Year 11 Chemistry Term 1 - Bonding and Structure

Atomic Radius

the distance from the nucleus to the outermost electron.

State the trend in atomic radius across a period

atomic radius decreases across a period (left to right),

Explain the trend in atomic radius across a period

Greater nuclear charge (more protons) for the same number of electron shells and the same amount of electron shielding means that the electron shells are attracted towards the nucleus, meaning a smaller atomic radius.

State the trend in atomic radius down the group

atomic radius increases down a group

Explain the trend in atomic radius down a group

More electron shells get added down a group, so naturally the atomic radius will increase

Ionisation Energy

The energy required to remove an electron from an atom or ion

First ionisation Energy

The energy required to remove the outermost electron from 1 mol of atoms of an element in its gaseous state.

State the trend in first ionisation energy across a period

first ionisation energy Increases across the period

Explain the trend in first ionisation energy across a period

As you move down the period increase in nuclear charge, an increase in core charge occurs as more protons are added to the nucleus but the amount of shells remains the same in the period. Therefore, the electrons are attracted greater to the nucleus and require more energy to remove an electron.

State the trend in first ionisation energy down a group

Ionisation energy decreases down a group

Explain the trend in first ionisation energy down a group

Because the atomic radius increases down a group, the valence electrons attraction to the nucleus down a group decreases as a result of more electron shielding and shells being added. Therefore, they require less energy to remove an electron from the valence shell.

Electronegativity

The tendency of an atom to attract electrons in a molecule.

State and explain the general trend of electronegativity

Metals have a low EN not only due to their weak hold on valence electrons but also due to low tendency to attract electrons. Non-metals have a high EN because they not only have a strong hold on their valence electrons but because they have a high tendency to attract electrons.

State the trend of electronegativity across a period

Electronegativity increases across a period.

Explain the trend of electronegativity across a period

As the core charge increases across a period due to more protons in the nucleus but the same number of shells, extra electrons in the outer shell for successive elements does not contribute to shielding. Additionally, the transition from metals to non-metals seen across a period is also why this trend occurs.

State the trend of electronegativity down a group

Electronegativity decreases down a group.

Explain the trend of electronegativity down a group

As more shells are added, the attraction the nucleus has to its valence electrons decreases, and electron shielding increases. Therefore, it is harder for the nucleus to attract electrons.

Metallic Character

a measure of how likely an element loses a valence electron, and therefore how similar the element's properties are to a metal.

Difference between metallic character and ionisation energy

metallic character is the likelihood of an element to lose a valence electron while ionisation energy is the energy required to remove the valence electron of an element.

Trend of reactivity and ionisation energy in non-metals.

Higher ionisation energy means higher reactivity for non-metals as more electrons in the valence shell means that a non-metal element will more vigourously want to gain electrons and also have a higher ionisation energy as it has a higher core charge.

Trend of reactivity and ionisation energy of metals.

Lower ionisation energy means higher reactivity for metals. This is because metals have a weak hold on valence electrons, so the fewer valence electrons it has (lower ionisation energy), the more vigourously it will want to lose its electrons, therefore becoming more reactive.

Na(g) + first ionisation energy required (E1) -\>

Na+(g) + e-

Formal charge word equation

valence electrons of middle atom - (number of bonds + remainder electrons after dividing by 8)

formal charge formula

FC \= VE - (B + d)

how to find number of bonds using Formal charge equation

make FC \= 0 and fill in the rest of the info to find B

using formal charge to find number of bonds of SO3

0 \= 6 - (B + 0), therefore B \= 6 (number of bonds \= 6)

Group 1

alkali metals

Group 2

alkaline earth metals

Group 17

Halogens

Group 18

Noble gases

Mass Spectrometry

a method used to determine the different masses, molecular structure and relative abundance of isotopes or elements (by relative mass) of atoms in a given sample.

Steps of Mass Spectrometry

Vapourisation, ionisation, acceleration, deflection, and detection

Vapourisation (Mass Spectrometry)

the sample is vapourised so it can be observed in a vacuum chamber, it must be in its gaseous state

Ionisation (Mass spectrometry)

because mass spectrometry only measures the mass of charged particles, the sample must be ionised. A high energy electron beam is shot at the sample, and electron(s) may be ejected from the molecule (forming cations) or captured (forming anions). Mass spectrometers can be set up to detect both positive or negative ions (but not both at once). The mass of the molecular ion (+ or -) is essentially the same as the mass of an electron is negligible.

Acceleration (mass spectrometry)

Acceleration by an electric field to form high speed ions. Uses the property of repulsion of like charges. Because it functions off creating electric and magnetic fields to manipulate ions, ionisation of the sample is required,

deflection (mass spectrometry)

Deflection is based on mass and charge by a magnetic field (created by an electromagnet). Ions too slow or too fast will not exit at the other end (meaning all exiting ions have the same speed). Separation is based on mass/charge ratio (heavier masses aren't deflected as much as lighter masses, more charged particles are deflected by a greater amount)

Detection (Mass spectrometry)

A negatively charged plate, therefore when a positive ion hits the plate a current is formed.

