1.2.S: Atoms, Molecules, and Ions (Study Guide)
This chapter introduced some of the fundamental concepts of chemistry, with particular attention to the basic properties of atoms and elements. These entities are the building blocks of all substances we encounter, yet most common substances do not consist of only pure elements or individual atoms. Instead, nearly all substances are chemical compounds or mixtures of chemical compounds. Although there are only about 115 elements (of which about 86 occur naturally), millions of chemical compounds are known, with a tremendous range of physical and chemical properties. Consequently, a major emphasis of modern chemistry focuses on understanding the relationship between the structures and properties of chemical compounds.
1.2.2: Early Ideas in Atomic Theory
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Dalton’s atomic theory of matter:
- Each element is composed of extremely small particles called atoms
- All atoms of a given element are identical; the atoms of different elements are different and have different properties
- Atoms of an element are not changed into different types of atoms by chemical reactions
- Compounds are formed when atoms of more than one element combine
- explains the law of constant composition, law of conservation of mass
1.2.3: Evolution of Atomic Theory
- subatomic particles – what atoms are composed of
- Like charges repel each other; unlike charges attract
Cathode Rays and Electrons
- cathode rays – radiation resulting from a high voltage
- cause certain materials to give off light (fluoresce)
- mass of an electron 9.10939*10 - 28 g
- 2000 times smaller than hydrogen
1.2.2 Radioactivity
- radioactivity – spontaneous emission of radiation
- three types of radiation: alpha ( a ), beta ( b ), gamma ( g )
- alpha and beta radiation are affected by an electric field
- beta particles have a charge of 1-
- alpha particles have a charge of 2+
- gamma radiation has no particles and no charge
1.2.3 The Nuclear Atom
- Rutherford determined that there was a nucleus in every atom
- Protons discovered by Rutherford in 1919
- Neutrons discovered by James Chadwich in 1932
1.2.4: Atomic Structure and Symbolism
- charge of an electron is –1.602*10 -19
- charge of a proton is +1.602*10 -19
- 1.602*10 -19 is called to electronic charge
| Particle | Charge | Mass (amu) |
|---|---|---|
| Proton | Positive | 1.0073 |
| Neutron | None | 1.0087 |
| Electron | Negative | 5.486*10 -4 |
- atomic mass unit (amu) – equals 1.66054*10 -24 grams
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angstrom (Å) – unit of length to measure atomic dimensions
- 1 angstrom = 10 - 10 m
- atoms have diameters of 1-5 Å
- nucleus – diameter of 10 -4 Å
- all atoms of an element have the same number of protons in the nucleus
- isotopes - atoms of the same element that have a different number of neutrons
- atomic number – the number of protons in an atom
- mass number – number of protons + number of neutrons
- nuclide – atom of a specific isotope
1.2.5: Chemical Formulas
- empirical formula - formula showing the composition of a compound given as the simplest whole-number ratio of atoms
- molecular formula - formula indicating the composition of a molecule of a compound and giving the actual number of atoms of each element in a molecule of the compound
- isomers - compounds with the same chemical formula but different structures
- spatial isomers - compounds in which the relative orientations of the atoms in space differ
- structural isomer - one of two substances that have the same molecular formula but different physical and chemical properties because their atoms are bonded differently
- structural formula - shows the atoms in a molecule and how they are connected
1.2.6: The Periodic Table
- periodic table – the arrangement of all the elements by atomic number and similarities into a table
- columns = groups
- metallic elements – all elements on the left side and in the middle of the periodic table
- nonmetallic elements – elements on the periodic table that are divided by a diagonal steplike line from boron to astatine
- metalloids – properties of metals and nonmetals
1.2.7: Molecular and Ionic Compounds
- molecule – two or more atoms bonded together
- ion – charged particle formed by the removal or addition of an electron
- cation – ion with a positive charge
- anion – ion with a negative charge
- metal atoms tend to lose electrons, nonmetal atoms tend to gain electrons
- monatomic ion - ions formed from one atom
- polyatomic ions – joined atoms that have a net positive or negative charge
- Oxyanions - polyatomic ions that contain one or more oxygen atoms
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Predicting Ionic Charges
- alkali metals (group 1) form 1+ ions
- alkaline earth metals (group 2) form 2+ ions
- halogens form 1- ions
- group 6A from 2- ions
- ionic compound – a compound that contains positively and negatively charged ions
- ionic bond - electrostatic forces of attraction between the oppositely charged ions of an ionic compound
- ionic compounds are generally combinations of metals and nonmetals, molecular compounds are generally nonmetals only
- chemical formula – way of representing molecule
- molecular compound or covalent compound – contains more than one type of atom, pairs of electrons are shared between bonded atoms
- covalent bond - attractive force between the nuclei of a molecule’s atoms and pairs of electrons between the atoms
1.2.8: Chemical Nomenclature
- chemical nomenclature – the naming of substances following a system of rules
- organic compounds – contain carbon as the principle element
- inorganic compounds – composed principally of elements other than carbon
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positive ion
- cations formed from atoms have the same name as the metal
- if a metal can form cations of differing charges, the positive charge is given by a roman numeral in parentheses following the name of the metal
- cations formed from nonmetal atoms have names that end in –ium
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negative ion
- monatomic anions have names formed by dropping the ending of the name of the element and adding the ending –ide
- polyatomic anions containing oxygen have names ending in –ate or –ite alled oxyanions
- anions derived by adding H + to an oxyanion are named by adding as a prefix the word hydrogen or dihydrogen, as appropriate
- names of ionic compounds are the cation name followed by the anion name
- names of ionic compounds containing a metal with a variable charge state the charge on the metal with a Roman numeral in parentheses after the name of the metal
- acids based on anions whose names end in –ide have associated acids that have the hydro- prefix and an –ic ending
- acids based on anions whose names end in –ate or -ite
| Anion | Acid |
|---|---|
| ____ide | Hydro____ic acid |
| ____ate | _____ic acid |
| ____ite | _____ous acid |
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Names and Formulas of Binary Molecular Compounds
- the name of the element farthest to the left in the periodic table is usually written first
- if elements in same group lower one written first
- name of second element is given an –ide ending
- greek prefixes used to indicate number of atoms of each element
- if prefix ends in a or o and the name of the anion begins with a vowel, the a or o is dropped
| Prefix | Meaning |
|---|---|
| Mono- | 1 |
| Di- | 2 |
| Tri- | 3 |
| Tetra- | 4 |
| Penta- | 5 |
| Hexa- | 6 |
| Hepta- | 7 |
| Octa- | 8 |
| Nona- | 9 |
| Deca- | 10 |