Atoms molecules and ions Daltons Atomic Theory 1808

Atoms, molecules and ions

Dalton’s Atomic Theory (1808) • The elements are formed of extremely small particles, called atoms. All the atoms of a given element are identical. The atoms of an elements are different than those of all other elements. • Compounds are formed of more than one element. In all compounds, the ratio between the number of atoms of two elements is either an integer or a simple fraction.

Dalton’s Atomic Theory (1808) • A chemical reaction is the separation, combination, or rearrangement of atoms; it causes neither the destruction nor the creation of atoms. • N. B. Dalton did not know the structure of the atom (i. e. , electron, proton, neutron, nucleus, etc. ) - Dalton imagined an atom was small and indivisible

Dalton’s Atomic Theory (1808) • Dalton’s work explained some empirical observations – The law of definite proportions – The law of multiple proportions – The law of conservation of mass

The Structure of the Atom: the Electron • With a cathode ray tube, the existence of a subatomic particle with a negative charge was discovered by Thomson…the electron. • Thomson established the charge/mass ratio of the electron.

The Structure of the Atom: the Electron • Millikan established the charge of an electron in the early 1900’s (-1. 60 x 10 -19 C) • With the help of Thomson’s work, Millikan deduced the mass of an electron

Radioactivity • Radioactivity: the spontaneous emission of particles and/or radiation • Three types of particle/rays are produced by the decay of radioactive substances : – α particles (helium nuclei) – β particles (electrons) – γ rays (rays of high energy, and no charge)

The Structure of the Atom: the Proton and the Nucleus • Before 1910, Thompson’s “plum pudding” model of the atom was the accepted model. • In this model, the electrons are dispersed within a much bigger sphere of uniform and positive matter. – This is “logical" considering the small mass of the electron compared to the overall mass of the atom

The Structure of the Atom: the Proton and the Nucleus • Rutherford bombarded thin sheets of gold with α particles • According to the “plum pudding” model, α particles should pass through without their trajectories being greatly affected • However, the trajectories of some particles are greatly perturbed

The Structure of the Atom: the Proton and the Nucleus • In Rutherford’s experiment, the deviations are the result of large repulsions • Because α particles are positively charged, the positive charge in the atom must be concentrated in a very small and solid nucleus in the center of the atom (about 10 -13 of the volume of the atom, but almost 100% of the mass) • The positive charges that are located in the nucleus are the protons • The charge of a proton is the same as the electron, but with the opposite sign (i. e. , positive) • The mass of the proton is 1. 67252 x 10 -24 g (about 1840 times that of the electron)

The Structure of the Atom: the Neutron • Before 1932, a great mystery was the fact that H had one proton and He had two, but He was four times more massive than H! • Many people predicted the existence of the neutron, but it was Chadwick that discovered it in 1932 • The neutron has no charge and is slightly larger than the proton (1. 67493 x 10 -24 g for the neutron vs 1. 67252 x 10 -24 g for the proton)

Atomic number, Atomic Mass, and Isotopes • Atomic number: The number of protons in the nucleus of each atom of a given element • Atomic mass: The total number of neutrons and protons contained in the nucleus of an atom • All atoms of an element have the same number of protons, but not necessarily the same number of neutrons • Atoms that have the same atomic number but different atomic masses are called isotopes

Atomic number, Atomic Mass, and Isotopes • To identify an isotope, we use the notation , where A is the atomic mass, Z is the atomic number and X is the element in question • Isotopes of an element have essentially the same chemistry since the number of neutrons has little influence on the chemistry of an element • We use the atomic mass to identify isotopes (e. g. . ; is uranium 235 and is uranium 238) • Hydrogen has three isotopes (only hydrogen has special names for its isotopes) hydrogen: deuterium: tritium:

The Periodic Table • Table in which elements are grouped based on similar physical and chemical properties. • The horizontal rows are called periods • The vertical columns are called groups (or families) • Three categories of elements: – metal (good conductor) – non-metal (poor conductor) – metalloid (between metal and non-metal)

Molecules and Ions • A molecule is an assembly of at least two atoms held together in a fixed arrangement, by chemical forces • An ion is an atom or molecule that has gained or lost electrons – Cation: ion with a positive charge – Anion: ion with a negative charge • An ionic compound is a composed of cations and anions – e. g. . ; Na. OH(s) is a group of Na+ and OH- ions and there are no distinct molecules of Na. OH

