TOPIC 2 - STRUCTURE AND BONDING Flashcards
(26 cards)
what is ionic bonding
the strong electrostatic attraction between oppositely charged ions
the effects that ionic radius and ionic charge have on the strength of ionic bonding
Ionic bonding is stronger and the melting points higher when the ions are smaller and/ or have higher charges
understand the formation of ions in terms of electron loss or gain
loss of electron = +ion
gain of electron =
-ve ion
reasons for the trends in ionic radii down a group and for a set of isoelectronic ions, e.g. N3- to Al3+
As you move down a column or group, the ionic radius increases. This is because each row adds a new electron shell. Ionic radius decreases moving from left to right across a row or period. … While the atomic radius follows a similar trend, ions may be larger or smaller than neutral atoms
ISOLECTRONIC IONS
number of protons in the nucleus of the ion increases, the electrons get pulled in more closely to the nucleus. The radii of the isoelectronic ions therefore fall across this series.
(the group 8 don’t fit in because they don’t form ions)
what is a covalent bond
the strong electrostatic attraction between two nuclei and the shared pair of electrons between them
the relationship between bond lengths and bond strengths for covalent bonds
Bond Strength: Covalent Bonds. … Generally, as the bond strength increases, the bond length decreases. Thus, we find that triple bonds are stronger and shorter than double bonds between the same two atoms; likewise, double bonds are stronger and shorter than single bonds between the same two atoms.
what is the shape of a simple molecule determined by
The electron pair repulsion theory
The shape of a molecule or ion is governed by the arrangement of the electron pairs around the central atom. All you need to do is to work out how many electron pairs there are at the bonding level, and then arrange them to produce the minimum amount of repulsion between them. You have to include both bonding pairs and lone pairs.
how do bonding pairs and lone pairs influence shape of molecule
bonding pairs of electrons which arrange themselves as far away as possible to minimise the electron repulsion between them
lone pairs repel more than bonding pairs and so reduce bond angles (by about 2.5o per lone pair
what is electronegativity
the ability of an atom to attract the bonding electrons in a covalent bond
what are London forces
Constant movement of electrons in a molecule causes temporary dipoles to form in a molecule
what are permanent dipoles
These occur when two atoms in a molecule have substantially different electronegativity: One atom attracts electrons more than another, becoming more negative, while the other atom becomes more positive
what is hydrogen bonding
A hydrogen bond is a partial intermolecular bonding interaction between a lone pair on an electron rich donor atom, particularly the second-row elements nitrogen, oxygen, or fluorine, and the antibonding orbital of a bond between hydrogen and a more electronegative atom or group
the interactions in molecules, such as H2O, liquid NH3 and liquid HF, which give rise to hydrogen bonding
The anomalously high boiling points of H2O, NH3 and HF are caused by the hydrogen bonding between these molecules in addition to their London forces. The additional forces require more energy to break and so have higher boiling points
why does water have a high melting and boiling point
H2O has an additional intermolecular forces of attraction, requiring significantly more heat energy to break. These relatively powerful intermolecular forces are described as hydrogen bonds on top of the London forces (dipole - dipole forces )
why is ice less dense than water
When the hydrogen bonds are broken as the ice melts, the water molecules get closer together. That means that ice is less dense than water, and so will float on the water.
how does the length of an alkane change affect the boiling point
As the length of an alkane chain increases, the boiling point increases. This is because there are more points of contact with each adjacent molecule. This means there are stronger induced dipole to dipole forces between adjacent molecules.
how does branching of a chain affect the boiling point
A straight chain alkane will have a boiling point higher than a branched chain alkane because of the greater surface area in contact with other molecules. Branching makes molecules more compact thus reduces the surface area
why do alcohols have a low volatility (high boiling point) compared to alkanes with similar number of electrons
The reason why alcohols have a higher boiling point than alkanes is because the intermolecular forces of alcohols are hydrogen bonds, unlike alkanes with van der Waals forces as their intermolecular forces.
trends in boiling temperatures of the hydrogen halides, HF to HI
The general increase in boiling point from H2S to H2Te or from HCl to HI is caused by increasing London forces between molecules due to an increasing number of electrons.
The anomalously high boiling points of H2O, NH3 and HF are caused by the hydrogen bonding between these molecules in addition to their London forces. The additional forces require more energy to break and so have higher boiling points
water, to dissolve some ionic compounds, in terms of the hydration of the ions
When an ionic lattice dissolves in water it involves breaking up the bonds in the lattice and forming new bonds between the metal ions and water molecules. The negative ions are attracted to the δ+ hydrogens on the polar water molecules and the positive ions are attracted to the δ - oxygen on the polar water molecules
The higher the charge density the greater the hydration enthalpy (e.g. smaller ions or ions with larger charges) as the ions attract the water molecules more strongly
water, to dissolve simple alcohols, in terms of hydrogen bonding
The smaller alcohols are soluble in water because they can form hydrogen bonds with water. The longer the hydrocarbon chain the less soluble the alcohol
water, as a poor solvent for compounds (to include polar molecules such as halogenoalkanes), in terms of inability to form hydrogen bonds
The solubility of haloalkanes in water. Haloalkanes aren’t very soluble in water because they can’t form hydrogen bonds, and the energy required to break hydrogen bonds in water etc is higher than the energy released when new bonds between the haloalkane and water are formed.
non-aqueous solvents, for compounds that have similar intermolecular forces to those in the solvent
Compounds which have similar intermolecular forces to those in the solvent will generally dissolve — Non-polar solutes will dissolve in non-polar solvents. e.g. Iodine which has only London forces between its molecules will dissolve in a non polar solvent such as hexane which also only has London forces.
what is metallic bonding
the strong electrostatic attraction between metal ions and the delocalised electrons
