Atomic Structure and Bonding Flashcards
Formation of ions (as a result of electron gain or loss)
An ion is a charged atom. During reactions, elements gain or lose electrons. –> anions, cations
Atoms form ions by gaining or losing electrons through electron transfer.
Cation
Metal atoms that lose electrons forming positive ions.
Anions
Non-metal atoms gain electrons forming negative ions known as anions.
exception: hydrogen = non-metal that forms positive ions.
Ionic Bond
The electrostatic force of attraction between oppositely charged ions.
Formation of ionic compounds (as a result of electron transfer)
Ionic compounds are formed during a reaction between a metal atom and a non-metal atom. Outer (valent) shell electron(s) are removed from a metal atom by a non-metal atom. The metal atom loses electron(s) forming positively charged ions called cations and the non-metal atom gains electron(s) forming negatively charged ions called anions. The ions are held together by the electrostatic attraction of the oppositely charged ions.
Ionic Bonding
e. g.
- Lithium atom donates/transfers electrons to chlorine atom.
- One Lithium atom loses an electron to chlorine.
- One chlorine atom gains an electron from lithium.
- Forming ions Li+ and Cl-.
- There is an electrostatic force of attraction between oppositely charged lithium and chloride ions.
- Forming an ionic bond/electrostatic attractions between oppositely charged ions.
Polyatomic Ions
Carbonate: CO3(2-) Sulfate: SO4(2-) Hydroxide: OH(-) Nitrate: NO3(-) Phosphate: PO4(3-) Hydrogen Carbonate: HCO3(-) Ammonium: NH4(+)
Reasons for properties of ionic compounds
- High melting and boiling points
- Mostly soluble in water
- Ionic solids do not conduct electricity (they are insulators)
- Conduct electricity when dissolved in water or molten (liquid)
- Brittle
- Hard
- High melting and boiling points: The strong electrostatic forces of attraction between the ions require a lot of heat energy to break.
- Mostly soluble in water: If the ionic compound is soluble, the water molecules are able to break the electrostatic force of attraction between ions because water is polar. The force of attraction between water molecules and ions is stronger than the attraction between ions.
- Ionic solids do not conduct electricity (they are insulators): Ions in a solid ionic compound are not free to move. But in a liquid or solution, ions move past each other and charge can flow, inducing current.
- Conduct electricity when dissolved in water or molten (liquid): As ionic compounds are made of charged ions, they can conduct electricity but only if the ions can move. If it is molten or dissolved, the ions are free to move and carry electrical charge.
- Brittle: When force is applied, the ions in the lattice shift so that like ions repel and the lattice breaks (snaps b/c same charge).
- Hard: Ionic compounds have strong electrostatic forces of attractions between oppositely charged ions.
Testing for flame test (metal ions)
Flame test to detect presence of metal ion.
Lithium, Li(+) = Red
Sodium, Na(+) = Intense yellow/orange
Potassium, K(+) = Purple (lilac/violet)
Calcium, Ca(2+) = Brick Red
Testing for ammonium, NH4(+) ions
- add dilute sodium hydroxide solution
- gently heat
- if NH4(+) is present, characteristic choking smell produced, The ammonia fumes turn damp red litmus paper or damp universal indicator from red to blue.
Testing for carbonate, CO3(2-) ions
- add dilute hydrochloric acid
- bubble gas through limewater
- If CO3(2-) present, limewater turns cloudy.
Testing for sulfate, SO4(2-) ions
- add dilute hydrochloric acid
- add a few drops of barium chloride solution
- If SO4(2-) present, a white precipitate forms.
Testing for halide ions, Cl(-), Br(-), I(-)
- add dilute nitric acid
- then add a few drops of silver nitrate solution
- If Cl(-) present, white precipitate is formed.
If Br(-) present, cream precipitate is formed.
If I(-) present, yellow precipitate is formed.
Covalent Bond
A covalent bond is a chemical bond that involves the sharing of valence electron pairs between atoms.
Covalent Bonding
A covalent bond forms between non-metal elements (non-metal to non-metal). Only valence electrons are involved in the bonding, so the inner shells are not always drawn in the diagrams (Dot and Cross, Solid Line). Electrons are shared so that each atom has a full outer shell of electrons. Covalent bonds are very strong bonds.
Dot and Cross Diagrams
Step 1. Identify which elements you are working with.
Step 2. How many valence electrons? (group number)
Step 3. How many more electrons does each atom need to have 8 in the outer shell?
The pair of electrons in the centre are shared by both valence shells.
Structural Formula
Structural formula of covalent compounds:
e.g. HCl: H – Cl
Simple molecular structural properties
Low melting and boiling points:
There are intermolecular forces between simple molecules. These intermolecular forces are much weaker than the strong covalent bonds in molecules. When simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome.
Do not conduct electricity:
A substance can conduct electricity if: it contains charged particles, and these particles are free to move from place to place.
