Unit 5 Essential Knowledge
Atomic radii increase going down a group (column). Going down a group on the Periodic Table, each element has one
more principal energy level filled with electrons than the element above it, so the outer electrons are farther away from the
nucleus. The inner electrons from the filled energy levels act as a shield to decrease the nuclear pull on the outer electrons.
This means the size of the atoms increases going down a group.
Atomic radii decrease going from left to right across a period (row). Going from left to right across a period of the
Periodic Table the valence electrons are all in the same principal energy level, but the number of protons in the nucleus increases
from one element to the next. This means that the nucleus becomes more positively charged and attracts the electrons
more strongly. Therefore, the size of the atom decreases across a row.
Electronegativity is the ability of an atom in a covalent bond to attract electrons. The electronegativity of an element can be
judged from its position on the periodic table.
The electronegativity increases across a period of the periodic table. (The atomic radius decreases, which means that the
valence electrons are closer to the nucleus and held more tightly by the nucleus.)
The electronegativity decreases down a group of the periodic table. (The valence electrons are further away from and more
loosely held by the nucleus. It is more difficult for the nucleus to attract electrons.)
Ionization energy is the energy needed to remove the most loosely held valence electron from a neutral atom.
Ionization energy increases going from left to right across a period of the periodic table because the atomic radius
decreases, which means that the valence electrons are closer to the nucleus and held more tightly by the nucleus.
Ionization energy decreases going down a group because the valence electrons are further away from the nucleus (in a
higher energy level) and are more loosely held by the nucleus.
Shielding effect is the tendency for the electrons in the inner energy levels to block the attraction of the nucleus for the
valence electrons.
Shielding effect is constant within a given period, because all elements on a period have the same number of electrons on
the inner (lower) energy levels protecting the valence electrons from the pull of the nucleus.
Shielding effect increases going down a group because there are more inner energy levels which means there are more
inner electrons blocking the attraction of the nucleus for the valence electrons.
Electron dot diagrams for an element show the number of valence electrons for that element.
The Lewis dot diagram for a covalent compound shows how the atoms in a molecule share electrons to gain a filled valence
level. A filled valence level is called an octet. Lewis dot diagrams for most molecules generally follow the octet rule.
Hydrogen is an exception, because it needs only two electrons to be stable. Each shared pair of electrons in a Lewis diagram
represents a single covalent bond.
A covalent molecule can also be represented by a structural formula in which each covalent bond is shown as a line joining
two atoms. In other words, a line in a structural formula represents two electrons (a pair) shared by two elements. structural
formula shows the arrangement of atoms and bonds in a molecule.
The other type of compound (other than ionic) is covalent. A covalent compound is formed by two NONMETALS. You name
them using prefixes. (mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octo-, nano-, and deca- are 1-10 respectively)
A covalent bond consists of electrons shared between atoms. This sharing is not always equal, because different atoms have
different electronegativities. Unequal sharing results in a polar bond.
The more electronegative atom in a covalent bond will attract the electrons more strongly and this will result in it having a slight
negative charge. The less electronegative atom will therefore be slightly deficient in electrons and so will have a slight
positive charge.
A covalent bond in which the atoms do not share their electrons equally and have slight electrical charges is known as a
polar covalent bond. (example: HF)
A covalent bond in which the atoms share their electrons equally and do not have slight electrical charges is known as a
nonpolar covalent bond (example: F2). Only two of the same atom can share electrons equally.
An ionic bond not only exists as binary compounds (ONE metal and ONE non-metal creating a TWO atom compound) but
also contains polyatomic ions. Polyatomic ions are tightly bonded groups of atoms that behave as a unit and carry a charge.
Know the name, formula and charge for the following polyatomic ions: ammonium (NH4)+, hydroxide (OH)-, nitrate
(NO3)-, carbonate (CO3)2-, sulfate (SO4)2- and phosphate (PO4)3-.
In chemical reactions bonds are broken and new bonds are formed. The energy absorbed in breaking the bonds is never
exactly equal to the energy released when the new bonds are formed. Therefore, all reactions are accompanied by a
change in potential energy that can be measured and is represented by the symbol  H.
A potential energy diagram represents the energy change during a chemical reaction.
