#structure and bonding

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saturday 28/05/22

end of year exams start friday!! i have three papers, history, math, and chem all paper 1. feeling: a lil wonky can’t lie. in all fairness its vibes but im a lil tired of this revision. so much content. so little time.

♫ westside - carpetgarden ♫

[miastudys]

08.01.17 my most hated chemistry topic :(

[iamrevisinghonest]

Electronegativity

Electronegativity: the power of an atom to attract the electrons (electron density) in a covalent bond towards itself

Atoms are held together by forces of attraction between opposite charges. In a covalent bond, the electrons shared between the atoms will be unevenly spread if one of the atoms attracts electrons more strongly than the other.

Electronegativity is measured on the Pauling scale, which runs from zero (no electronegativity) to four (the most electronegative).

Some common values to learn are:

H -- 2.1

C -- 2.5

Br -- 2.8

Cl -- 3.0

F -- 4.0

Electronegativity depends on the charge and the atoms;

The higher the nuclear charge, the more attraction between the nuclear charge and the pair of electrons in the covalent bond, so the more electronegative the atom.

The larger the atom, the less electronegative the atom -- because the covalent bond is further from the nucleus so there is less attraction

The more shells in an atom, the less electronegative the atom -- because there is more shielding between the nucleus and the covalent bond.

Trends in Electronegativity

Electronegativity increases going across a period -- because the nuclear charge increases, the number of inner electrons remains the same and the atoms become smaller

Electronegativity decreases going down a group -- because there is more shielding by electrons in inner shells and the atoms get larger (this has a greater effect on the electronegativity than the increase in nuclear charge).

The most electronegative atoms are in the top right-hand corner of the periodic table (but not the Noble Gases; they have zero electronegativity).

N, O and F are the most electronegative elements

H is one of the most electropositive elements

C and H have very similar electronegativity.

Bond Polarity

When both atoms in a covalent bond have the same electronegativity, the electron pair is shared equally and the bond is non-polar. (Usually, the two atoms have to be the same element for this to occur.)

When the atoms in a covalent bond have different electronegativity values, the more electronegative atom has a greater attraction to the electron pair. This means that the electron pair is pulled towards the more electronegative atom, which leads to a polar bond with an unequal sharing of the electron pair. This is shown using the symbols δ+ and δ-. (This is not a full ion.)

When an electron pair is shared unequally between two atoms, each atom gains a partial charge, shown by a lower-case Greek letter delta (δ). The more electronegative atom will gain a δ- charge and the less electronegative atom will gain a δ+ charge.

The polarity of the covalent bond depends on the difference in electronegativity;

Atoms with a large difference in electronegativity are ionic

Atoms with a small difference in electronegativity are polar covalent

Atoms with no difference in electronegativity are pure covalent.

Not all molecules that contain polar bonds are polar overall; sometimes there is no overall separation of charge from left to right, which occurs when a molecule is symmetrical.

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Covalent Bonding

Covalent bond: a pair of electrons shared between two atoms

Co-ordinate bond: a covalent bond in which both electrons are provided by one atom

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Simple Covalent Compounds

Simple covalent compounds are made up of small molecules, and are usually gases or volatile liquids. They occasionally form molecular crystals.

The covalent bonds between atoms in the molecules (intramolecular bonds) are strong, but the forces of attraction between the molecules (intermolecular forces) are weak (usually Van der Waals' forces).

Properties of Simple Covalent Compounds

Simple covalent compounds tend to:

Have low melting and boiling points -- because there are only weak forces of attraction holding the molecules together, so not much energy is needed to overcome them

Have strong covalent bonds between the atoms within molecules

Be poor conductors of electricity -- because the atoms are neutral overall so there are no charged particles to carry the charge

Be insoluble, or only very sparingly soluble, in water -- because the forces of attraction between the water molecules cannot be overcome. Those that do not dissolve often react with water.

Iodine

Iodine:

Is made up of I2 molecules, with Van der Waals' forces -- because iodine molecules have a large number of electrons, so the Van der Waals' forces are strong enough to hold the diatomic molecules together in a solid

Has crystals that are soft and break easily -- because the forces holding them together are Van der Waals' forces, which are weak

Has low melting and boiling points -- because it does not take a lot of energy to break the weak forces between molecules

Does not conduct electricity -- because there are no charged particles to carry the charge

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Giant Covalent Macromolecular Compounds

Some elements -- for instance, diamond and graphite (two allotropes of carbon) naturally form giant covalent crystals.