Simple molecules have no overall charge, or charged particles that can separate, so simple molecular substances cannot conduct electricity, even when liquid or dissolves in water.
Polar Molecules
The negative electrons are held in their shells by the attraction of the positive protons in the nucleus. The nucleus has a pull on the electrons.
The strength of the pull depends on the type of atom.
The electrons shared in a covalent bond are pulled closer to the atom with a stronger nuclear pull.
This produces a slight charge on each side of the bond (represented by the lower case delta and + or -).
Allotropes
A different form of the same element in the same state.
e.g. diamond and graphite are allotropes of carbon (C).
Allotropes of carbon - Diamond
Diamond is extremely hard and has a high melting point (often used in cutting tools to cut things like bricks and concrete). It is insoluble in water and does not conduct electricity.
Every atom in a diamond is bonded to its neighbours by 4 strong covalent bonds, leaving no free electrons and no ions.
This explains why diamond does not conduct electricity, is hard and has a high melting point.
Allotropes of carbon - Graphite
Graphite is insoluble in water. It has a high melting point and is a good conductor of electricity, which makes it suitable material for the electrodes needed in electrolysis. It is also a very slippery material (used in pencil leads because layers easily slide onto the paper, leaving a black mark).
Each carbon atom is bonded into its layer with 3 strong covalent bonds. This leaves each atom with a spare electron, which together form a delocalised ‘sea’ of electrons loosely bonding the layers together. These delocalised electrons can all move along together, making graphite a good electrical conductor.
Giant Covalent Structures
Giant covalent structures contain very many (undefined numbers) of atoms, each joined to adjacent atoms by covalent bonds.
The atoms are usually arranged into giant regular lattices, extremely strong structures because of the many bonds involved.
Properties of Giant Covalent Structures
Very high melting points: this is because a lot of strong covalent bonds must be broken.
Variable electrical conductivity: Diamond does not conduct electricity, whereas graphite contains free electrons so it does conduct electricity. Silicon is a semi-conductor; it is a midway between non-conductive and conductive.
Metallic bonding
Electrostatic attraction between a lattice of metal cations and a sea of delocalized ions.
General properties of metals
- High melting and boiling point
- Good conductors of heat and electricity
- Hard
- High density
- Malleable
- Ductile
- Sonorous
- Lustrous
Alloy
A mixture composed of two or more elements, at least one of which is a metal.
How are metals able to conduct electricity?
Delocalized electrons can move through the structure and carry charge.
Why are metals malleable?
The arrangement of ions can change but the attraction between cations and delocalized electrons isn’t broken.
How are metals able to conduct heat?
Delocalized electrons can move faster and pass energy on quickly. The lattice of atoms transmits energy efficiently, as the atoms vibrate and bump into other atoms.
Why do metals have a high melting and boiling point?
The electrostatic attractions between cations and delocalized electrons are strong, and it requires lots of heat energy to break them.
Intermolecular forces
Intermolecular forces are found between molecules. They are weaker than intramolecular bonds.
There are 3 types of intermolecular forces (weakest to strongest force):
- Dispersion forces
- Dipole-dipole forces
- Hydrogen bonds
Dispersion
Weakest type of intermolecular bond. They exist between all atoms and molecules.
Occur between all molecules due to the constant movement of electrons.
At any one instant, there may be more electrons at one end of the molecule than the other (an instantaneous dipole is formed). This is a temporary situation.
Dipole-dipole
Some molecules contain atoms with a large difference in nuclear pull.
e.g. the chlorine atom has a stronger nuclear pull and therefore the electrons are attracted to the Cl end of the molecule more than the H end.
This results in a polar molecule.
Hydrogen bonds
This is an ‘extra strong’ dipole-dipole attraction that occurs between molecules with Hydrogen covalently bonded to a NOF and attracted to a NOF on another molecule.
N- Nitrogen
O- Oxygen
F- Fluorine
Boiling and melting points in intermolecular forces.
The properties of molecular substances are determined by their intermolecular forces. These are responsible for boiling point melting point and hardness.
The strength of intermolecular forces reflects on the amount of energy it takes to break the bonds between molecules.
Molecules that exist as a solid have low kinetic energy and therefore, cannot overcome the intermolecular forces that are holding the molecules together.
Boiling point (intermolecular)
With enough kinetic energy, molecules can break the intermolecular forces to exist as a gas. This is known as the boiling point (temperature when a liquid transitions into a gas.)
Melting point (intermolecular)
When there is enough energy to overcome the forces the substance transitions into a liquid state. This is known as the melting point (temperature when solid transitions into liquid.)
Explain why nitrogen has a low boiling point
(Weak) dispersion forces exist between molecules.
Little energy is needed to break the intermolecular forces.
Explain why graphite is soft and is a good conductor of electricity
Graphite has a giant covalent structure. It is a good conductor of electricity as it has delocalized electrons. It is soft b/c layers can slide over one another due to weaker forces
