Atoms, Ions, and the Periodic Table

Transcription

1 Atoms, Ions, and the Periodic Table Dalton s Atomic Theory 2-2 1

2 2.1 Dalton s Atomic Theory The scanning tunneling microscope, STM, invented in 1981, allows us to create images of matter at the atomic level Dalton s Atomic Theory : Mass is not gained or lost in a chemical reaction. Proposed by Antoine Lavoisier in What would happen to the mass reading if the reaction was done without the balloon (an open system)? Sodium carbonate + hydrochloric acid Carbon dioxide + Sodium chloride 2-4 2

3 2.1 Dalton s Atomic Theory : A compound always has the same relative amounts of the elements that compose it. Proposed by Joseph Proust ~1800 For example, when water is broken down by electrolysis into oxygen and hydrogen, the mass ratio is always 8 to Dalton s Atomic Theory In 1808 John Dalton published a paper on atomic theory, summarized by the following postulates: All matter is composed of small, indivisible particles, called atoms. All atoms of a given element are identical both in physical and chemical properties. Atoms of different elements have different physical and chemical properties. Atoms are not created or destroyed in chemical reactions. Atoms combine in simple, fixed, whole-number ratios to form compounds. In a chemical reaction, atoms rearrange into new combinations, obeying the Law of Conservation of Mass

4 2.1 Dalton s Atomic Theory Is the law of conservation of mass obeyed here? Is this process possible? Dalton s Atomic Theory : Atoms actually are divisible. They are composed of subatomic particles. Subatomic particles found in the (dense center of the atom) - positively charged subatomic particles - uncharged subatomic particles One kind of subatomic particle found outside the nucleus - negatively charged subatomic particles 2-8 4

5 2.2 Structure of an Atom Protons, p +, are located in a tiny core at the center of the atom, the nucleus. - charge = C (expressed as +1) - mass = g Neutrons, n, are also located in the nucleus - no charge - mass = g (~same mass as proton) The nucleus of the atom contains the p + & n, and most of the mass of the atom. The diameter of an atom is about meter. The diameter of the nucleus is about m, 10,000 times smaller! The electrons, e -, occupy the empty space around the nucleus. - charge = C (expressed as -1) - mass = g (insignificant to 4 SF s) Although small in size, the nucleus accounts for about 99.9% of the mass of an atom Structure of an Atom Atoms of different elements differ by the number of protons in their nucleus. All atoms of an element have the same number of protons. : the number of protons in an atom. The atomic number is indicated in most periodic tables above the element symbol. : How many protons does a gold atom have?

6 2.2 Structure of an Atom : the number of protons in the nucleus of an atom : the number of protons and neutrons in the nucleus of an element s atom A = Z + N the number of neutrons in the nucleus of an element s atom N = A Z : an atom that contains a specific number of neutrons. Many elements have multiple isotopes. There are three isotopes of hydrogen: Structure of an Atom : a notation that adds the mass number, A, and atomic number, Z, to the elemental symbol, X. # " X Representations for a carbon atom with 6 neutrons: &' % C carbon-12 C-12 : Determine the number of protons and number of neutrons for this atom Write the isotopic symbol for this atom

7 : Write the isotopic symbol Write two representations for the following isotope Determine the number of protons and neutrons in each of the following isotopes: *+ &) Cl carbon 11 &67Au : Write the isotopic symbol Practice writing the isotope symbols for the following isotope pairs. 1. carbon-13 and carbon chlorine-35 and chlorine uranium-235 and uranium lithium-6 and lithium

8 : Heavy water One ice cube is made with water that contains only the hydrogen- 2 isotope. The other ice cube is composed of water with normal water which contains mostly hydrogen-1. Which ice cube is made with H-2? How can you tell? Ions Ions differ from atoms in that they have a charge; the number of electrons do not match the number of protons. : positively charged ions. They have fewer electrons than in the neutral atom. Cationsare named by adding ion to the elemental name - Na + is sodium ion : negatively charged ions. They have more electrons than in the neutral atom. Anions are named by adding ide ion to the prefix of the elemental name - Cl - is chloride ion

9 2.3 Ions Ions

10 : Counting Protons & Electrons Write the name, number of protons & number of electrons for the following ions: 1. Li + 2. F 3. O 2 4. Al : Ion Names & Symbols Write the name and symbol for the ion that have the following: protons and 18 electrons protons and 18 electrons protons and 23 electrons

