Periodic Table Trends: CHM 101

Periodic Table and Trends

• FSC112/CHM 101 covers trends in atomic radii, ionization energies, and electronegativity based on elements' positions in the periodic table.
• Chemical equations, chemical reactions, stoichiometry, calculations based on chemical equations, and the limiting reactant concept are also covered.
• The periodic table arranges elements in a tabular format by increasing atomic number which represents the number of protons.
• Periodicity notes the recurrence of similar properties at regular intervals with increasing atomic number.

A Brief History

• Some books refer to main group and transition elements as A and B subgroups, respectively.
• The s-block and p-block are numbered as groups I to VII and zero (0).
• The d-block and f-block also exist.
• The work of chemists in the 19th century, including Newland, Lother, and Mendelev, led to the periodic table.

Blocks in the Periodic Table

• The periodic table has an s-block, p-block, d-block, and f-block.
• Lanthanide and actinide series exist in the f-block.

Trends and Properties

• Elements are arranged in order of their atomic number.
• Elements in the same group (column) have similar properties.
• Elements show changing trends across a period (row).
• Elements in the same group have the same number of electrons in their valence shell.
• Elements arranged in groups (columns) have related chemical and physical properties.
• Elements arranged in periods (rows) have progressively different physical and chemical properties.

Ground State Electron Configurations

• The ground state electron configurations for the outermost electrons differ across the periodic table as shown in the blocks.

Periodicity

• Periodicity refers to the recurring trends observed in the properties of elements.
• Periodicity results from regular and predictable variations in element atomic structure.
• Periodic properties rely on electronic configuration, repeating at certain intervals or down a group and showing gradual changes.
• Periodic properties include atomic radius, ionic radius, ionization energy, electron affinity, and electronegativity.

Atomic Radii

• Atomic radius is half the distance between two nuclei in a homonuclear diatomic molecule.
• Atomic radius decreases across a period because of the increase in positive charge from protons.
• Each added electron experiences a greater positive charge due to the protons pulling in the same direction.

Trends in Atomic Radii

• Trends in the electron radii are influenced by three factors: energy level of the valence electron, nuclear charge, and shielding effect.
• The higher the energy level, the further the distance from the nucleus.
• Distance from the nucleus follows an order of energy level of 1<2<3<4<5<6<7.
• The higher the charge, the closer the electrons are to the nucleus and a nuclear charge is the same as the atomic number.
• With higher atomic number, the element size is smaller.
• Shielding or screening effect by inner electrons is the screening of outermost electrons from the nucleus by the inner electrons, representing electron repulsion.
• The greater the shielding or screening effect, the larger the atom size.

Ionization Energy

• Ionization energy (IE) is the energy (in kJ/mole) required to remove the most loosely bound electron from an isolated gaseous atom to form a positive gaseous ion.
• If a small amount of energy is supplied to an atom, electrons may be promoted to a higher energy level.
• If a sufficient amount of energy supplied, then the electron may be completely removed.

First Ionization Energy

• First ionization energy is the minimum energy (kJ/mol) required to remove the first electron from a gaseous atom in its ground state to form a gaseous ion.
• Ionization energy is higher with subsequent electrons: 1₁ < 1₂ < 1₃
• Ionization energy is always endothermic, and energy is added to the atom to remove the electron.
• The energy to remove an electron from an atom reduces or decreases as the size of the atom increases.
• Ionization energy and atomic radius are inversely proportional.
• The larger the atom, the smaller the ionization energy and the larger the atomic radii.

Factors That Affect Ionization Energy

• Nuclear Charge: The larger the nuclear charge, the greater the ionization energy.
• Shielding Effect: The greater the shielding effect, the less the ionization energy.
• Radius: The greater the distance between the nucleus and the outer electrons of an atom, the less the ionization energy.
• Sublevel: Removing an electron from a full or half-full sublevel requires additional energy.

Full and Half Full Orbitals and Ionization Energy

• Full and half full orbitals are stable
• Filled ns and np orbitals i.e. ns² and np⁶
• Half filled np³ orbitals
• The higher ionization energy is expected due to reluctance to lose its stability.

Group Trends in Ionization Energy

• Group trends are based on the first, second, third, fourth, etc. ionization energies.
• First ionization energy decreases down a group.
• Atomic radius of the atoms increases.
• The shielding effect increases.

