An element is a fundamental substance that cannot be broken down by chemical means into a simpler substance.
Elements are the building blocks of matter.
Elements can occur naturally or be synthesized in labs. The smallest unit of an element that retains its
properties and chemical behavior is called an atom. Atoms are made up of subatomic particles,
but they do not have the properties of the element.
Elements and Atoms
Currently, 118 elements are known.
Ten elements make up almost 99 % of the mass of the Earth’s crust, seawater and atmosphere.
Oxygen accounts for about 20 % of the atmosphere and is found in nearly all rocks, sand and soil.
Elements
The most prevalent elements in the human body
are oxygen (65 %), carbon (18 %) and hydrogen (10 %). Some elements in the body are present in tiny amounts,
like chromium and copper, but are crucial for life.
Elements
Many element names are derived from Greek, Latin or German words that describe a property of the element.
Iodine comes from the Greek word iodes meaning “violetlike” and iodine is violet colored as a vapor.
Some elements are named for the location of their discovery, such as germanium which was
discovered in 1886 by a German chemist.
Other elements commemorate famous scientists, like curium which is named after Marie Curie.
Names of Elements
The first letter of a symbol must always be capitalized. If a second letter is needed, it should be lowercase.
Consider the element cobalt: Cobalt has the symbol Co
Writing CO would symbolize elements C (carbon) and O (oxygen) and would represent carbon monoxide.
Symbols of the Elements
The Periodic Table was designed by Dimitri Mendeleev in 1869 to organize elements based on their properties.
Elements with similar chemical properties are placed in columns called groups.
Four groups have special identifying names, like Noble Gases, which are all unreactive gases.
Introduction to the Periodic Table
The eight tall columns are called representative elements, or main group elements. These are shown in red.
The elements in the center are called the transition metals. These are shown in purple.
Introduction to the Periodic Table
Elements can be further classified as metals, metalloids and nonmetals.
Introduction to the Periodic Table
Metals are:
• solids at room temperature (except mercury) • shiny
• good conductors of heat and electricity • malleable (can be shaped)
• ductile (can be drawn into wires)
Most metals have a high melting point and density. Common metals include aluminum, gold, platinum,
silver, tin and iron.
Metals
Metals readily combine with nonmetals to form compounds but rarely combine with other metals.
Some metals are found in their free states, like copper, gold and silver.
Some metals are mixed to form homogeneous mixtures called alloys, like brass, bronze, steel and coinage metals.
Metals
Nonmetals:
• are not shiny
• have fairly low melting points and densities • are poor conductors of heat and electricity
Nonmetals combine with each other to form molecular compounds and with metals to form ionic compounds.
Common nonmetals found naturally include carbon, nitrogen, oxygen and sulfur.
Nonmetals
Metalloids have properties between metals and nonmetals. These elements are positioned diagonally on the
Periodic Table separating the metals and nonmetals.
Certain metalloids, like boron, silicon and germanium are the materials used in electronic devices.
Metalloids
Elements tend to be reactive and combine with other elements to form compounds.
It is rare to find elements in nature in their pure forms. Gold, silver and platinum (the noble metals) have low reactivity and are found uncombined in nature.
Air is composed mainly of nitrogen and oxygen, a homogeneous mixture of diatomic elements.
Natural States of the Elements
Seven elements exist as diatomic molecules. These molecules contain exactly two atoms.
Diatomic elements can be separated.
In water (H2O), neither hydrogen or oxygen is diatomic.
Diatomic Elements
A compound is a substance containing two or more elements that are chemically combined in a
definite proportion by mass.
Compounds, unlike elements, can be decomposed chemically into simpler substances.
Elements in compounds are always combined in whole number ratios.
Compounds and Formulas
Chemical Formulas
•
AxByCz
Elemental
Compounds fall into two general types, molecular and ionic.
• Molecular compounds are held together by covalent bonds (2 nonmetals)
• ionic compounds are held together by attractive forces between the positive and negative charges (a metal & a
nonmetal)
Molecular and Ionic Compounds
Molecules are the smallest unchanged individual unit of a compound formed by two or more atoms.
A molecule cannot be further divided without destroying its identity.
Ions are charged atoms or groups of atoms. Ions can be positively or negatively charged.
A positively charged ion is called a cation. A negatively charged ion is called an anion.
Molecules and Ions
More than 50 million compounds are currently known. Each compound has a unique set of properties.
Consider water (H2O) and hydrogen peroxide (H2O2).
H2O H2O2
color clear pale blue
boiling point 100 °C 150 °C
density 1.00 g/mL 1.44 g/mL
Property
Compounds
Chemical formulas are abbreviations for compounds. A chemical formula contains the symbols and ratio of
the atoms of the elements in a compound. Consider sulfuric acid.
1 molecule contains 2 H atoms, 1 S atom and 4 O atoms. One possible formula for sulfuric acid is HHSOOOO.
A better formula is H2SO4 – “H-two-S-O-four”.
The numbers after the element symbols are subscripts.
Writing Formulas of Compounds
1. The formula of a compound contains the symbols of all the elements in the compound.
2. When a formula contains only one atom of an element, the number 1 is not needed as a subscript.
3. When a formula contains more than one atom of an element, the number is indicated by a subscript
written after the symbol.
Writing Formulas of Compounds
4. When the formula contains more than one of a group of atoms that occur as a unit, parentheses are placed around the group and the number of units is
represented by a subscript outside the parentheses.
5. Formulas show the number and kind of atoms in a compound, but not the connectivity of the elements.
Writing Formulas of Compounds
How many of each element is present in K3PO4? 3 K atoms
How many of each element is present in Mg(OH)2?
