Chapter 2 Atoms, molecules, and ions The Atomic Theory History The Structure of the Atom Proton, Neutron, Electron Atomic Number, Mass Number Isotopes The Periodic Table DAB/CAS/CSU/2010

Matter

Brownian Motion

Atomic theory Atoms are incredibly small. What we know about them is based on indirect evidence.

History of atomic theory 500 BC Democritus proposed the idea of atom but had no experimental data to support Believed that all matter consists of very small, indivisible particles, which he named atomos (meaning uncuttable or indivisible). 1800 Dalton proposed an atomic theory supported by an experimental data

Dalton’s atomic theory: Support Support for his theory came when he began looking at the masses of elements that combined with each other He found that elements combine by simple ratio by mass

Dalton’s atomic theory: All matter is composed of tiny indivisible particles called atoms. All atoms of the same element are identical (having same size, mass, and chemical properties). The atoms of one element are different from the atoms of all other elements. Atoms of two or more elements can combine in small whole number ratios to form compounds. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.

Dalton’s atomic theory: Support Law of Definite Proportion Different samples of the same compound always contain its constituent elements in the same proportion Law of Multiple Proportions If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers. Example: CO & CO2

Dalton’s atomic theory: Support Law of Conservation of Mass Matter can neither be created nor destroyed The mass of atoms before and after reaction is conserved

Dalton’s atomic theory: Support Law of Conservation of Mass

Dalton’s atomic theory: Problems with Daltons Theory Atoms of the same element have different masses Atoms have subatomic particles (protons, electrons, neutrons.) So, Dalton’s theory is not completely correct!

The structure of the atom

Subatomic Particles Particle Symbol Charge Relative Mass (amu) Electron e- 1- 0 Proton p+ + 1 Neutron n 0 1

Subatomic Particles: Discovery Electron was discovered by JJ Thomson in ～1900 using his cathode ray tube. Electron charge was found by American Robert Millikan ～ 1910 by an oil-drop experiment. Proton was found by the adaptation of cathode ray tube ～ 1900. Neutron was discovered by Chadwick in 1932.

The Electron Discovery 1890’s discovery of radiation lead to discovery of electron Radiation – the emission and transmission of energy through space in the form of waves Used cathode ray tube Cathode – negatively charged plated, emits an invisible ray Anode – positively charged tube

Thomson’s plum pudding model of the atom

Subatomic Particles: Discovery Rutherford’s Gold Foil Experiment: Plum pudding model: atom has a weak positive charge. Rutherford tested the idea by positively charged particles at gold foil. He expected particles to pass through. His results surprised him.

Movie on Gold Foil Experiment

Subatomic Particles: Discovery Rutherford’s Gold Foil Experiment: Results Atoms is mostly empty space. Protons are found in the dense nucleus in the center of the atom.

Location of Subatomic Particles 10-13 cm electrons protons neutrons 10-8 cm nucleus

Rutherford’s Model: Problem: Could not explain why electrons would not spiral into the nucleus.

Implications of Rutherford’s Exp’t. Nucleus – at the center of the atom Dense central core within the atom Contains positively charged particle (protons) Protons carry the same charge as that of electron Mass of protons is 1840 times that of electron Neutrons – neutral particles within the nucleus having a mass slightly greater than that of protons

Particle Mass (g) Coulumb Charge unit Electron 9.10939 x 10 -28 -1.6022 x 10-19 -1 Proton 1.67262 x 10 -24 +1.6022 x 10-19 +1 Neutron 1.67493 x 10 -24

ATOMS TO ELEMENTS What makes elements different from one another? The number of protons, electrons, and neutrons in them. Example: Gold and Mercury

Counts the number of protons in an atom Atomic Number, Z Counts the number of protons in an atom

Atomic Number on the Periodic Table 11 Na Atomic Number Symbol

All atoms of an element have the same number of protons 11 Na 11 protons Sodium

Learning Check State the number of protons for atoms of each of the following: A. Nitrogen 1) 5 protons 2) 7 protons 3) 14 protons B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons C. Barium 1) 137 protons 2) 81 protons 3) 56 protons

Counts the number of protons and neutrons in an atom Mass Number, A Counts the number of protons and neutrons in an atom Mass Number, A = p + n n = A - p

Atomic Symbols Show the mass number and atomic number Give the symbol of the element mass number 23 Na sodium-23 atomic number 11

Number of Electrons Number of protons = Number of electrons An atom is neutral The net charge is zero Number of protons = Number of electrons + 2 + (-2) = 0 Atomic Number =Number of protons Atomic number = Number of electrons

Subatomic Particles in Some Atoms 16 31 65 O P Zn 8 15 30 8 p+ 8 n 8 e- 15 p+ 16 n 15 e- 30 p+ 35 n 30 e-

Subatomic Particles in Some Atoms 16 31 65 O P Zn 8 15 30 8 p+ 8 n 8 e- 15 p+ 16 n 15 e- 30 p+ 35 n 30 e-

What happens when atoms losses electrons? When Na losses 1 electron? 11 p+ 11 e- 2311Na+ 2311Na +11 -10 1 11 p+ 11 e- 12 n 11 p+ 10 e- 12 n

What happens when atoms gains electrons? When Cl gains 1 electron? 17 p+ 17 e- 3517Cl- 3517Cl +17 - 18 - 1 17 p+ 17 e- 18 n 17 p+ 18 e- 18 n

What happens when atoms gains electrons? When O gains 2 electron? 8 p+ 8 e- 168O 168O + 8 - 10 - 2 8 p+ 8 e- 8 n 8 p+ 10 e- 8 n

Isotopes Atoms with the same number of protons, but different numbers of neutrons. Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 37Cl 17 17 chlorine - 35 chlorine - 37 17 p+ 18 n 17 e- 17 p+ 20 n 17 e-

Learning Check 6 6 6 #p _______ _______ _______ Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 #p _______ _______ _______ #n _______ _______ _______ #e _______ _______ _______ 6 6 7 6 8

Learning Check An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 2) 35 3) 65 B. Number of neutrons in the zinc atom C. What is the mass number of a zinc isotope with 37 neutrons? 1) 37 2) 65 3) 67

Learning Check Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 e- ___________ B. 17p+, 20n, 17e- ___________ C. 47p+, 60 n, 47 e- ___________ 16O 8 37Cl 17 107Ag 47

Atomic Mass on the Periodic Table 11 Na 22.99 Atomic Number Symbol Atomic Mass

Atomic Mass Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.

Example of an Average Atomic Mass Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine. 35 x 75.5 = 26.4 100 35.5 37 x 24.5 = 9.07