11 2.4 Atomic Mass We can use mass number to compare the approximate relative masses of different isotopes. A carbon-12 atom is about twelve times the mass of a hydrogen- 1 atom. An oxygen-16 atom is about four times the mass of a helium-4 atom. But keep in mind that the mass number is NOT an actual mass! : A technique used to determine: the individual masses of the isotopes relative amounts of the isotopes in a sample Atomic Mass: AMU Scale : atomic mass units, 1/12 mass of 1 C-12 atom The mass of a carbon-12 atom is g. Because this is such a small number, the amu scale was developed and based on the mass of carbon-12: 1 amu = 1/12 mass of 1 C-12 atom = g Carbon-12 has a mass of exactly 12 amu per atom

12 2.4 Atomic Mass: Relative Atomic Mass Most elements have more than one isotope. If we average the masses of all the isotopes of carbon, taking into account the relative amounts of each, we get a relative atomic mass of amu. - This value is pretty close to 12 since carbon-12 is the most abundant isotope. : the average mass of the individual isotopes of an element, taking into account the naturallyoccurring relative abundance of each This is the mass represented on Periodic Tables. : The relative atomic mass of silver is amu, as found on the Periodic Table If silver is composed of only Ag-107 and Ag-109, which isotope is most abundant? Atomic Mass: Relative Atomic Mass Mass contribution from isotope (MC) = Isotope mass relative abundance Relative Atomic Mass = MC 1 + MC 2 + : An unknown element (X) discovered on a planet in another galaxy was found to exist as two isotope, with masses and abundances as listed in the table. What is the relative atomic mass of the element? Isotope Mass (amu) Natural Abundance 22 X % 20 X %

13 2.5 The Modern Periodic Table Our current periodic table shows elements in order of increasing atomic number (number of protons.) or : Elements in the same column having similar properties. Groups are designated in 2 ways: 1. A Roman numeral (I through VIII) and a letter (A or B) 2. An Arabic number (1-18) : Horizontal row of elements having properties that tend to vary in a regular fashion. Periods are designated by Arabic numbers. K reacts violently with water to producing a basic solution and flammable hydrogen gas. All alkali metals react with water to produce hydrogen gas The Modern Periodic Table: Groups Some groups have descriptive names that are commonly used instead of their group numbers. : Group 1 (IA) metals (not hydrogen) are considered reactive because they react readily with other elements and compounds React with water to produce hydrogen gas : Group 2 (IIA) metals are more reactive than the transition metals but less reactive than alkali metals : Group 17 (VIIA) nonmetals exist naturally as diatomic molecules (2 atoms) : Group 18 (VIIIA) nonmetals also called inert gases they do not chemically react with other elements under normal conditions

14 2.5 The Modern Periodic Table The periodic table has many classifications: Groups (columns) and periods (rows) Groups have similar properties Periods have properties that vary in a pattern Metals, nonmetals, and metalloids. Metals are lustrious, malleable, ductile, and conduct heat & electricity Nonmetals are dull, brittle and heat & electrical insulators. Metalloids are elements that have physical properties resembling a metal, but the chemical reactivity of a nonmetal The Modern Periodic Table

15 2.5 The Modern Periodic Table : contain any element in the eight groups designated with the letter A. (In the Arabic numbering, groups 1, 2, and 13-18) : contain any element in the 10 groups designated with the letter B. (In the Arabic numbering, groups 3-12) : contain the lanthanides and actinides listed separately at the bottom of the table The Modern Periodic Table : occur in pairs when not combined with other atoms Most elements do not exist as single atoms. Notable exceptions: Noble gases 7 elements exist naturally as diatomic molecules in their elemental form. N 2, O 2, F 2, Cl 2, Br 2, I 2 & H

16 2.5 The Modern Periodic Table: Ions When alkali metals, such as sodium, reacts to form ions, the ions have a 1+ charge. The alkaline earth metals, such as magnesium, form ions with a 2+ charge. Oxygen and other chalcogens react to form ions which have a 2 charge. The fluorine and other halogens form ions with a 1 charge. The noble gases do not form ions. How can we explain or predict this pattern? The Modern Periodic Table: Ions The noble gases are the most stable (least reactive) elements on the periodic table. Their stability is associated with the number of electrons they contain. Many atoms in the main-group elements gain or lose electrons to obtain the same number of electrons as the nearest noble gas. Metals tend to lose electrons - become cations - to have the same number of e - s as the last noble gas. Nonmetals tend to gain electrons, - become anions - to have the same number of e - s as the next noble gas

17 2.5 The Modern Periodic Table: Common Monatomic Ions