For Exothermic reactions (H), the energy required to break the bonds of the reactants is less than the energy
released in making the bonds of the products. The products are more stable because they have less potential energy than
the reactants.
For Endothermic reactions (+H), the energy required to break the bonds of the reactants is greater than the energy
released in making the bonds of the products. The reactants are more stable because they have less potential energy than
the products.
The coefficients in a balanced equation indicate the mole ratios between each substance in the equation. These ratios can be
used to predict the amount of product that can be formed from a given amount of reactant.
The theoretical yield (maximum amount of product that can be produced) can be predicted from the mass of a reactant. Molar
masses from the periodic table and mole ratios from the balanced equation can be used to calculate the mass of a reactant or
product.
An empirical formula shows the smallest whole number ratio of elements in a compound. The molecular formula shows the
actual ratio of the elements in a compound. The The molecular formula is usually a whole number multiple of the
empirical formula. The molecular mass is therefore the same whole number multiple of the empirical mass.
Ionic solids are composed of oppositely charged ions arranged in a regular, repeating, crystal lattice structure; the empirical
formula always gives the ratio of positive to negative ions.
Covalent compounds are often in the form of individual molecules; the empirical formula gives the ratio of atoms in one
molecule. Example: The molecular formula for glucose is C6H12O6; the empirical formula is CH2O.
A reactant that is used up first in a reaction is called a limiting reactant; it determines the maximum amount of product
that can be formed.
In a single replacement reaction one element takes the place of another in a compound. The general form for a single
replacement reaction is
A + BX  AX + B
Where A and B are elements and BX and AX are compounds. Example: Cu + FeCl2  CuCl2 + Fe
In a double replacement reaction the positive portions of two compounds are interchanged. The general form for a double
replacement reaction is
AX + CY  AY + CX
For two ionic compounds, the cations A and C swap partners. Example: BaCl2 + Na2O  BaO + 2NaCl
Combustion reactions are exothermic reactions in which oxygen combines with the elements of an organic compound
producing carbon dioxide and water. One example is the reaction between methane and oxygen in a Bunsen burner:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + energy
Neutralization reactions result from the reaction of an acid with a base to form a salt and water. These reactions are
usually double replacement reactions. Salts are ionic compounds.
Ex: HCl + NaOH  NaCl + HOH and
HNO3 + KOH  KNO3 + HOH
The concentration of a solution is the amount of solute contained in a certain volume of solution. If a solution contains a small
amount of solute it is called dilute, and if it contains a large amount of solute it is called concentrated.
In chemistry, concentration is given as molarity, (M), the number of moles (mol) of the solute in one liter (L) of
solution and expressed as moles/liter or just M.
Adding a solute to water increases its boiling point and decreases its freezing point. The degree of change in the boiling and
freezing points is directly related to the amount of solute added to water.
The general rule for predicting solubility is “like dissolves like”. Water is a polar substance, so it will dissolve polar
solutes. For example, ammonia (NH3) is covalent and dissolves in water, therefore ammonia must be a polar molecule. Water
will also dissolve ionic solutes because the positive end of a water molecule attracts the anion and the negative end of water
attracts the cation.
Oil is non-polar, so oil will not dissolve in water. Oil and water do not mix but different oils will mix with each other because
all oils are nonpolar. Nonpolar solutes will dissolve in a nonpolar solvent. For example, carbon tetrachloride (CCl4) dissolves in oil,
therefore carbon tetrachloride must be a nonpolar molecule.
Diatomic molecules (Cl2, F2, O2…) do not dissolve in water so they must dissolve in oil and are nonpolar molecules.
Temperature affects solubility. Solubility graphs relate solubility to temperature. For most solid solutes, solubility
increases as temperature increases. For most gas solutes, solubility decreases as temperature increases.
Given a graph of experimental data, interpret the relationship (inverse, direct or constant) between the IV and DV variables.
Mixtures can be separated based on the physical properties of the components of the mixture.
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Centrifugation (using a centrifuge) separates mixtures of undissolved solids from liquids by density.
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Distillation separates mixtures of liquids by boiling point
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Filtration separates mixtures of undissolved solids from liquids by particle size.
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Chromatography separates mixtures based on attractive forces between particles.