Periodic Trends in Ionization Energy

• Atoms in the same period have valence electrons in the same energy level.
• Shielding is the same
• Nuclear charge increases across the period.
• Ionization energy generally increases from left to right.
• Exceptions occur at full and 1/2 full orbitals.
• For elements in the second and third row of the periodic table, successive ionization energies increase.
• As electrons are removed from the valence orbitals (2s or 2p & 3s or 3p.)
• A large increase in ionization energy occurs when electrons are removed from filled core levels.
• First ionization energies tend to increase across the period because the valence electrons do not screen each other, causing the effective nuclear charge to increase steadily across the row.
• Valence electrons are attracted more strongly by the nucleus, so atomic sizes decrease and ionization energies increase.
• Changes seen in the second, third, fourth, fifth, and sixth rows of the s and p blocks follow the same pattern.
• Ionization energies increase from left to right across each row.
• First ionization energies decrease down a group.

Electronegativity Trends

• Electronegativity measures an atom's ability in a molecule to attract electrons.
• Linus Pauling first proposed the concept, which won him a Nobel Prize.
• Electronegativity refers to the ability of an atom to attract toward itself the electrons in a covalent bond.
• Metals display a low tendency to attract electrons and a high tendency to release them.
• Non-metals exhibit the opposite which is high electron attraction and a low tendency to release.
• Small atoms attract electrons more strongly than large ones, hence they are more electronegative.
• Atoms with nearly filled electron shells tend to have higher electronegativity than those with sparsely occupied shells.
• If two atoms have similar electronegativity, the bond between them will be predominantly covalent.
• A large difference in electronegativity leads to a bond with high polar character, which is predominantly ionic.

Group Electronegativity Trends

• As you progress down a group in the periodic table, the size of the atom increases.
• Valence electrons are further away from the nucleus and have a better shielding.
• This results in less nuclear attraction for valence electrons.
• Electronegativity decreases as you go down the group for this reason.

Periodic Electronegativity Trends

• The atoms have same energy levels but size decreases across the period.
• Nuclear attraction for valence electrons increases.
• Electronegativity increases from left to right across a period.
• Fluorine (F) is the element with the highest electronegativity and is the most electronegative element.

Errors in Measurements

• All measurements that involve a numerical answer involve an error.
• The error can come from the instrument used or from personal error (inability of an analyst to read the instrument accurately).
• The error of a measurement is the difference between the observed/measured value and the true value.
• This is also called the absolute error.
• Absolute error = measured value - true value
• Relative error = absolute error - true value.
• % relative error = |absolute error| - true value x 100

Accuracy and Precision

• Two important terms for measurement are accuracy and precision, which have different meanings.
• Accuracy is the degree of agreement between the measured value and the true value.
• Precision is the extent to which results agree with one another, the degree of repeatability of a result meaning how close the values are to each other.

Relative Atomic Masses

• Atoms are very small, making it impossible to weigh a single isolated atom.
• The mass of an atom can only be determined relative to another through experimentation.
• Carbon 12 (¹²C) is the standard and assigned a mass of exactly 12 atomic mass units providing a standard for measuring the atomic mass of other elements.

Average Atomic Mass

• The atomic mass of carbon is 12.01 instead of 12.00 because of isotopes.
• Isotopes are two atoms with the same atomic number but different mass numbers.
• The average atomic mass calculation uses the relative abundance of each isotope.
• Average atomic mass computation example: ₁₇³⁵Cl has a relative mass contribution (75.53%) with a relative mass being 34.968amu and for ₁₇³⁷Cl the relative mass contribution is (24.47%) at 36.956 amu.
  • Solution: (75.53/100)x34.968 + (24.47/100)x36.956 = 35.454

Avogadro's Number and the Mole

• A mole of substance is the amount containing as many atoms, molecules, or particles as there are in exactly 12 g of carbon-12 isotope.
• Samples contain so many atoms, thus a unit of measure called the mole counts the atoms.
• It is the number equal to the number of carbon atoms in exactly 12 g of pure ¹²C.
• 12 g of ¹²C contains 6.023 x 10²³ atoms, which is Avogadro's Number.
• 1 mole of ¹²C = 6.023 x 10²³ atoms
• 1 mole of H₂O molecules = 6.023 x 10²³ H₂O molecules
• 1 mole of NO₃⁻ ions = 6.023 x 10²³ NO₃⁻ ions
• 1 mole of H atoms = 6.023 x 10²³ atoms
• 1 mole of O atoms = 6.023 x 10²³ atoms
• 1 mole of H₂ molecules = 6.023 x 10²³ molecules

Molar Mass

• The molar mass of a substance is the mass (in grams) of one mole of a compound.
• This is obtained by summing the atomic masses of all the component atoms in a compound or substance.
• One (1) mole of Mg atom is 24.31 g; therefore, the molar mass is 24.31 g/mol.
• The molar mass of methane (CH₄) is (12.01 x 1) + (1.008 X 4)= 16.04 g/mol.
• no of moles = mass/molar mass is the relationship between mass, number of moles, and molar mass.
• Percentage composition determines the percentage of which elements comprise the substance.

