CHAPTER 2. ATOMS 2.2 COMPOSITION OF MATTER

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1 CHAPTER 2. ATOMS 2.2 COMPOSITION OF MATTER 1. Pure substance: Consist of 1 Substance with fixed composition and distinct properties; cannot be separated by physical means A. Element: Made up of unique atoms & cannot be chemically separated into simpler substances. E.g. H, O, C B. Compound: Consist of 2 or more different elements & can be chemically broken down into simpler substances. E.g. H 2 O, NaCl Compounds have different properties than elements contained in compound. E.g. H 2 O vs. H 2 & O 2 2. Mixture: Physical combination of 2 or more substances; variable composition; components can be separated by physical means E.g. sand, rocks, egg, salt water, air, vodka tonic ELEMENTS & COMPOUNDS 112 known elements ( are radioactive) ~88 naturally occurring elements Each element has a 1-3 letter symbol derived from its name; 1st letter & only 1st letter is always capitalized. * Memorize names & symbols of the selected elements on handout Chemical formula of an element or compound tells us type & number of atoms present. E.g. Ne, H 2, H 2 O, Ca(OH) 2 subscript indicates how many atoms are present; 1 is omitted 2.3 ATOM: smallest particle of an element that retains the chemical identity of the element (John Dalton). Atomic size: nm STM (scanning tunneling microscope) used to "see" atoms Dalton's Atomic Theory ) Each element is composed of small particles called atoms. 2) All atoms of an element are identical to each other. 3) Atoms cannot be created or destroyed. 4) Atoms of one element are different than atoms of another element. 5) Compounds are formed when atoms of different elements combine. molecule: 2 or more atoms bonded together; the atoms may be identical or different from one another. Many elements exist as diatomic molecules: O 2, N 2, H 2, F 2, Cl 2, Br 2, I 2 Law of Constant Composition: A compound always consists of the same combination of elements (% composition is fixed). E.g. 100 g of H 2 O always contains 11.1 g H, 88.9 g O

2 Law of Conservation of mass: during a chemical reaction, mass is conserved. Total mass of the reactants = Total mass of the products 2.4 SUBATOMIC PARTICLES The 3 main subatomic particles are the proton, electron and neutron: Particles charge mass (g) mass (amu) location electrons x g = 1/1835 amu outside nucleus protons x g = 1 amu nucleus neutons x g = 1 amu nucleus Nuclear Model (Rutherford): 1) Each atom has a very small nucleus. Protons & neutrons are located in the small nucleus; it contains most of the mass. 2) Electrons are located in region outside of the nucleus. Most of the atom is empty space. 3) Atoms possess neutral overall charge - equal number of protons & electrons. ATOMIC NUMBER, MASS NUMBER, ISOTOPES Every atom of an element has the same # of protons The # of protons defines an element The number of protons is called the atomic number, Z. (top # on per. table) Mass number = A = total number protons & neutrons in nucleus A = number p's + number n's n = A - Z Isotopes are atoms of an element that have a different number of neutrons. Isotopes of an element have the same atomic number, but a different mass number. For neutral atoms: # of protons = # of electrons nuclear symbol(isotope symbol) for element X: Name of isotope: name-a A X Z A = p + n; Z = p Ex Cl Z = p = 17 A = 35 n = A - Z = = 18 e- = 17 name: chlorine-35 Ex Zn Z = 30 A = 66 p = 30 e- = 30 n = 36 name: zinc-66 Note that Z is sometimes omitted from the nuclear symbol: 13 6 C or 13 C can be used for carbon-13 IONS - atoms that have lost or gained electrons. Charge is written after symbol. Ions are discussed in more detail in Chapter 3. E.g. Mg +2 ion has 12 protons and 10 electrons (positively charged ion has more p) F - ion has 9 protons and 10 electrons (negatively charged ion has more e-)