Chemical Formulas • Molecular formula: indicates the exact number of atoms of each element contained in the smallest unit of a substance • Empirical Formula: indicates the lowest whole number ratio of atoms present in a substance • e. g. ; hydrogen peroxide is a molecule that contains two atoms of oxygen and two atoms of hydrogen. • The molecular formula is H 2 O 2 • The empirical formula is HO • For many molecules, the molecular and empirical formulas are identical (e. g. ; H 2 O)

Chemical Formulas • For an ionic compound, there are no distinct molecules, so we use their empirical formula to identify them. • In the figure, each Na+ is associated with six Cl-, each Cl- is associated with six Na+, and the whole solid is electrically neutral. • Other ionic compounds may have different structures and different charges (as long as the crystal remains electrically neutral) – ex. ; Al 3+ and O 2 - forms Al 2 O 3

Nomenclature of Inorganic Compounds: Ionic Compounds • For ionic compounds, we name the cation followed by the anion • The anion or the cation is sometimes a polyatomic ion – ex. ; NH 4+ : ammonium CO 32 - : carbonate OH- : hydroxide PO 43 - : phosphate NO 3 - : nitrate SO 42 - : sulfate NO 2 - : nitrite SO 32 -: sulfite • ex. ; KBr : potassium bromide Zn. I 2 : zinc iodide Al 2 O 3 : aluminium oxide Na. OH : sodium hydroxide KCN : potassium cyanide NH 4 Cl : ammonium chloride

Nomenclature of Inorganic Compounds: Covalent Compounds • Covalent compounds consist of distinct molecules • For a binary covalent compound, we name the first element in the formula first – ex. ; HCl : hydrogen chloride NO : nitric oxide Si. C : silicon carbide

Nomenclature of Inorganic Compounds: Covalent Compounds • When two elements form several compounds, Greek prefixes are used to specify the number of atoms of each element – e. g. ; CO : carbon monoxide CO 2 : carbon dioxide NO 2 : nitrogen dioxide N 2 O 4 : dinitrogen tetroxide • Many covalent compounds containing hydrogen are referred to by their common name – e. g. ; B 2 H 6 : diborane CH 4 : methane NH 3 : ammonia H 2 O : water Si. H 4 : silane PH 3 : phosphine

Nomenclature of Inorganic Compounds: Acids and Bases • A definition of an acid is: a substance that releases an H+ when dissolved in water. • If the acid is not an oxyacid, i. e. , does not contain oxygen, we use the prefix “hydro” and the suffix “ic” – ex. ; HF : hydrofluoric acid HI : hydroiodic acid H 2 S : hydrosulfuric acid • N. B. HCl(g) is hydrogen chloride, but once it is dissolved in water, it become hydrochloric acid and releases H+

Nomenclature of Inorganic Compounds: Acids and Bases • For an oxyacid (general formula Hm. XOn) it often happens that there are multiple possible values of n for each element X, and as such, within this series of compounds, – There is always an acid in the series that ends with “ic” • Adding another oxygen to the “ic” acid produces the “per…. ic” acid • Removing an oxygen atom to the “ic” acid produces the “ous” acid • Removing two oxygen atoms to the “ic” acid produces the “hypo…ous” acid

Nomenclature of Inorganic Compounds: Acids and Bases • ex. ; H 2 CO 3 : carbonic acid HNO 3 : nitric acid HNO 2 : nitrous acid H 2 SO 4 : sulfuric acid H 2 SO 3 : sulfurous acid HCl. O 4 HCl. O 3 HCl. O 2 HOCl : : perchloric acid chlorous acid hypochlorous acid

Nomenclature of Inorganic Compounds: Acids and Bases • When an oxyacid loses its H+ to produce an oxyanion – The “ic” acids produce the “ate” anions e. g. ; Cl. O 4 - : perchlorate Cl. O 3 - : chlorate – The “ous” acids produce the “ite” anions e. g. ; Cl. O 2 - : chlorite OCl- : hypochlorite • When not all of the possible H+ ions are lost, we have to specify the number of H’s present e. g. ; H 3 PO 4 : phosphoric acid H 2 PO 4 - : dihydrogenphosphate HPO 42 - : hydrogenphosphate PO 43 - : phosphate