Topic 4 : Chemical Bonding & Structure Flashcards
Metals and non-metals on the periodic table
As a general rule, metals are on the left of the Periodic Table and non-metals are on the right-hand side
Forming Ionic bonds
- Ionic bonds involve the transfer of electrons from a metallic element to a non-metallic element
- Transferring electrons usually leaves the metal and the non-metal with a full outer shell
- Metals lose electrons from their valence shell forming positively charged cations
- Non-metal atoms gain electrons forming negatively charged anions
- Cations and anions are oppositely charged and therefore attracted to each other
- Electrostatic attractions are formed between the oppositely charged ions to form ionic compounds
- This form of attraction is very strong and requires a lot of energy to overcome
Cation
Positive ion
Anion
Negative ion
Ionic Compounds (structure)
- The ions form a lattice structure which is an evenly distributed crystalline structure
- Ions in a lattice are arranged in a regular repeating pattern so that there are equal positive and negative charges
- Therefore the final lattice is overall electrically neutral
Examples of ionic bonding
- Sodium Chloride (salt)
- Magnesium Oxide
Ionic Bonding Physical Properties
- Strong
- Brittle
- High melting and boiling points
- Soluble in water
- Conduct electricity when molten or in solution
Ionic bonding - strength
- Ionic compounds are strong
- The strong electrostatic forces in ionic compounds keep the ions strongly together
Ionic bonding - brittle
- Ionic compounds are brittle as ionic crystals can split apart
- When a layer of ions is pushed, it will repel with the ions on top causing the material to break
Ionic bonding - melting + boiling points
- Ionic compounds have high melting and boiling points
- The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
- Melting and boiling points increase with charge density of the ions due to the greater electrostatic attraction of charges
- e.g. Mg2+O2- has a higher melting point than Na+Cl-
Ionic bonding - solubility
- Ionic compounds are soluble in water as they can form ion - dipole bonds
- Basically, water has a positive and negative end so ionic ions attract and dissolve
- Water has a positive and negative end because of the oxygen and hydrogen
Ionic bonding - electricity
- Ionic compounds only conduct electricity when molten or in solution
- When molten or in solution, the ions can freely move around and conduct electricity
- In the solid state they’re in a fixed position and unable to move around
Metal ions
- All metals form positive ions
- There are some non-metal positive ions such as ammonium, NH4+, and hydrogen, H+
Transition metals charges
- The charge on the ions of the transition elements can vary which is why Roman numerals are often used to indicate their charge
- Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions
- Eg. in copper (II) oxide, the copper ion has a charge of 2+ whereas in copper (I) nitrate, the copper has a charge of 1+
Non-metal ions
- The non-metals in group 15 to 17 have a negative charge and have the suffix ‘ide’
- Eg. nitride, chloride, bromide, iodide
Compound negative ions
- Also known as polyatomic ions
- negative ions made up of more than one type of atom
7 polyatomic ions
- Ammonium : (NH4)+
- Hydroxide : (OH)-
- Nitrate : (NO3)-
- Sulfate : (SO4)2-
- Carbonate : (CO3)2-
- Hydrogen carbonate : (HCO3)-
- Phosphate : (PO4)3-
Covalent Bonds
- Covalent bonding occurs between two non-metals
- A covalent bond involves the electrostatic attraction between nuclei of two atoms and the electrons of their outer shells
- No electrons are transferred but only shared in this type of bonding
- When a covalent bond is formed, two atomic orbitals overlap and a molecular orbital is formed
- In a normal covalent bond, each atom provides one of the electrons in the bond. A covalent bond is represented by a short straight line between the two atoms, H-H
- Covalent bonds should not be regarded as shared electron pairs in a fixed position; the electrons are in a state of constant motion and are best regarded as charge clouds
Purpose of a covalent bond
- Non-metals are able to share pairs of electrons to form different types of covalent bonds
- Sharing electrons in the covalent bond allows each of the 2 atoms to achieve an electron configuration similar to a noble gas
- This makes each atom more stable
Octet rule
The octet rule refers to the tendency of atoms to prefer to have eight electrons in the valence shell.
Breaking the octet rule
- In some instances, the central atom of a covalently bonded molecule can accommodate more or less than 8 electrons in its outer shell
- Being able to accommodate more than 8 electrons in the outer shell is known as ‘expanding the octet rule’
- Accommodating less than 8 electrons in the outer shell means than the central atom is ‘electron deficient’
How to predict between covalent and ionic bond character
- The differences in Pauling electronegativity values can be used to predict whether a bond is covalent or ionic in character
- A difference of less than around 1.0 in electronegativity values will be associated with covalent bonds, although between 1.0 and 2.0 can be considered polar covalent
- A difference of more than 2.0 will be considered an ionic bond
- All bonds have ionic or covalent character but some have less than others
Diatomic molecules (electronegativity difference)
- In diatomic molecules the electron density is shared equally between the two atoms
- Eg. H2, O2 and Cl2
- Both atoms will have the same electronegativity value and have an equal attraction for the bonding pair of electrons leading to formation of a covalent bond
Coordinate bonds
- In simple covalent bonds the two atoms involved share electrons
- Some molecules have a lone pair of electrons that can be donated to form a bond with an electron-deficient atom
- An electron-deficient atom is an atom that has an unfilled outer orbital
- In other words, both of the “shared” electron come from the same atom
- This type of bonding is called dative covalent bonding or coordinate bond
- An example is an ammonium ion