Chapter 4 Section 2 Notes
Combination of Atoms
- compound – a substance made up of atoms of two or more
different elements chemically combined
- properties of a compound differ from those of the
individual elements (NaCl)
Molecules
- molecule – a group of atoms that are held together by
chemical forces; a molecule is the smallest unit of matter
that can exist by itself and retain all of a substance’s
chemical properties
- diatomic molecules – made up of only two atoms of the
same element
o H2, O2, F2, I2, Br2, Cl2, N2
Chemical Formulas
- elements occur in the same relative proportions
- chemical formula – a combination of letters and number
that show types and amounts of elements
- water
o H2O
o Subscript
 small number in front of an element
 related to the preceding element
 identifies how many of that element
o two hydrogen atoms
o one oxygen atom
Chemical Equations
- a formula that describes a reaction
o H2 + O2 → H2O
Equation Structure
- reactants
o elements/molecules on the left side of the equation
o elements/molecules that react chemically
- product
o elements/molecules on the right side of the equation
o elements/molecules that are produced by a chemical
reaction
- arrow (→)
o separates the reactants from the products
o means “gives”, “yields” or “produces”
Balanced Equations
- equations must be balanced
- equations are balanced when number and kinds of elements
on the reactant side equal the number and kinds of
elements on the product side
- balancing
o cannot change chemical formulas
o use coefficients
 number placed in from of an element/molecule
 coefficient distributes to entire molecule
Chemical Bond
- forces that hold the atom in a molecule together
- form because the attraction between the positive and
negative charges
- form by the sharing or transferring of electrons
Ions
- an atom or group of atoms that has a positive or negative
charge caused by the losing or sharing of electrons
- cation
o positive ion
o loses electrons
o metals form cations
o Na+1
- anion
o negative ion
o gains electrons
o nonmetals form anions
o Cl-1
Ionic Bonds
- the attractive force between oppositely charged ions, which
form when electrons are transferred
- electrons are transferred from cations to anions
- sodium chloride (NaCl), common table salt
- occurs between a metal and a nonmetal
Covalent Bond
- a bond formed when atoms share one or more pairs of
electrons
- positive nucleus is attracted to shared negative electrons
- covalent compound – compound form the sharing of
electrons
Polar Covalent Bonds
- a covalent bond that is formed by the unequal sharing of
electrons
- one atom will have a stronger attraction for electrons than
another atom
- water (H2O)
o oxygen attracts electrons stronger than hydrogen
o oxygen end is more negative
o hydrogen end is more positive
Mixtures
- a combination of two or more substances not chemically
combined
- substances in a mixture keep their individual properties
- can be separated by physical means
Heterogeneous Mixture
- substances are not uniformly distributed
- granite is an example
Homogeneous Mixture
- mixtures where individual components are uniformly
distributed
- known as solutions
- any sample of a solution will be the same as any other
sample (i.e. sea water)
- gases and solids can be solutions
- alloy – a solution composed of two or more solids
o brass
o 10K gold
o steel