3 ATOMIC MASS Masses of atoms are so small that we define the atomic mass unit (amu) to scale up the numbers The carbon-12 isotope was assigned a mass of exactly 12 amu, masses of other elements are scaled relative to C amu = x g, so p & n have masses of approximately 1 amu. Atomic mass of an element is the average weighted mass of all isotopes of an element. (Atomic mass is found at bottom of box on periodic table) 2.5 THE PERIODIC TABLE 1st table 1869 produced by Mendeleev & Meyer Features of modern periodic table: 1. Elements arranged in order of increasing Z (atomic #), not mass. 2. Horizontal Rows in periodic table are called periods. 7 periods exist 3. Vertical Columns on table correspond to groups or families; elements in a group have similar properties. representative elements: A Group transition elements: B Group 4. Metals are located to the left of the stairstep line. Metals are shiny, malleable, ductile, good conductors, solids (except Hg) 5. Nonmetals are located to the right of the stairstep. Nonmetals are dull, insulators, can be gas, liquid or solid 6. Elements located at the diagonal line are intermediate in character. These are semiconductor or metalloid elements. Si, Ge, As, Sb, Te, Po, At Individual boxes on periodic table have atomic # at top, chemical symbol in middle, atomic mass at bottom E.g. As Atomic # = z = 33; As = arsenic; mass = amu 2.6 Electronic Stucture Bohr model ~1913 1) charged e's travel rapidly in orbits around the tiny + charged nucleus 2) Electrons are contained in specific energy levels called principle energy levels or shells. These energy levels are quantized which means only certain energies are allowed. There is not a continuum of energy. Analogy: stairway (quantized) ramp (continuum) Energy levels are designated by the quantum number n. n = 1,2,3. n = 1 is ground state level - this is level closest to nucleus. 3) Electrons can jump from one level to another by absorbing or emitting energy.

4 QUANTUM MODEL - SCHRODINGER 1) n = principal quantum number, where n is energy shell. Values for n = 1,2,3,4... (n = 1 closest shell to nucleus) Generally, energy increases with increasing n. 2) Principle energy levels can be subdivided into subshells. Electrons within a subshell have identical energy. There are 4 known subshells: s, p, d, and f. These correspond to blocks of elements on periodic table. 3) Each subshell contains a specific number of orbitals Orbitals are regions in space where an electron is most likely to be found. Sublevel Block on Chart Max # of e- # of orbitals s 1A, 2A 2 1 p 3A-8A 6 3 d transition metals 10 5 f inner transition 14 7 Electron Configurations and Orbital Diagrams Important Rules 1) Pauli exclusion principle - no more than 2 e- can occupy the same orbital 2) Aufbau principle: Orbitals are filled in order of increasing energy. At ground state, electrons fill lowest energy orbitals available 3) Hund's rule: When filling degenerate orbitals (same subshell), 1 e- is placed in each orbital before pairing up. An electron configuration describes how many e- an atom has in each of its subshells. Suggestions for writing electron configurations 1) Locate element on periodic table. 2) List all subshells in order of increasing energy. 3) Use superscript #'s to indicate # of e-'s in each subshell. 4) Double check. Sum of superscript #'s = Z Be able to write electron configurations for 1 st 20 elements E.g. Write electron configurations & orbital diagrams for the following H (Z = 1) 1s 1 He (Z = 2) 1s 2 Li (Z = 3) 1s 2 2s 1 C (Z = 6) 1s 2 2s 2 2p 2 F (Z = 9) 1s 2 2s 2 2p 5 Mg (Z = 12) 1s 2 2s 2 2p 6 3s 2

5 S (Z = 16) 1s 2 2s 2 2p 6 3s 2 3p 4 Noble gas core - can use noble gas from previous row as short cut. Na (Z = 11) 1s 2 2s 2 2p 6 3s 1 [Ne]3s 1! [Ne] Cl (Z = 17) [Ne]3s 2 3p PERIODIC PROPERTIES VALENCE ELECTRONS The outer shell s and p electrons are called the valence electrons. These electrons dictate the properties of an element. All elements in a family have identical valence shell e-configurations - same # of valence electrons. Group 1A 2A 3A 4A 5A 6A 7A 8A e- conf. ns 1 ns 2 ns 2 np 1 ns 2 np 2 ns 2 np 3 ns 2 np 4 ns 2 np 5 ns 2 np 6 valence e Ionization Energy - energy required to remove an electron from an atom. IE increases periodic table - Harder to remove e-'s from Noble gases & nonmetals (don't want to lose e-'s) IE decreases periodic table - Easier to remove e- from higher energy shell (further from nucleus)