Module 2 Section 2: Electrons, Bonding and Structure Flashcards
Nature of electrons and how they move around the nucleus
Electrons have fixed energies
They move around the nucleus in shells ( sometimes called energy levels )
These shells are all given numbers known as principal quantum numbers
How does energy change in shells with distance from the nucleus
Shells further from the nucleus have a higher energy ( and a larger principal quantum number ) than shells closer to the nucleus
Arrangement of electrons
The electrons shells are divided into sub - shells
Different electron shells have different numbers of sub-shells, each of which has a different energy
Sub-shells are called s-, p-, d- or f sub shells ( just need to acknowledge f-sub-shell )
These sub-shells have different numbers of orbitals, which can hold up to 2 electrons
The numbers of electrons that can fit into each sub shell
S sub-shell: 1 orbital, 2 max electrons ( 1 x 2 = 2)
P sub-shell: 3 orbitals, 6 max electrons (3 x 2 = 6)
D sub-shell: 5 orbitals, 10 max electrion (5 x 2 = 10)
F sub shell: 7 orbitals, 14 max electrons (7 x 2 = 14)
Structure of first 4 shells
1st: 1s - 2 total electrons
2nd: 2s 2p - 8 total electrons
3rd: 3s 3p 3d - 18 total electrons
4th: 4s 4p 4d 4f - 32 total electrons
What is an orbital
A region within an atom (around the nucleus - spec) that can hold up to two electrons with opposite spin
Orbitals within the same sub-shell have the same energy
What happens when there are two electrons in the same orbital
They must ‘ spin ‘ opposite directions
This is called spin-pairing
Shapes of orbitals
s orbitals are spherical
p orbitals have dumbbell shapes
There are three p orbitals and they’re at right angles to one another
What is electron configuration
The number of electrons an atom or ion has, and how they are arranged
Different ways to show electron configuration
Sub shell notation: 1s^2 2s^2 2p^6
Big number: energy shell level ( principal quantum number )
Letter: sub shell
Superscript: number of electrons
Electrons in boxes
Each box represents an orbital and each arrow is an electron
The up and down arrows show electrons spinning in opposite directions
Two electrons can only occupy the same orbital if they have opposite spin
E.g. 1s: ⬆️⬇️, 2p: ⬆️⬇️ ⬆️⬇️ ⬆️⬇️
Exception with 4s and 3d sub shell
Even though 4s’ quantum number is bigger than 3d, it has a lower energy level than 4s
This means 4s fills up first
This has exceptions as well ( copper and chromium )
How to fill out an atoms electron configuration
Electrons fill up the lowest energy sub-shells first
Electrons fill orbitals with the same energy singly before they start sharing
For the configuration of ions from the s and p blocks of the periodic table, just remove or add the electrons to or from the highest energy occupied sub shell
E.g. Mg2+: 1s2 2s2 2p6
E.g. Cl-: 1s2 2s2 2p6 3s2 3p6
note that when losing electrons, electrons are lost from 4s before 3d (the energy levels are very close, and when electrons fill them, 4s goes above 3d). “
How can noble gases be used in electron configuration
Can be used in shorthand configuration
E.g. calcium: 1s2 2s2 2p6 3s2 3p6 4s2 can be written as [Ar]4s2
( Ar: 1s2 2s2 2p6 3s2 3p6 )
How are ions formed
When electrons are transferred from one atom to another so as to have full outer shells
They may positively or negatively charged
How are ions held together
Electrostatic attraction holds the positive and negative ions together
When atoms are held together like this, it’s called ionic bonding
What is ionic bonding
An ionic bond is an electrostatic attraction between two oppositely charged ions
What happens when two oppositely charged ions bond
When oppositely charge ions form an ionic bond, you get an ionic compound
The formula for an ionic compound tells you what ions that compound has in it
E.g. KBr is made of K+ and Br- ions
Why are ionic compounds neutrally charged
The positive charges in the compound balance the negatively charges exactly so the overall charge is zero
How are dot and cross diagrams helpful
They show the arrangement of electrons in an atom or ion
Each electron is represented by a dot or a cross
They can show which atom the electrons in a bond originally came from
Structure of ionic compound e.g. sodium chloride
The Na+ and Cl- ions are packed together alternately in a regular structure called a lattice
The structure’s called ‘ giant ‘ because it’s made up of the same basic unit repeated over and over again
It forms because each ion is electrostatically attracted in all directions to ions of the opposite charge
The sodium chloride lattice is cube shaped - different ionic compounds have different shaped structures
Sodium chloride’s got very strong ionic bonds, so it takes lots of energy to break up the lattice
This gives it a high melting point (801°C)
When do ionic compounds conduct electricity
Ionic compounds conduct electricity when they’re molten or dissolved - but not when they’re solid
The ions in a liquid are mobile (and they carry a charge)
In a solid they’re fixed in position by the strong ionic bonds
Why do ionic compounds have high melting and boiling points
The giant ionic lattices are held together by strong electrostatic forces
It takes a lot of energy to overcome these forces, so their melting and boiling points are very high
Why do ionic compounds dissolve in water
Water molecules are polar - part of the molecule has a small negative charge and the other parts have small positive charges
The water molecules are attracted to the charged ions
They pull the ions away from the lattice and cause it to dissolve
(When ions in compound have a large charge it will be insolvable and sparingly soluble because the attraction may be too strong for the molecules to break down the ionic structure.)
