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Elements, Atoms, and Ions Dalton’s Atomic Theory (1808) 1. Elements are composed of extremely small particles called atoms. 2. All atoms of a given element are identical, having the same size, mass and chemical properties. 3. The atoms of one element are different from the atoms of all other elements. 4. Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms. 5. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together. 2 Each water molecule has 2 hydrogen atoms (2 g) and 1 oxygen atom (16 g). The mass ratio of oxygen to hydrogen is always 8:1. 16 X + 8Y 8 X2Y J.J. Thomson, measured charge/mass of e(1906 Nobel Prize in Physics) Measured mass of e(1923 Nobel Prize in Physics) e- charge = -1.60 x 10-19 C Thomson’s charge/mass of e- = -1.76 x 108 C/g e- mass = 9.10 x 10-28 g Rutherford Experiment (1902) (Uranium compound) Shown to be incorrect by Rutherford’s experiments (1908 Nobel Prize in Chemistry) particle velocity ~ 1.4 x 107 m/s (~5% speed of light) 1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron (-) 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g) Elements, Atoms, Molecules, Ions and Isotopes Rutherford’s Model of the Atom atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m What are atoms? • Atoms are the smallest particle of an element that retains the chemical identity of that element. • Atoms are composed of protons, neutrons, and electrons. – Even the subatomic particles are made up of particles themselves. Subatomic Particles Mass (g) Particle - -28 Charge (units) -19 -1 1.67 x 10-24 +1.6 x 10-19 +1 Electron (e ) 9.1 x 10 Proton (p+) Charge (Coulombs) Neutron (n) 1.67 x 10-24 -1.6 x 10 0 0 mass p = mass n = 1840 x mass e- Units of Mass • Particles’ masses measured in grams are extremely small. • For convenience, atomic mass units are used to express masses instead. • 1 AMU = 1/12 the mass of a Carbon-12 atom – Scientists massed a sample of carbon-12, determined the mass of a single carbon-12 atom, and divided it by 12 to find the mass of each proton and neutron. Subatomic Particles Particle Mass Charge Location proton 1 AMU + in the nucleus neutron 1 AMU none in the nucleus - surrounding the nucleus electron 0 AMU Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Mass Number A ZX Atomic Number 1 1H 235 92 2 1H U Element Symbol (D) 238 92 3 1H U (T) Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei Do You Understand Isotopes? How many protons, neutrons, and electrons are in 146 C? 6 protons, 8 (14 - 6) neutrons, 6 electrons How many protons, neutrons, and electrons are in 116 6 protons, 5 (11 - 6) neutrons, 6 electrons C? A molecule is an aggregate of two or more atoms in a definite arrangement held together by chemical bonds H2 H2O NH3 CH4 A diatomic molecule contains only two atoms H2, N2, O2, Br2, HCl, CO A polyatomic molecule contains more than two atoms O3, H2O, NH3, CH4 An ion is an atom, or group of atoms, that has a net positive or negative charge. cation – ion with a positive charge If a neutral atom loses one or more electrons it becomes a cation. Na 11 protons 11 electrons Na+ 11 protons 10 electrons anion – ion with a negative charge If a neutral atom gains one or more electrons it becomes an anion. Cl 17 protons 17 electrons Cl- 17 protons 18 electrons A monatomic ion contains only one atom Na+, Cl-, Ca2+, O2-, Al3+, N3- A polyatomic ion contains more than one atom OH-, CN-, NH4+, NO3- Do You Understand Ions? + 27 3 How many protons and electrons are in 13 Al ? 13 protons, 10 (13 – 3) electrons 2- ? Se How many protons and electrons are in 78 34 34 protons, 36 (34 + 2) electrons