The size of the current gives you the relative abundance

Detection of the isotope abundance (ions that strike) based on detector count and beam deflection related to the isotopic mass and charge (m/z ratio), which is displayed in a graphical format as a mass spectrum.

Uses of Mass Spectrometry

it is an analytical instrument, that is used in scientific research, industry and forensic analysis (PEDs, illegal substances in hair, blood or urine samples). Used to Identify unknown compounds, find relative abundance of each isotope of an element, determine structural information

Advantages of Mass spectrometry

It is very sensitive, so a very small amount of a sample can be used for analysis. It detects different isotopes and molecular masses, which can be used to detect unknown compounds and purity of the sample.

Disadvantages of mass spectrometry

Very expensive, Large in size, Requires lots of maintenance, hydrocarbons that produce similar ions are not detected.

Rutherford's Experiment

Alpha particles were shot at a thin piece of gold foil. The pass-through of most of these alpha particles allowed him to conclude that atoms are mostly empty space. The deflection of some of these rays also allowed him to conclude that positively charged particles exist in a concentrated manner, known as the nucleus where most of the atom's mass exists,

Absorption Spectrum

Occurs when light is passed through an atom or molecule and some light is absorbed, leaving dark lines at some wavelengths.

Atomic Absorption Spectroscopy

An analytical technique used for quantitative analysis for determining the unknown concentration of an element based on the unique property of different elements in the gaseous state each absorbing different wavelengths of light, which can be compared to standards. Used to find metals in compounds.

Calibration curve

A graph showing the value of absorbance versus concentration of the substance being analysed. When the corresponding property of an unknown is measured, its concentration can be determined from the graph.

Uses of Atomic Absorption Spectroscopy

analyse metals as low as 1 part per billion (ppb), and analyse the concentration of toxic heavy metals or other metal ions in water samples such as from bore water, streams, lakes or drinking water, finding concentrations of gold, copper and silver in mineral samples (for mining).

Emission Spectra of Atoms

Each element emits different frequencies of light and absorb those same frequencies in its gaseous state. The colour that the gas produces depends on the colour emitted and the quantity of each colour emitted.

How does AAS work (detailed)

- When testing substances containing many elements, normal light will not suffice as they will all absorb light - the lamp is therefore made of the same element being tested. E.g if zinc being tested lamp is zinc.
- Because the lamp is made of a single element, the lgiht emitted has the unique set of wavelengths particular to that element.
- The sample is vapourised by the burner, and as the light from the lamp passes through the vapourised sample, only the element being tested will absorb light from the lamp while other elements in the sample will not absorb this light because the energy levels if all other atoms are different and their electrons can't absorb energies of the element specific light.
- The amount of light absorbed will depend on how much of the element being tested is found in the sample.
- The light is focused before entering a monochromator, where only one wavelength of the light is selected for analysis by the detector.
- The detector measures the intensity of the light, which is then displayed as a number that is a measure of the amount of light that passed through the sample without being absorbed, known as an absorbance value. Absorbance is then compared to standards on a calibration curve.

How does AAS work (simple)

The light from the emission source passes through a sample that has been aspirated and atomised into a flame.

A monochromator is used to select a unique wavelength of light for analysis of the metallic element.

A detector is then used to measure the amount of light that has been absorbed by the metallic element from the sample.

- A hollow cathode lamp is used as a light source, with the metal of the cathode being the metal being analysed from the test substance. The light produced is of precise wavelengths that can be absorbed by the target metal atoms alone.
- The test sample is aspirated (sucked up) into the gas/air stream supplying the flame, which ensures the sample is decomposed and atomised. In this form, target metal atoms freely absorb light from the light beam directed through it.
- A wavelength filter selects a desired wavelength of light from the exiting beam and the amplifier/signal detector determines the extent to which it has been absorbed, called the absorbance value, which is proportional to concentration of target metal atoms

How do flame tests work?

When energy is added to electrons in an atom (either by direct heating or light) they jump up to a higher energy level, known as being in the excited state. As these electrons return to their ground state energy level (may be more then one energy level) they release extra energy as a photon of electromagnetic radiation.

Monochromator

A device for isolating individual wavelengths or frequencies of light.

AAS Brief Explanation

1. electron(s) are excited and return to the ground state, releasing photon(s) of energy it absorbed as light.
2. Selected sample is vaporised and atom chosen for detection
3. Monochromator selects desired wavelengths of light
4. Detector provides absorbance value for desired atom
5. Absorbance value is compared to standards to determine concentration

Purpose of Bonding

Atoms will try and to become stable by obeying the octet rule; that is, the valence electron configuration of the nearest noble gas. Atoms will either gain or lose electrons.

Metallic Bonding

The electrostatic attraction between delocalised electrons and positive metal ions arranged in a 3D lattice. These electrons join up to form a mobile cloud which prevents newly-formed positive ions flying apart due to repulsion of similar charge. Non-directional bond, meaning the attraction is equal in all directions between all metal cations.