When do molecules form
Molecules form when two or more atoms bonds together
It doesn’t matter is the atoms are the same or different
Examples of common molecules
Chlorine gas: Cl2
Carbon Monoxide: CO
Water: H2O
Ethanol: C2H5OH
How are molecules held together
Molecules are held together by covalent bonds
In covalent bonding, two atoms share electrons, so they’ve both got full outer shells
What is a covalent bond
A covalent bond is the strong electrostatic forces of attraction between a shared pair of electrons and the nuclei of the bonded atoms
What are the exceptions that we must know for covalent molecules
A few compounds have less than 8 electrons in their outer shell ( electron deficient )
In boron trifluoride, boron only has 6 electrons in its outer shell
Some compounds can use d orbitals to expand the octet
This means they have more than 8 electrons in their outer shell
In sulfur hexafluoride, sulfur has 12 electron in its outer shell
What is average bond enpalthy
Average bond enpalthy measures the energy required to break a covalent bond
The stronger the bond is, the more energy is required to break it, and so the greater the value of the average bond enpalthy
Covalent double and triple bonds
Some atoms can form double and triple covalent bonds
These multiple bonds contain more than one shared pair of electrons between two atoms
An example of a double bond is oxygen ( O2 )
What type of covalent bonds is measured in nitrogen and carbon dioxide molecules
Nitrogen has 5 electron in it’s outer shell, so it needs another 3 to have a full outer shell
This is done by each nitrogen atom shares 3 electrons - resulting in a triple bond
Carbon has 4 elections in its outer shell, so it needs another 4 to have a full outer shell
This means each oxygen atom must share 2 electrons
Atoms and how many covalent bonds they make
Hydrogen: 1
Oxygen: 2
Nitrogen: 3
Carbon: 4
What is dative covalent bonding and how are ammonium ions formed
Dative covalent bonding is where both electrons come from one atom
The ammonium ion ( NH4+ ) is formed by dative covalent ( or coordinate ) bonding
It forms when the nitrogen atom in an ammonia molecules donates a pair of electrons to a proton ( H+ ion )
How is dative covalent bonding shown in a displayed formula
An arrow pointing towards the dative covalently bonded hydrogen atom
What do the shapes of molecules depend on
Molecules and molecular ions come in different shapes
The shape depends on the number of pairs of electrons in the outer shell of the central atom
How do electron pairs repel each other and how does this affect the shape of the molecule
Electrons are negatively charged, so electron pairs will repel eachother as much as they can
The type of electron pair affects how much it repels other electron pairs
Lone pairs repel more than bonding pair
This means the greatest angles are between lone pairs of electrons, and bond angles between bonding pairs are often reduced because they are pushed together by lone pair repulsion
How do the different pairs compare with repulsion
Lone pair/lone pair angles are the biggest
Lone pair/bonding pair angles are the second biggest
Bonding pair/bonding pair angles are the smallest
What does the shape of the molecule depend on
The shape of the molecule depends on the type of electron pair surrounding the central atom as well as the number
This way of predicting molecular shape is known as ‘electron pair repulsion theory’
What are some examples of electron pair repulsion theory being used
Methane, ammonia and water molecules all have 4 pairs of electrons in their outer shells, but they’re all different shapes
Methane: no lone pairs, all bond angles are 109.5° - tetrahedral
Ammonia: 1 lone pair, all three bond angles are 107° - pyramidal
Water: 2 lone pairs, the bond angle is 104.5° - non-linear
The bond angle decreases by 2.5° with every lone pair
How to use electron pairs to predict the shape of molecules
Find the central atom (the one all the other atoms are bonded to)
Work out the number of electrons in the outer shell of the central atom ( using periodic table )
The molecular formula tells you how many atoms the central atom is bonded to and from this you can work out how many electrons are shared with the central atom
Add up the electrons and divide by 2 to find the number of electron pairs on the central atom
Remember to account for the charge if it is an ion
Compare the number of electron pairs with the number of bonds ( if there is a double bond it counts as two bonds )
You can then use the number of electron pairs and the number of lone pairs and bonding centres around the central atom to work out the shape of the molecule
List of the most electronegative atoms
Fluorine: 4
Oxygen: 3.5
Nitrogen: 3
Chlorine: 3
Carbon: 2.5
Hydrogen: 2.1
How is electronegativity measured
Measured on the Pauling scale
The greater the elements Pauling value, the higher it’s electronegativity
