Chemistry Unit
Review Summary
Atoms and Elements
An atom is composed of 3 particles:
1. protons which have a positive charge;
2. neutrons which have no charge (they are
neutral); and
3. electrons which have a negative charge.
The modern atomic model differs from the
diagram shown above – it consists of protons
and neutrons in a nucleus surrounded by a cloud
of moving electrons. Valence electrons are
nearest to the outside of the cloud.
Elements are the simplest substances, they can not be broken down into other substances. Every atom
of an element has the same number of protons; however, the number of neutrons may differ. Atoms
of the same element with different mass numbers (different numbers of neutrons) are called isotopes.
Atoms are of neutral charge – they have an equal number of protons and electrons.
The Periodic Table - Originally developed by Mendeleev after organizing elements by atomic mass, then by
atomic number.
A period is a horizontal row of the table – properties vary as you move across a period.
A group or family is a vertical column of the table – properties are similar. Group 1 are the alkali
metals, the most reactive metals with 1 valence electron. Group 2 are the alkaline earth metals, the next
most reactive metal group, which each have 2 valence electrons. Groups 3 through 12 are the transition
metals. Groups 13 through 16 are mixed groups made up of metals (toward the bottom), metalloids along
the division line, and non-metals to the right. Group 13 is the Boron group with 3 valence electrons,
Group 14 is the Carbon group with 4 valence electrons. Group 15 is the Nitrogen group with 5 valence
electrons. Group 16 is the oxygen group with 6 valence electrons. Group 17 are the halogens which are
the most reactive non-metals with 7 valence electrons. Group 18 are the noble gases, a group of nonmetals with 8 valence electrons which are generally unreactive.
Physical properties of metals include shininess, malleability, ductility and conductivity. Non-metals tend
to be brittle, dull, and non-conductive.
An electron dot diagram shows the valence electrons around the element symbol. A Bohr diagram shows
the orbits of electrons around the atom. The first orbital holds a maximum of two electrons, the second
and third orbitals hold a maximum of eight electrons each. Orbitals of elements in groups 1, 2, 13, 14, 15,
16, 17, and 18 fill from the innermost orbital to the outer orbitals.
Matter – anything that has mass and takes up space.
Physical properties of matter are those properties that can be observed without changing the matter
into another substance. Examples of chemical properties include melting point, boiling point,
solubility, texture, flexibility, conductivity, hardness, and color.
Chemical properties of matter are those properties that describe the matters ability to change into
different substances. Examples are burning, rusting, tarnishing, baking, etc.
Chemistry Unit
Review Summary
Compounds are substances made up of two or more elements combined in a set ratio.
Mixtures are made up of substances that are not chemically combined. Mixtures may be
heterogeneous (not evenly mixed) or homogeneous (evenly mixed with undistinguishable parts).
Density is the mass of a material in a given volume. It is calculated by dividing the mass of a
substance by the volume of that substance. The density of water = 1 g/mL = 1 g/cm3. A substance
more dense than water will sink in water and a substance less dense than water will float on top of it.
Matter may undergo change. Matter may undergo physical changes where the substance remains
the same substance before and after the change (like phase change, dissolving, crushing, bending,
breaking, filtering). Matter may also undergo chemical changes where a different substance is
produced as a result of the change. Examples of chemical changes would be burning, electrolysis,
tarnishing and oxidation. Chemical and physical changes include a change in energy. Endothermic
change occurs when energy is taken in or absorbed. Exothermic change occurs when energy is
released.
States of Matter – there are three states of matter:
1. Solids which may be crystalline or amorphous. Solids have a definite shape and volume.
Molecules of solids move slightly in a vibrating form. Solids are low energy. Solids may melt and
become liquids or may sublimate and become gases.
2. Liquids have an indefinite shape and a definite volume. Liquids flow with particles moving over
each other. Liquids are higher energy than solids, but not as much as gases. Liquids may freeze
and become solids or vaporize to become gases. Liquids that heat at the surface only vaporize by
evaporation. Those that heat all the way through vaporize by boiling.
3. Gases have indefinite shape and volume. Gas particles move freely and have high energy. Gases
may turn to liquids by condensation.
Charles Law describes the relationship between the volume and the temperature of gases. It states that the
volume of a gas increases with the temperature of the gas (a direct relationship).
Boyles Law describes the relationship between the volume and the pressure of gases. It states that the
pressure of a gas will increase as the volume decreases (an inverse relationship).
Bonding and Chemical Reactions
Ions are formed when an atom gains or loses an electron. Ions have an electronic charge. Ions of
opposite charge attract forming ionic bonds and ionic compounds. Ionic compounds are neutral in
charge. When writing an ionic compound, the positive ion is always written first. Ionic compounds
are named by their metal component followed by their non-metal component using the ending –ide
(example: sodium chloride). Some groups of atoms function as groups – these are called polyatomic
ions.
Covalent bonds form when atoms share electrons. Covalent bonds typically form between two nonmetals and are named using prefixes that identify the number of atoms of each element involved in
the compound.
Chemical equations summarize reactions. They are written with the reactants on the left, the products
on the right with an arrow between the reactants and the products. Chemical equations must be
balanced (with equal numbers of each type of element on each side) to show that mass is conserved
(neither created nor destroyed) in the reaction. This concept is called the Principle of Conservation of
Mass.