What is the Periodic Law
When elements are arranged in order of increasing atomic number, repeating properties are seen at regular intervals
What is Effective Nuclear Charge
Increases across a period
Stays the same down a group
The net positive charge (of nuclear protons) attracting the electron is the Zeff
More positive nucleus (more attractive force from the nucleus pulling electrons closer) = higher Zeff
What is the Shielding Effect
The lessening of attractive electrostatic charge difference between nucleur protons and valence elections by partially or fully filled inner shells
Zeff = Z - S
Zeff: Effective nuclear charge
Z: Atomic number
S: The screening constant
What is Atomic Radius. When does it increase/decrease and why?
Increases across a period
Decreases down a group
Across a Period
The more protons/electrons, the stronger the attraction between the nucleus and electrons, thus higher Zeff and smaller Atomic Radius
–> the electron cloud shrinks until the electron-electron repulsion overcomes nuclear attraction and it stops contracting
Down a Group
There are more orbitals, placing the electrons further from the nuclei
The inner energy levels are filled with electrons, which shield the outer energy levels, decreasing attractionq
What is Ionization Energy. When does it increase/decrease and why?
Increases across a period
Decreases down a group
It is the amount of energy needed to remove the weakest-held electron from a neutral atom
Across a period, the radius between the nucleus and electrons in the outer energy level decreases
Increases Zeff = Higher IE
What is Electron Affinity? When does it increase/decrease and why?
Increases across a period
Decreases down a group
It is the energy released when a gaseous atom gains an electron to form a gaseous anion
High EA means adding an electron releases a large amount of energy, making it more stable.
The greater the nuclear attraction (small atom) = greater EA = higher IE
EA decreases down a group (as atoms get larger) as the increased distance from the nucleus with each increasing principal energy level makes the nucleus farther from the valence level and more shielded
EA increases across a period as atoms get smaller, and the increasing Zeff more strongly attracts the electrons
What is electronegativity?
The measure of the relative attraction of an atom for the bonding electrons
Increases across a period
The higher the EN, the more likely it is to attract or remove an electron from another atom
The difference in the EN between 2 atoms determines the type of intramolecular bond between them (ionic or covalent)
What is Radioactive Decay?
When a radioactive nucleus decomposes to form another nucleus and produces one or more
Both A (Mass number A = Z + N) and Z (Atomic number = # of protons) are conserved
All nuclides (unstable elements) with 84+ protons are. unstable with respect to radioactive decay
What is Alpha Decay
A decomposition that involves a change in the mass number of the decaying nucleus
- the nucleus left behind has Its atomic mass reduced by 2 and Its mass number reduced by 4 (2 protons, 2 neutrons)
An alpha particle is a helium nucleus (He2^4)
What is Beta Decay?
The mass number of a decaying nucleus remains constant (electron has no mass)
- The atomic number goes up by 1, mass stays the same
– Proton comes in to balance the equation after the electron leaves
- The atomic numbers on both sides add up to the same value, and the same for mass number
What is the Half-Life of Radioactive Isotopes?
A = A0(1/2)^t/h
The half-life (t1/2) of a radioactive sample is defined as the time required for the number of nuclides to reach half of their original value
A = Final amount
A0 = Initial amount
T = time elapsed
H = Half-life
What is a Chemical Bond?
Bonds are attractive forces that hold groups of atoms together. The system is trying to achieve the lowest possible energy state by bonding. When a bond is formed energy is released, to break a bond energy is required.
What is Bond energy
The energy required to break a bond
What is a Lewis Structure
Shows the arrangement of electrons and bonds in a molecule or polyatomic ion. The type of intramolecular force can be determined by the difference in electronegative values
EN > 1.7: Ionic bond
0.5 < EN < 1.7: Polar covalent bond
En < 0.5: Non-polar covalent bond
What are ionic bonds
An electrostatic attraction between ions, usually between metals and non-metals. Bonding orbital is strongly displaced toward one nucleus (The higher EN nucleus) as the valence electrons are transferred
There are no physical bonds between atoms
Use brackets when writing Lewis symbols of ions
What are the Ionic Compound properties
Ionic compounds form crystalline (crystal lattice) structures that are solids at room temperature
Opposite charge holds the ions in place, thus the bonds are very strong and require large amounts of energy (heat) to break
When melted or in solution, they form free ions, which allows them to conduct electricity
What is Coulomb’s Law?
The energy interaction between a pair of ions. If one charge is positive, and the other negative, there will be a negative sign on the Force (energy) once calculated. Thus means that the two ions will have lower energy (more stable) when they are close together than when further apart
Formula of Coulomb’s Law?
F = (2.31 x 10^-19 J(nm))(Q1Q2/r)
F = Electrostatic force, lattice energy
r = the distance between the ion centres in nanometers
J = energy in Joules
Q1 and Q2 = the numerical ion charges
Rules of Coulomb’s law
More energy is required to remove successive electrons from the atom
The greater the charge:
- The stronger the attraction of opposite charges
- The stronger the repulsion of like charges
The closer the particles are to each other, the stronger the attraction/repulsion
What is Charge Density (Related to Coulomb’s law)
How much charge is in a space
Larger charges will have stronger interactions and lower energies. Likewise, smaller ions will be able to get closer and thus will have lower energies
Smaller electron clouds = high charge density
Large ionic radius = lower charge density = smaller attractive forces
Attractive forces between molecules decrease as you go up a group (increased radius), resulting in lower melting points.
Small attractive forces = lower melting point
What are Attractive Forces?
a natural, non-contact interaction that pulls objects, particles, or molecules together
What are Lattice Structures
Larger lattice structures mean stronger ionic bonds
Electrostatic attraction (and thus lattice energy) increases as ionic charges increase and as ionic radii decreases
All solids will have a lattice structure
- Ionic compounds will involve ions
- Covalent compounds rely on other attractive forces
When a solid melts, the attractive force is too weak to hold the lattice together
- The temperature at which the energy of the molecule overcomes the attractive forces
What are covalent bonds
Unlike ionic compounds, covalent compounds are not reduced (i.e., N2O4 does not equal NO2)
Can be solid, liquid or gas
Will not produce ions in solution
- Neither element is gaining or losing electrons, but sharing the sharing of electrons is unequal
- No or very weak electrical conductivity
What are polar covalent bonds
The valence electrons are shared unequally and partial charges are formed