Pattern with electronegativity and the periodic table
Increases across periods and decreases down groups
This ignores the noble gases
How can covalent molecules become polar
In a covalent bond between two atoms of different electronegativities (generally with a difference of 0.4 or more on the Pauline scale), the bonding electrons are pulled towards the more electronegative atom
This makes the bond polar
What is a dipole
A dipole is a difference in charge between the two atoms caused by a shift in electron density in the bond
The greater the difference in electronegativity, the more polar the bond
How are permanent dipoles created
In a polar bond, the difference in electronegativity between the two atoms causes a permanent dipole
What is the polarity of a diatomic gas
The covalent bond in diatomic gases (e.g. H2 and Cl2) are non-polar because the atoms have equal electronegativities and so the electrons are equally attracted to both nuclei
Why are bonds between elements such as carbon and hydrogen non-polar
Carbon and hydrogen have similar electronegativities, so bonds between them are essentially non-polar
What happens if the polar bonds are arranged symmetrically
The dipoles will cancel eachother out
This means that the molecule has no overall dipole and is non-polar
E.g. carbon monoxide contains two polar bonds but has no overall dipole moment
What happens if the polar bonds are arranged so they don’t cancel eachother out
The charge is arranged unevenly across the whole molecule
It will have an overall dipole
Molecules with an overall dipole are polar
E.g. water is polar because the negative charge is positioned more towards the oxygen atom
How to work out if a molecule is polar
You must draw it in 3D
Then label the partial charges on each atom, and look to see if they cancel eachother out
What are the only purely covalently bonded molecules
Only bonds between atoms of a single element, like diatomic gases such as hydrogen (H2) or oxygen (O2)
This is because the electronegativity difference between the atoms is 0
So the bonding electrons are arranged completely evenly within the bond
If only a few compounds are completely ionic, what are the properties of most compounds
Most compounds are somewhere in between the two extremes
They’ve got ionic and covalent properties
E.g. covalent hydrogen chloride gas molecules dissolve to form hydrochloric acid, which is an ionic solution
How can electronegativity be used to predict bonding between two atoms
The higher the difference in electronegativity, the more ionic in character the bonding becomes
Table to decide what type of bond is present in a molecule from the difference in electronegativity
Covalent: 0
Polar Covalent: 0 - 1.8
Ionic: above 1.8
Names of the shapes of molecules depending on the number of bonds
3 bonds made, no lone pairs: trigonal planar ( each angle 120° ) e.g. boron trifluoride
2 bonds made, no lone pairs: linear ( each angle 180° ) e.g. carbon dioxide
4 bonds made, no lone pairs: tetrahedral ( each angle 109.5° ) e.g. methane
6 bonds made, no lone pairs: octahedral ( each angle 90° ) e.g. sulfur hexafluoride
What are intermolecular forces
Forces between molecules
They’re much weaker that covalent, ionic or metallic bonds
There are three types:
induced dipole-dipole or London dispersion forces
Permanent dipole-dipole interactions
Hydrogen bonding
List of the intermolecular forces in order of strength
Induced dipole-dipole or London dispersion forces - weakest
Permanent dipole-dipole interactions
Hydrogen bonding - strongest
What are induced dipole-dipole forces
These forces cause all atoms and molecules to be attracted to each other
Noble gases are affected as well, despite not needing to form other types of bonds
How do induced dipole-dipole interactions work
Electrons in charge clouds are always moving quickly
At any one moment, the electrons in an atom are likely to be more to one side than the other. At this moment, the atom would have a temporary dipole
This dipole can cause another temporary ( induced ) dipole in the opposite direction on a neighbouring atom. The two dipole are then attracted to one another
This second dipole can cause another dipole in a third atom
Because the electrons are constantly moving, the dipoles are being created and destroyed all the time
Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to eachother
When are induced dipole-dipole forces stronger
Larger molecules have larger electron clouds, meaning stronger induced dipole-dipole forces
Molecules with greater surface areas also have stronger induced dipole-dipole forces because they have a bigger exposed electron cloud
What is the pattern with substances with stronger induced dipole-dipole forces and boiling points
The stronger the induced dipole-dipole forces, the higher the boiling point
