Download Chapter 2: Elements, Compounds, and Chemical Reactions

Chapter 2:
Elements, Compounds, and Chemical Reactions
Elements and Atoms
Ellen Fraser
The Periodic Table
Elements and atoms can be explained by Dalton’s atomic
theory, which in turn also explains the chemical laws:
Law of Conservation of Mass: No detectable gain or
loss of mass occurs in chemical reactions. Mass is
conserved.
Law of Definite Proportions: In a given chemical
compound, the elements are always combined in the same
proportions by mass.
Laws of Multiple Proportions: Whenever 2 elements
form more than 1 compound, the different masses of 1
element that combine with the same mass of the other
element are in the same ratio of small whole numbers.
Atomic Structure:
Atoms are composed of 3 types of subatomic particles
called protons, neutrons and electrons. The dense nucleus
of an atom contains positively charged protons and
neutrally charged neutrons, while negatively charged
electrons surround the nucleus and fill the remaining
space in an atom.
As seen in the image above, the modern periodic table arranges
elements in rows (periods) in order of increasing atomic number. The
rows are arranged so that the columns (groups or families) have
similar chemical and physical properties.
Group 1 (with the exception on hydrogen) is classified as the alkali
metals, Group 2 is classified as the alkaline earth metals, Group 17 is
classified as the halogens, Group 18 is classified as the noble
gasses and many of the elements between Groups 3-12 are
classified as transition metals.
Metals, Nonmetals and Metalloids
Most elements are metals and they occupy the entire left hand side of the periodic table (with the exception of hydrogen). Nonmetals
can be found in the upper right hand region of the table, while metalloids occupy a narrow band between metals and non metals (also
known as the “ladder”). As for individual properties, metals exhibit metallic luster and tend to be ductile and malleable as well as good
conductors of electricity. Nonmetals, which are often gases, tend to be the opposite, being brittle, lacking in metallic luster and poor
conductors of electricity. Metalloids contain properties that are somewhere in between the properties of metals and nonmetals.
Reactions of Elements to form
Compounds
Almost every element has the ability to form a
compound, however not all combinations of
elements are possible. When a chemical reaction
occurs, the properties of the substances present in
the reactant begin to disappear and are replaced
with chemical properties of the new substances in
the product. Chemical formulas are used to show
what happened during a chemical reaction. Element
symbols are used as well as subscripts (little
numbers) for either free elements (diatomic
molecules), or chemical compounds. These
subscripts show how many atoms of each element
are present. Some compounds can also form solids
called hydrates which contain water molecules in
definite proportions.
Chemical Equations
A chemical equation is a scientific equation that shows a before-andafter description of what happened in a chemical reaction. Chemical
equations need to be balanced on both sides, and coefficients (big
numbers) allow you to balance an equation. Coefficients have the
ability to change the number of atoms among both the reactants and the
products. This allows the equation to support the law of conservation of
mass. The physical state of the reactants and products can be identified
by placing symbols beside the chemical formulas. The symbols are s
(solid), l (liquid), g (gas), and aq (aqueous solution or something that
can be dissolved in water).
Example of a Balanced Chemical Equation:
3Ba9OH02 + 2Na3PO4  Ba3(PO4)2 + 6NaOH
There are also different types of chemical reactions that can make
different chemical equations such as combination, decomposition,
single replacement, double replacement, and combustion.
Ions and Ionic Compounds
Binary ionic compounds are formed when metals react with nonmetals. In this type of reaction, electrons are transferred from the
metal to the nonmetal. The metal atom becomes a positive ion (a cation) while the nonmetal atom becomes a negative ion (an anion).
The formula of an ionic compound needs to be in the smallest whole number ratio that is possible. There are also many ionic
compounds that contain polyatomic ions, which are atoms that are composed of 2 or more atoms. When creating a compound out of
2 different elements (a metal and a nonmetal) you need to switch or transfer the charges.
Ex: Mg 2+ Cl 1- = MgCl2
Naming Ionic Compounds
When naming ionic compounds (nomenclature), you need to specify the cation first, then follow with the anion. If there
are 2 non metals in the compound, you can add prefixes such as mono, di, tri, penta to the beginning of the elements
name depending on how many atoms there are of it. If there is a metal within the compound, you cannot add any
prefixes. When naming, you also add “ide” onto the end of the name of the nonmetal, however you keep the metal
name the same, for example, magnesium sulfide. There are some more complex rules to naming compounds as well
(see below).
Nomenclature Summary
Formula Begins With….
Metal
Non-metal
Ionic Substance
Molecular Substance
Refer to
common ions
Use Prefixes:
1= mono
2= di
3= tri
4= tetra
5= penta
6= hexa
7= hepta
8= octa
9= nona
10= deca
Hydrogen
Acid
Binary
Hydro
ic Acid
Oxo-acids
ate-ic
ite-ous
acids