Exambusters Chemistry Study Cards 3

Law of Definite Composition
A compound always contains the same elements combined in the same proportions by mass. e.g. H2O is 88% oxygen no matter where it is found.
Mass number
The number of protons plus neutrons in the nucleus of an atom.
Atoms which contain the same number of protons but different numbers of neutrons. e.g. hydrogen has three isotopes with mass numbers of 1, 2, 3.
A group of two or more atoms held together by chemical covalent bonds
An atom or group of atoms having a positive or negative electrical charge. e.g. Na+, Cl, SO42-
Law of Conservation of Mass
During a physical change or reaction, or a chemical reaction, matter is neither created nor destroyed.
Valence electrons
The electrons found in the outermost energy level of an atom
Oxidation Number
A number, positive or negative, representing the charge on an ion, or the distribution of electrons in an atoms involved in a chemical bond.
Atomic Mass Unit
The mass of a proton or neutron is equal to one atomic mass unit. symbol is amu 1 amu = 1.66 X 10-24 g
Three postulates of Dalton’s Atomic Theory

1. An element is composed of identical atoms.

2. Atoms of different elements have different properties. 3. Compounds are atoms of two or more elements chemically combined.

positively charged ion: Na+, Fe2+, NH4+, Ag+
negatively charged ion: Cl, OH, P3-
Proton: Location in atom
atomic nucleus
Proton: Mass in amu
1 amu
Proton: Charge
Proton: Symbol
Neutron: Location in atom
atomic nucleus
Neutron: Mass in amu
1 amu
Neutron: Charge
Neutron: Symbol
Electron: Location in atom
electron cloud surrounding the nucleus
Electron: Mass in amu
1/1837 amu (sometimes expressed as zero)
Electron: Charge
Electron: Symbol
Atomic number
Number of protons in the nucleus of an atom of an element.
Atomic number: helium
Atomic number: hydrogen
Atomic number: carbon
Atomic number: oxygen
Describe the research of: Rutherford
Nucleus. Gold foil experiment. Found positively charged atomic nucleus. 1911
Describe the research of: Millikan
Electron. Oil drop experiment. Calculated the electric charge and the mass of an electron. 1909
Describe the research of: J.J. Thomson
Electron. Cathode ray tube experiment. Identified electron paths in cathode ray tubes. 1897
Law of Multiple Proportions
The same elements may combine to form more than one compound. The ratios of atoms are in small whole numbers. Proposed by Dalton. e.g. H2O and H2O2
Which elements exhibit several oxidation states, and why?
The transition elements. Their highest energy sub-levels are close in energy (s and d) and easily become involved in bonding.

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