This is because you must overcome the intermolecular forces to boil a substance
So you need more energy to break stronger intermolecular forces
How can induced dipole-dipole forces create lattices
Example: induced dipole-dipole forces are responsible for holding iodine molecules together in a lattice
Iodine atoms are held together in pairs by strong covalent bonds to form molecules of I 2
The molecules can then be held together in a lattice arrangement by weak induced dipole-dipole interactions
What are permanent dipole-dipole interactions
The δ+ and δ- charges on polar molecules cause weak electrostatic forces of attraction between molecules
These are called permanent dipole-dipole
These can happen in addition to induced dipole-dipole interactions
When do hydrogen bonds occur
Can only happen when hydrogen is covalently bonded to fluorine, nitrogen or oxygen
e.g. H-F, H-N, H-O
Nitrogen, oxygen or fluorine must have a lone pair
How do hydrogen bonds occur
Hydrogen has a high charge density because it’s so small, and fluorine, nitrogen and oxygen are very electronegative
The bond is so polarised that a weak bond forms between the hydrogen of one molecule and a lone pair of electrons on the fluorine, nitrogen or oxygen in another molecule
How do hydrogen bonds effect the properties of substances
They are soluble in water and have high boiling and freezing points than molecules of a similar size that are unable to form hydrogen bonds
E.g. water, ammonia and hydrogen fluoride generally have the highest boiling points if you compare them with other hydrides in their groups, because of the extra energy needed to break hydrogen bonds
How do hydrogen bonds make the properties of water anomalous
In ice, molecules of H2O are held together in a lattice by hydrogen bonds
When ice melts, hydrogen bonds are broken, so ice has more hydrogen bonds than liquid water
Since hydrogen bonds are relatively long, this makes ice less dense than liquid water
How do intermolecular forces explain trends in boiling points
Main factor to determine boiling points are the strength of the induced dipole-dipole forces ( unless molecules can form hydrogen bonds )
This can explain why BP of group 7 hydrides increase from HCl to HI:
Although permanent dipole-dipole interactions are decreasing, the number of electrons in molecules increases, so the strength of induced dipole-dipole interactions also increases
If you have two molecules with a similar number of electrons, the strength of their induced dipole-dipole interactions will be similar
So if one of the substances has molecules that are more polar than the other, it will have stronger permanent dipole-dipole interactions and so a higher boiling point
Why do simple covalent compounds have low melting and boiling points
Intermolecular forces that hold together the molecules in simple covalent compounds are weak so do not need much energy to break
So melting and boiling points are normally low - often liquids or gases are room temperature
As intermolecular forces get stronger, melting and boiling points increase
How are polar molecules soluble in water, what about molecules with only induced dipole?
Water is a polar molecule, so only tends to dissolve other polar substances
Compounds with hydrogen bonds, such as ammonia or ethanoic acid, can form hydrogen bonds with water molecules, so will be soluble
Molecules that only have induced dipole-dipole forces, such as methane, will be insoluble, but will be soluble in other substances with induced dipole-dipole forces
Like dissolves like
Why don’t simple covalent compounds conduct electricity
Even though some covalent molecules have permanent dipoles, overall covalent molecules are uncharged
This means they can’t conduct electricity
What is electronegativity
An atoms ability to attract the electron pair in a covelant bond
Exceptional structure of copper and chromium
Cr: 1s2 2s2 2p6 3s2 3p6 3d5 4s1
Prefers to have one election in each orbital of 3d subshell and one in the 4s subshell to make it more stable
Cu: 1s2 2s2 2p6 3s2 3p6 3d10 4s1
Prefers to have full 3d subshell and one electron in the 4s subshell for stability
Definition of a salt
The substance formed when the H+ in an acid is replaced by a metal ion
Definition of a base
A substance that accepts a proton/H+ ion when reacting with an acid
What is an ionic equation
An ionic equation is a chemical equation in which the formulas of dissolved aqueous solutions are written as individual ions.
How to tell if a molecule will be polar (cb)
Molecules of type AXn (atom A is bonded to two or more atoms of X) may be nonpolar if atom A is at the center of the group of X atoms and therefore the distribution of charge is symmetrical. If this is true, there is no dipole moment because the center of partial negative charge and partial positive charge are both at central atom A.
(When non polar can be thought of as the sum of the vectors being 0)
