Chapter 8 Flashcards
Electronegativity
is the ability of an atom to attract a pair of electrons towards itself in a covalent bond
- This phenomenon arises from the positive nucleus’s ability to attract the negatively charged electrons, in the outer shells, towards itself
- The Pauling scale is used to assign a value of electronegativity for each atom
the most electronegative atom on the Periodic Table, with a value of 4.0 on the Pauling Scale is
fluorine
It is best at attracting electron density towards itself when covalently bonded to another atom
Electronegativity: Affecting Factors
- Nuclear charge
- Atomic radius
- Shielding
Electronegativity: Affecting Factors: Nuclear charge
- Attraction exists between the positively charged protons in the nucleus and negatively charged electrons found in the energy levels of an atom
- An increase in the number of protons leads to an increase in nuclear attraction for the electrons in the outer shells
- Therefore, an increased nuclear charge results in an increased electronegativity
Electronegativity: Affecting Factors: Atomic radius
- The atomic radius is the distance between the nucleus and electrons in the outermost shell
- Electrons closer to the nucleus are more strongly attracted towards its positive nucleus
- Those electrons further away from the nucleus are less strongly attracted towards the nucleus
- Therefore, an increased atomic radius results in a decreased electronegativity
Electronegativity: Affecting Factors: Shielding
- Filled energy levels can shield (mask) the effect of the nuclear charge causing the outer electrons to be less attracted to the nucleus
- Therefore, the addition of extra shells and subshells in an atom will cause the outer electrons to experience less of the attractive force of the nucleus
- —-Sodium (Period 3, Group 1) has higher electronegativity than caesium (Period 6, Group 1) as it has fewer shells and therefore the outer electrons experience less shielding than in caesium
-Thus, an increased number of inner shells and subshells will result in a decreased electronegativity
As nuclear charge increases
the nucleus has a greater attractive force on the electrons in shells given that the shielding doesn’t increase.
As a result of this, the atomic radius decreases.
Electronegativity: Trends: down a group
- There is a decrease in electronegativity going down the Group
- The nuclear charge increases as more protons are being added to the nucleus
- However, each element has an extra filled electron shell, which increases shielding
- The addition of the extra shells increases the distance between the nucleus and the outer electrons resulting in larger atomic radii
-Overall, there is decrease in attraction between the nucleus and outer bonding electrons
Electronegativity: Trends: Across a period
- Electronegativity increases across a Period
- The nuclear charge increases with the addition of protons to the nucleus
- Shielding remains reasonably the same across the Period as no new shells are being added to the atoms
- The nucleus has an increasingly strong attraction for the bonding pair of electrons of atoms across the Period of the Periodic Table
-This results in smaller atomic radii
Electronegativity & covalent bonds
-Single covalent bonds are formed by sharing a pair of electrons between two atoms
—-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
-The equal distribution leads to a non-polar molecule
Electronegativity & ionic bonds
-When atoms of different electronegativities form a molecule, the shared electrons are not equally distributed in the bond
-The more electronegative atom (the atom with the higher value on the Pauling scale) will draw the bonding pair of electrons towards itself
-A molecule with partial charges forms as a result
The more electronegative atom will have a partial negative charge (delta negative, δ–)
-The less electronegative atom will have a partial positive charge (delta positive, δ+)
-This leads to a polar covalent molecule
If there is a large difference in electronegativity of the two atoms in a molecule
the least electronegative atom’s electron will transfer to the other atom
- This in turn leads to an ionic bond – one atom transfers its electron and the other gains that electron
- —-The cation is a positively charged species which has lost (an) electron(s)
- —-The anion is a negatively charged species which has gained (an) electron(s)
Ionic Bonding: Definition
-As a general rule, metals are on the left of the Periodic Table and nonmetals are on the right-hand side
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
Once the atoms become ions, their electronic configurations are the same as a stable noble gas.
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
- —–This causes high melting points in ionic compounds
Sodium chloride
- Sodium is a Group 1 metal
- It loses its outer electron to form a sodium ion with a +1 charge (Na+)
- Chlorine is a Group 7 non-metal
- It gains 1 electron to form a chloride ion with a -1 charge (Cl–)
- The oppositely charged ions are attracted to each other by electrostatic forces to form NaCl
- The final ionic solid is neutral in charge
Magnesium oxide
- Magnesium is a Group 2 metal
- It loses its 2 outer electrons to form a magnesium ion with a +2 charge (Mg2+)
- Oxygen is a Group 6 non-metal
- It gains 2 electrons to form an oxide ion with a -2 charge (O2-)
- The oppositely charged ions are attracted to each other to by electrostatic forces to form MgO
- The final ionic solid is neutral in charge
Calcium fluoride
- Calcium is a Group 2 metal
- It loses its 2 outer electrons to form a calcium ion with a +2 charge (Ca2+)
- Fluorine is a Group 7 non-metal
- It gains 1 electron to form a fluoride ion with a -1 charge (F–)
- As before, the positive and negative ions are attracted to each other
- To cancel out the 2+ charge of the calcium ion, 2 fluoride ions are needed
- Calcium fluoride is made when 1 calcium ion and 2 fluoride ions form an ionic bond, CaF2
- The final ionic solid of CaF2 is neutral in charge
Metallic Bonding: Definition
Metal atoms are tightly packed together in lattice structures
When the metal atoms are in lattice structures, the electrons in their outer shells are free to move through the structure
The free-moving electrons are called ‘delocalised electrons’ and they are not bound to their atom
metallic bonding
- When the electrons are delocalised, the metal atoms become positively charged
- The positive charges repel each other and keep the neatly arranged lattice in place
- There are very strong electrostatic forces between the positive metal centres and the ‘sea’ of delocalised electrons
The strength of electrostatic attraction can be increased by
- Increasing the number of delocalised electrons per metal atom
- Increasing the positive charges on the metal centres in the lattice
- Decreasing the size of the metal ions
- Due to the delocalised ‘sea’ of electrons, metallic structures have some characteristic properties
features of metallic bonds
- high melting and boiling points
- electrical conductivity
high melting and boiling points why
- positive metal atom centres and the delocalized electrons in a metallic lattice have strong electrostatic forces between them
- Therefore high energy is needed to overcome the strong forces of attraction
- As the number of mobile charges increase from left to right of he Periodic Table, the melting and boiling points increase as the electrostatic forces become stronger
electrical conductivity why
-When a potential difference is applied to the metallic lattice, the delocalised electrons repel away from the negative terminal and move towards the positive terminal
- As the numbers of valance electrons increases across the Period, the number of delocalised charges also increases
- —sodium= 1 valance electron
- —Magnesium= 2 valance electrons
- —Aluminum= 3 valances electrons
-Therefore the ability to conduct electricity also increases