Elements, Compounds, and the Periodic Table
Classified in Chemistry
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Pure Substances and Mixtures
Pure substances: A pure substance has a constant composition that does not vary when subjected to physical changes.
Mixtures: A mixture can be separated by physical changes, such as filtering or boiling.
Elements and Compounds
Element: An element is a pure substance consisting of a single type of atom.
Compound: A compound is a pure substance consisting of two or more types of atoms linked together by chemical bonds.
Composition of the Universe, Earth, and Humans
- Universe: Hydrogen (73.9%), Helium (24%), Other (2%)
- Earth: Oxygen (29.7%), Iron (31.9%), Silicon (16.1%), Magnesium (15.4%), Other (6.4%)
- Human Being: Oxygen (65%), Carbon (18%), Hydrogen (10%), Nitrogen (3%), Other (3.4%)
The Periodic Table
Periodic Table: The periodic table is a classification system that arranges chemical elements according to their atomic number and their physical and chemical properties. It was proposed by Dmitri Mendeleev in 1869.
- Groups: 18 vertical columns
- Periods: 7 horizontal rows
Physical and Chemical Properties
.Whas proppoused by: Dmitri Mendeleev in 1869.
18 verical colums, called grups.
7 horizontal rows, called periods.
An element's physical properties are properties that can be measuret quantitatively.
The metallic character of an element is determinated by its physical properties and the ease which it gives off electrons.
A chemical formula: is an equation that expresses the composition of a pure substance.
molecular substances: atoms in these substances are linked together forming molecules, structures consisting of a fixed number of atoms.
Crystals: atoms in these substances ara linked together forming crystal structures, group with an identifie number of atoms in fixed proportions, which are repeated regulary in three dimensions on a a lattice.
Under ordinary conditions, the atoms of cretain elements-noble gases-donot form bons: monatomic substances.
A molecule: is a stable group of a fixed number of atoms linked together by covalennt bonds.
Intermolecular forces: are less intense and less stable than those of chemical bonds.
Nonmetals: except for de group of noble gases-are the elements most prone to forming molecules.
The chemical properties: of molecular substances are widely diverse and depend on their specific composition.
Their physical properties: are more regular, such as for example their state of aggretion and electrical conductivity.
Crystals: are compounds and elements whose atoms are arrenged into lattice structures.
Crystal lattice structure: is a group of atoms, ions ore molecules arranged in space forming a regulary repeating patterns.
Emperical formula: CaF