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ATOM, MOLECULE, AND IONS by Hak Sok Chea Academic year: 2008 ~ 2009 Atomic theory Democritus concept: Greek philosopher, 5th Century BC ¾ All matter is composed of very small, indivisible particles, which he called Atomos. Dalton theory: English Teacher, 1808 ¾ Elements are composed of extremely small particles, called atoms. ¾ All atoms of given element are identical, having the same size, mass, and chemical properties. The atoms of one element are different from the atoms of all other elements. ¾ Compounds are composed of atoms of more than one element. In any compound, the ratio of the numbers of atoms of any two of the elements present is either an integer or a simple fraction. ¾ A chemical reaction involves only the separation, combination, or rearrangement of atom; it does not result in their creation or destruction Schematic representative for the first two hypothesis Atom of element X Atom of element Y Compound of element X and Y Atomic structure ATOM ATOM Electron Electron Nucleus Nucleus Proton Proton Neutron Neutron - Atom: define as the basic unit of an element that can enter into chemical combination. - Electron: is a fundamental subatomic particle that carries a negative electric charge. (Thomson, 1898-1903) Atomic structure The structure of the Atom Electron configuration in atoms The electron configuration is the arrangement of electrons in an atom, molecule, or other physical structure (e.g., a crystal). The state of an electron in an atom is given by four quantum numbers: ¾ Principle quantum number (n) ¾ Azimuthal quantum number (l) ¾ Magnetic quantum number (m) ¾ Spin quantum number (s) The structure of the Atom Principle quantum number (n) The principal quantum number (n) describes the size of the orbital. Ex: Orbitals for which n = 2 are larger than those for which n = 1,. Energy must therefore be absorbed to excite an electron from an orbital in which the electron is close to the nucleus (n = 1) into an orbital in which it is further from the nucleus (n = 2). The structure of the Atom Principle quantum number (n) n definite the electronic shell: - n =1 correspond to K shell, - n =2 correspond to L shell, - n =3 correspond to M shell, - n =4 correspond to N shell, The maximum electron in the shell correspond to 2n2 (where n is the number of the energy level) Ex: n =1, the electrons in the K shell are maximum 4; n = 2, the electrons in L shell are maximum 2 x 22 =8 The principal quantum number (n) cannot be zero. The allowed values of n are therefore 1, 2, 3, 4, and so on. The structure of the Atom Azimuthal quantum number (l) The angular quantum number (l) describes the shape of the orbital. Orbitals have shapes that are best described as spherical (l = 0), polar (l = 1), or cloverleaf (l = 2). They can even take on more complex shapes as the value of the angular quantum number becomes larger. The angular quantum number (l) can be any integer between 0 and n - 1. If n = 3, for example, l can be either 0, 1, or 2. The structure of the Atom l definite the electronic sub-shell; l =0 sub-shell s (1 orbital, maximum 2 electrons), l =1 sub-shell p (3 orbitals, maximum 6 electrons), l =2 sub-shell d (5 orbitals, maximum 10 electrons), l =3 sub-shell f (7 orbitals, maximum 14 electrons), ..... Although there is no pattern in the first four letters (s, p, d, f), the letters progress alphabetically from that point (g, h, and so on). The structure of the Atom Magnetic quantum number (m) The magnetic quantum number (m), to describe the orientation in space of a particular orbital. (It is called the magnetic quantum number because the effect of different orientations of orbitals was first observed in the presence of a magnetic field.) The magnetic quantum number (m) can be any integer between -l and +l. If l = 2, m can be either -2, -1, 0, +1, or +2. The three quantum numbers (n, l, and m) that describe an orbital. And an orbital contain only two electrons. The structure of the Atom To distinguish between the two electrons in an orbital, we need a fourth quantum number. This is called the spin quantum number (s) because electrons behave as if they were spinning in either a clockwise or counterclockwise fashion. One of the electrons in an orbital is arbitrarily assigned an s quantum number of +1/2, the other is assigned an s quantum number of -1/2. The spins of the electrons have to be opposed. Thus, it takes three quantum numbers (n, l, m) to define an orbital but four quantum numbers (n, l, m, s) to identify one of the electrons that can occupy the orbital. Mass and charge of subatomic particles Proton: The positively charge particles in the nucleus. Neutron: electrically neutral particles having a mass slightly greater than that of proton. Particles mass, g mass, u Charge Symbol Electron 9.1093897 × 10–28 5.48579903 × 10–4 1– –10e Proton 1.6726231 × 10–24 1.007276470 1+ 10H+, 10p Neutron 1.6749286 × 10–24 1.008664904 0 10n Mass relationship of Atoms Atomic number, Mass Number, and Isotopes Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutron AX A: mass number, Z: atomic number Z X: symbol of an atom of element - The atomic number (Z): is the number of protons in nucleus of each atom of an element. - In neutral atom the number of proton = the number of electron. - The mass number (A): is the total number of neutrons and protons present in the nucleus of an atom of an element. Mass relationship of Atoms Isotope: Atoms that have the same atomic number (same number of proton) but different mass number (different number of neutron). Ex: The isotope of the hydrogen: 1 H 2H 3H 1 1 1 hydrogen deuterium tritium Two common isotopes of uranium: 235 U 238 U 92 92 The chemical properties of an element are determined by the protons and electrons in its atoms; neutrons do not take part in in chemical changes under normal condition. Mass relationship of Atoms Hydrogen Isotope Atomic Mass - The mass of an atom is related