoms of elements in a chemical formula?

Covalent Molecules The subscripts in the formula of covalent molecules tell you exactly how many atoms of each element is present. They do not merely state a ratio. e.g. CO2 tells us the molecule contains one C atom and two O atoms.

Ionic Compounds Positively charged ions are called cations Negatively charged ions are called anions Subscripts in the formula always specify the smallest whole-number ratio of the ions needed to make a neutral combination (formula unit, or f.u.) It does not mean 2 Fe ions are bonded to 3 O ions. Subscripts tell us the ions are in a ratio of 2Fe to 3 O. Fe O O2- Fe3+ 3 2 + See Chem FAQ: Ionic Compound Construction Kit

The subscripts in the formula of covalent molecules tell you the exact number of the atoms of each element is present. ionic compounds tell you the ratio of the atoms of each element that is present.

Overview: Molecules vs. Formula Units Electrically neutral, discrete particles called molecules. (H2O) Neutral groups of charged particles called formula units. (NaCl) FIG. 2.26 Molecular and ionic substances. (a) In water there are discrete molecules that each consist of one atom of oxygen and two atoms of hydrogen. Each particle has the formula H2O. (b) In sodium chloride, ions are packed in the most efficient way. Each Na is surrounded by six Cl, and each Cl is surrounded by six Na. Because individual molecules do not exist, we simply specify the ratio of ions as NaCl.

Why are ions charged? How does Ca form Ca2+? # p+ = # e- if neutral # p+ < # e- if negative # p+ > # e- if positive The number of p+ never changes when ions form. Ions are formed when the atom gains or loses electrons. Protons are not affected. How does Ca form Ca2+? Ca lost 2 electrons How is N3- formed? N gained 3 electrons

Symbol neutrons protons electrons 60Co3+ 81Br- Fill in the blanks: Symbol neutrons protons electrons 60Co3+ 81Br- 36 29 27 24 27 33 36 46 35 Chem FAQ: How do I count the number of electrons present in an atom or elemental ion?

Summary of Properties Hardness and brittleness Molecular compounds tend to be soft and easily crushed because the attractions between molecules are weak and molecules can slide past each other. Ionic compounds are hard and brittle because of the strong attractions and repulsions between ions Figure 2.15. An ionic crystal shatters when struck

Melting Points To melt the a solid, there must be sufficient kinetic energy to overcome the attractions between particles Molecular compounds have weak attractions between particles and so tend to have low melting points. Many molecular compounds are gases at room temperature. Ionic compounds tend to have strong attractions so they have high melting points. Nearly all ionic compounds are solids at room temperature.

Electrical Conductivity Requires the movement of electrical charge Ionic compounds: Do not conduct electricity in the solid state Do conduct electricity in the liquid and aqueous states-The ions are free to move. Molecular compounds: Do not conduct electricity in any state. Molecules are comprised of uncharged particles. Exception: Strong acids are molecular substances that become ionic when dissolved in water. Figure 2.16 See video: electrical conductivity apparatus See Chem FAQ: How can I distinguish ionic and molecular compounds given their properties?

Which of the following is likely true of NO2? It conducts electricity well. It has a low melting point. It is likely a solid in its pure form. None of these The formula tells you it’s a molecular compound, so it is not expected to conduct electricity, should have a relatively low mp, and therefore not likely to be a solid in its pure form. You should know how the formula tells you that it’s a molecular formula. See Slide 18!