Chemical Equations

• Chemical reactions which are symbolically represented.
• Symbols and formulae represent the elements and compounds involved
• CH₄ + 2O₂ ----> CO₂ + 2H₂O
• CH₄ and O₂ are the reactants, and CO₂ and H₂0 are the products.
• Note that atoms are rearranged and that bonds are broken (reactants) and new bonds are formed (products).

Balancing Chemical Equations

• Atoms are neither created nor destroyed in reactions, so the same number of each type of atom must exist on both sides of the reaction.
• The process of ensuring this rule is obeyed is balancing a chemical equation.
• Identities of the reactant and products cannot be changed when balancing equations.
• Most equations are balanced by inspection i.e trial and Error.
• It is best to with the most complicated molecules (most atoms).

Balancing Equations Example

• Balance each element by placing coefficients as per the reaction

• Reactants: K=2, Cl=2, and O=6.

• Products: K=2, Cl=2, and O=6.

Stoichiometric Calculations

• Before performing any calculations, the equation must be balanced.
• A balanced equation is C3H8 (g) + 5O2 (g) --> 3CO2 (g) + 4H2O (g)
• The equation requires one mole of C3H8 to reacts with 5 moles of O2 and produces 3 moles of CO2 and 4 moles of Water.

Stoichiometric Calculation Example

• From a balanced equation, set up the appropriate mole ratios.
• 1 mole of propane reacts with 5 moles of oxygen at a ratio of 1:5.
• Convert the known mass of the reactants or products to the moles of that substance or vice versa.
• 1 mole of C3H8 = 44.1 g
• 1 mole of O2 = 32 g
• 5 moles of O2 = 160g
• If 44.1 grams of C3H8 reacts with 160 grams of O2 , therefore, 96.1 grams will then react with 160 x 96.1 / 44.1 at a quantity of 349 g.

Limiting Reagent

• The limiting reagent in a chemical reaction is a reactant that is totally consumed when the chemical reaction is complete.
• It limits the amount of product formed and prevents the reaction from proceeding without it.
• One reagent example requires making toast bread which requires to slices of bread (Bd) and 1 egg.
• Here with 20 slices of bread and 12 eggs one can only make 10 toasts because there are not enough slices of bread to accommodate all the eggs.
• In this case the bread represents the limiting reagent.
• One general reaction can be written as 2Bd + 1 egg --> Bd₂egg
• This has shown bread represents the limiting reagent.
• Another example of N2(g) + H2(g) -> NH3(g) is, if with 35.0 kg of N₂ and 15 kg of H₂.
• To determine which is the limiting Reagent, the chemical equation must be balanced
• N2(g) + 3H2(g) -> 2NH3(g) means that 1 mole of N₂ reacts with 3 moles of H₂.
• Converting it to mass, it means that 28g of N₂ reacts with 6g of H₂.
• Therefore, 35 kg should react with 6/28 x 35 = 7.5 kg.
• Since 35kg of N₂ will only react with 7.5 kg of H₂, there will be excess H₂ meaning that the N₂ finishes first and is therefore that limiting compound.

Example Problems

• If nitric oxide (NO) reacts with oxygen gas to form nitrogen dioxide (NO₂). In an experiment, 0.886 mole of NO is mixed with 0.503 mole of O2 in which the equation setup --> NO + O2 = NO₂.
• Balance the equation ---> 2NO + O2 > 2NO₂, which yields the set up of the stoichiometric ratios being 2NO + O₂ = 2NO₂.
• This means that 2 moles, 1 mole and 2 moles all equal 2NO + O₂ ---> 2NO₂ so therefore, 0.866 moles will react with 0.433 moles. Therefore, the NO finishes first while the O₂ is still left making it that limiting reagent. Since NO is the limiting reagent, it determines the amount of NO₂ that will be produced and results in 2 moles being produced in that second step.
• Therefore, .0866 moles will produce the result of that set at 0.866 moles.

Yield and Percentage Yield

• Yield is the mass of the product formed in a chemical reaction.
• Percent yield refers to the percent ratio of actual yield to the theoretical yield.
• The amount of a product formed when the limiting reactant is completely consumed is called the theoretical yield while the actual amount of product obtained in an experiment is called the actual yield.
• In most cases, the actual yield is not equal to the theoretical yield.
• To measure the efficiency and results of a chemical reaction , the percentage yield is calculated with % Yield = actual yield/ theoretical yield x 100.
• When the actual yield equals the theoretical yield, the reaction process is a stoichiometric process.