to the number of electrons, protons, and neutrons in the atom. - The first information of atomic mass came from the work of Dalton, Gay-Lussac, Lavoisier, Avogadro, and Berzelius. - In the modern system of atomic mass (1961), is based on 12C (carbon-12) as the standard. - In this system 12C has six protons and six neutrons that is assigned a mass of exactly 12 atomic mass units (amu), and the mass of all other atoms are given relative to this standard. Atomic Mass - This carbon-12 atom serve as standard, mass of one carbon-12 atom so 1 atomic mass unit = 12 Ex: Experiment has shown that, on average, a hydrogen atom is only 8.4 percent as massive as the standard carbon-12 atom, then the atomic mass (that is, the mass of atom in atomic mass unit) of hydrogen must be 0,084 x 12 = 1.008 amu. Average atomic mass In the natural, the occurring elements have more than one isotope, so the atomic mass of element, we must generally settle for the average mass Ex: The natural abundances of carbon-12 and carbon-13 are 98.89 % and 1.11%, respectively. And the atomic mass of C-13 is 13.00335. The average atomic mass of carbon can be calculated as follows: average atomic mass of natural carbon= (0.9889 x12 amu) + (0.0111 x 13.00335 amu) = 12.0 Molar mass of an element and Avogadro’s Number • Mole: is the amount of substance that contains as many elementary entities (atoms, molecules, or other particles) (Unit defined by SI). 1 mole = 6.022045 x 1023 particles ( called Avogadro’s number. • Molar mass is the mass of one mole of a chemical element or chemical compound. Ex: 1 mol of H contains 6.022 x 1023 H atoms 1 mole of C-12 atoms has exact mass 12 grams (called molar mass) and contains 6.022 x 1023 atoms. Molecules: Atoms in combination A molecule is an aggregate of at least two atoms in definite arrangement held together by chemical forces. Ex: O2, O3, Cl2, F2, N2, CO, NH3... Molecular Formula simple molecule (diatomic molecule) Molecule: polyatomic molecule Molecules: Atoms in combination Molecule Hydrogen is diatomic molecule Molecules: Atoms in combination Molecules: Atoms in combination Molecular Mass The molecular mass is the sum of the atomic masses (in amu) in the molecule. Ex: the molecular mass of H2O is 2 atomic mass of H + atomic mass of O 2 (1.008 amu) + 16.00 amu = 18.02 amu The molar mass of a compound is the mass (in gram or in kilogram) of 1 mole of the compound. Notice: Molar mass of a compound (in gram) is numerically equal to its molecular mass. Ex: Molar mass of 1 mole of H2O is 18.02 g and 1 mole of H2O (weight 18.02 g) contains 6.022 x 1023 molecules Molecules: Atoms in combination EX: Calculate the molecular masses of the following compounds: - sulfur dioxide (SO2) - ascorbic acid or vitamin C (C6H8O6) Answer: Ex: Methane (CH4) is the principle component of natural gas. How many moles of CH4 are present in 6.07 g of CH4? Answer: Ex: How many Hydrogen atoms are present in 25.6 g of sucrose or table sugar (C12H22O11)? The molar mass of sucrose is 342.3 g. Answer:... Ions and Ionic compound The octet rule: atoms gain, lose or share valence shell electrons (valence electrons) in order to end up with eight valence electrons. (an octet “8 valence electrons” is a stable electron arrangement). An ion is an atom or group of bonded atoms which have lost or gained one or more electrons, making them negatively or positively charged. To reach an octet, metal lose electrons and nonmetal gain electrons. that bears a net positive charge is called a cation Ion whose net charge is negative is called an anion. Ions and Ionic compound An atom can lose or gain more than one electron. Ex: Na can lose one electron to become the cation (Na+) Na Na+ + eCl atom can gain an electron to become an anion (Cl-) Cl + eCl- Monoatomic ion Halogen group have seven valence electrons, and require just one more to reach octet. Oxygen and other group VIA (S, Se) nonmetal atoms always form monoatomic anions with a charge of 2- (each has six valence electrons when neutral) Nonmetal atoms gain electrons to reach an octet Ions and Ionic compound Electronegative of an element: is a tendency of the element to attire the electrons (or the ability of an atom in a molecule to attract shared electron to itself ). The elements the most electro negativity are: the elements have a tendency to captor a or the electrons to form the cation. Ex: Halogen, oxygen... Electropositive: the elements less electro negativity to form the cation is called electro positive. The metal is strongly electropositive, which mean it has a strong tendency to give up electrons. Periodic table Ions and Ionic compound In contrast, metal atoms lose electrons to form cation. Alkaline metal atoms form cation with a 1+ charge. Alkaline earth metal atoms (two valence electrons) and group IIIA metal atoms (three valence electrons) form cation with 2+, 3+ charges. Ex: Mg lose two valence electrons in n =3 energy to form Mg2+ Al lose three valence electrons in n =3 energy to form Al3+ O gain two valence electrons to form O2P gain 3 valence electrons to form P3- Ions and Ionic compound Polyatomic and molecular ions are often formed by the combination of elemental ions such as H+ with neutral molecules or by the loss of such elemental ions from neutral molecules. Hydrogen Carbonate HCO3− Ammonium ion NH4+, Hydronium ion H3O+ Ions and Ionic compound - Many ions are polyatomic ions, which mean they are built from two or more atoms. - When the electrons are removed or added to a group of atoms is called polyatomic ions. - The charge on polyatomic ions is due to an imbalance in the total number of protons and electrons that are present. Ex. NH4+ has 11 protons, only 10 electrons CO32- has 30 protons and 32 electrons. Ions and Ionic compound Polyatomic ion also form ionic compound. Ex: Food preservative Na2SO3 (sodium sulfite) is the combination of Na+ and SO32-. Table salt (sodium chloride) NaCl is an ionic compound contains Na+ and Cl- Concept map