Diamond

Diamond:

Is very strong -- because in the structure of diamond, the carbon atoms are arranged in a tetrahedral structure, which means diamond is a very rigid structure and is the hardest naturally occurring substance. Each carbon atom is covalently bonded to four other carbon atoms, so the co-ordination number of the carbon atoms is four

Has very high melting and boiling points -- because it takes a lot of energy to break the large number of strong covalent bonds

Does not conduct electricity -- because there are no electrons that are free to carry the charge as all the electrons in the outer shells of the atoms are used in covalent bonding

Is insoluble in water and organic solvents -- because there are no forces of attraction between solvent molecules and the carbon atoms that outweigh the forces of attraction between the covalently bonded carbon atoms.

Graphite

Graphite:

Has layers -- because the carbon atoms are arranged in a trigonal planar formation, which means that layers are formed. Each layer is made up of regular hexagons of carbon atoms. Each carbon atom is bonded to three other carbon atoms, so the co-ordination number of the carbon atoms is three

Conducts electricity -- because although there are four electrons in carbon's outer shell, only three are covalently bonded to other carbon atoms in graphite so there are some delocalised electrons that are free to move throughout the structure and carry the charge

Is soft -- there are only weak Van der Waals' forces between the adjacent layers, so the layers can easily slide over each other

Has high melting and boiling points -- a lot of energy is required to break the covalent bonds between carbon atoms

Is insoluble in water and organic solvents -- there are no forces of attraction between the molecules of the solvent and the carbon atoms.

[iamrevisinghonest]

Metallic Bonding

Metallic Bonding

Metallic Bonding: the force of attraction between the delocalised electrons and the positive ions in metals.

The strength of a metallic bond depends on the charge and the ions;

The higher the charge on the ion, the higher the number of delocalised electrons and the stronger the attraction between the cations and the delocalised electrons

The smaller the ion, the closer the electrons are to the nucleus and the stronger the bond

Structure of Metallic Compounds

In a solid metal, the atoms are arranged in a regular pattern called a giant lattice. The outer electrons are delocalised, so are free to move throughout the structure, but metals cannot transfer electrons unless there is a non-metal present to receive them. The delocalised electrons leave the metal atoms as positive metal ions.

Properties of Metals

Metals tend to:

Be good conductors of heat -- because of the sea of electrons and also the vigorous vibrations of closely packed ions

Be good conductors of electricity -- because of the delocalised electrons that are free to move throughout the structure; an electron from the negative terminal of the supply joins the electron sea at one end of a metal wire and a different electron leaves the wire at the positive terminal simultaneously

Be strong -- the giant lattice is a very strong structure because it is held together by delocalised electrons so there are no individual bonds to break

Have high melting points -- because of the giant structures and strong attractive forces between the positive ions and the delocalised electrons, making atoms difficult to separate

Be malleable [bendable] and ductile [possible to make into wires] -- because after being bent or distorted, each metal ion is in the same environment as before, so the new shape is retained

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Ionic Bonding

Ionic Bonding

Ionic Bond: the force of attraction between oppositely charged ions

Ions are formed by a transfer of electrons from metal atoms to non-metal atoms.

Only outer shell electrons are involved in bonding.

Metal atoms lose electrons and form positive ions (cations); Non-metal atoms gain electrons and form negative ions (anions).

Both species achieve a more stable electron structure -- often a full outer energy level, like the noble gases.

Electrovalency: the number of electrons lost or gained.

Structure of Ionic Compounds

In an ionic compound, there are forces of electrostatic attraction acting between oppositely charged ions throughout the compound. every cation is attracted to every anion and vice versa. However, this is somewhat counteracted by the repulsive force acting between identically charged ions, and this causes the structure of an ionic compound to be a lattice, in which all the forces are balanced. The ions are arranged in a regular, repeating pattern called a giant ionic lattice.

Properties of Ionic Compounds

Ionic compounds tend to:

Always be solid at room temperature

Conduct electricity when molten or dissolved in water (aqueous) but not when solid -- because, in a liquid state, the ions have the ability to move freely through the compound and carry the charge

Be brittle and easily shatter when hit with a sharp blow -- because the blow might displace the ions in the lattice, causing the pattern to be broken and ions of the same charge to come into contact with each other, causing repulsion and consequently causing the structure to shatter

Dissolve in water -- because there is a strong enough force of attraction between the water molecules and the ions to overcome the attraction between the ions themselves

Have high melting and points -- because it requires a lot of energy to overcome the strong electrostatic forces of attraction between oppositely charged ions in an ionic lattice. 

The strength of the electrostatic forces depends on the charge and the ions;

The higher the charge on the ions, the stronger the electrostatic forces

The smaller the ions, the stronger the electrostatic forces