Strength of Metallic Bonding Factors

- depends on the number of outer electrons donated to the cloud and size of the metal/ion. The strength of metallic bonding in sodium is relatively weak due to sodium atoms only donating one electron to the cloud. Potassium has a weaker metallic bond due to a larger atomic radius so the electron cloud has a bigger volume to cover so is less effective at holding ions together. The metallic bonding of magnesium is stronger then sodium because each atom has donated two electrons to the cloud (greater electron density). More attraction means more energy needed to break bonds (melting and boiling points).

Properties of Metals

Good conductors of electricity and heat, malleable, ductile, lustrous

Why are metals good conductors of heat and electricity

For a substance to conduct electricity it requires the flow of mobile charge carriers (ions or electrons). Becausd the electron cloud is mobile in metals, electrons are free to move throughout its structure. Electrons become attracted to the positive terminal. The mobile electron cloud also conducts heat well as the mass of electrons is negligible compared to the rest of the lattice, so any collisions between these electrons will cause a very large vibrational velocity, allowing the transfer of kinetic energy to be very fast along a metal.

Why can metals have their shape changed easily.

- Metals are malleable and ductile: They can be hammered into sheets. When enough force is applied metallic bonds are non-directional, and the layers of metal cations can simply slip over each other, but the electrostatic attraction between delocalised electrons and metal cations still operate so the overall structure does not change.

Why do metals have high melting and boiling points?

Melting point is a measure of how easy it is to separate individual particles. The strong electrostatic bonding means that melting and high boiling points will be very high as the attraction of opposite charges is very strong. The difference in melting and boiling points across many metals is due to the number of valence electrons and atomic radius.

Why do metals have high density?

Strong electrostatic bonding and close packing of the ions means that metals are generally very dense.

Ionic Bonding

Electrostatic attraction between oppositely charged ions, causing the transfer of electrons from one to the other so both can reach the octet state. Arranged in a large 3D lattice by elecrostatic attraction. positive cations are placed between negative anions and distance between like charged ions is greater then distance of oppositely charged ions.

Cation

A positively charged ion that has a lower atomic radius as a result of removal of electrons

Trend of melting points in metals

Increases across the periods and decreases down the groups

Anion

a negatively charged ion that has gained electrons

Core Charge

A measure of the attractive force felt by the valence shell electrons towards the nucleus

Electron Shielding

the reduction of the attractive force between a positively charged nucleus and its outermost electrons due to the cancellation of some of the positive charge by the negative charges of the inner electrons

Physical Properties of Ionic Compounds

High melting point: large amount of energy must be put in to overcome the strong electrostatic attraction to separate ions.
Electrical conductivity: Does not conduct when solid as there is no mobile charge carriers; ions are held strongly in the lattice. Conducts electricity when molten or aqueous solution - ions become mobile and conduction takes place as they are free to move.
Solubility: Insoluble in non-polar solvents but can be soluble in water. Soluble in water as it is polar and stabilises separated ions.
Brittle: If enough force is applied, the lattice will move so that like charged ions are placed next to each other. These like charges will repel each other, and the 3D lattice will shatter.

Covalent Bonding

a directional bond that results from the sharing of electron pairs between two non-metal atoms as a result of both atoms nuclei having a strong attraction on valence electrons and therefore sharing instead of transferring.

Covalent Molecular

Composed of discrete molecules in which the atoms are held together by covalent bonding. Strong covalent bonds within molecules are known as intramolecular bonds while weak bonds between neighbouring molecules are intermolecular forces.

Covalent Network

A large 3D network of covalently bonded atoms in a continuous array with strong covalent bonds throughout the structure.

Properties of Covalent Molecular Substances

low melting and boiling points (many liquid or gas at room temp): when a covalent molecular substance melts or boils, it is only the weak intermolecular forces that are being broken, the strong intramolecular forces remain bonded in all states. This results in low melting and boiling points
non-conductors of electricity in solid and liquid states (some exceptions): No mobile charge carriers results in a poor conductivity of electricity, and no charged particles exist
form solids that are generally soft: weak intermolecular forces can be easily separated from eachother and structure can be manipulated.
Solubility: tend to be more soluble in organic solvents then in water; some are hydrolysed.

Properties of Covalent Network Substances

Very high melting point (usually solid at room temp): covalent bonds that hold bonds together is strong, lack of intermolecular forces makes it need a lot of energy to break the bond.
Non-conductors of electricity in solid and liquid states: there are no charged particles that are free to move throughout the structure
extremely hard and brittle solids: the bonds are very strong between atoms so it is difficult to scratch, but an impact force disrupts the positions of the atoms and causes the network to shatter
insoluble in water and most other solvents: there is no attraction between the atoms in the network and water molecules.

Trend about intermolecular forces in covalent molecules

Intermolecular forces increase down groups because more electrons are added.

Nanoparticle

a particle that has at least one dimension in the range of 1-100 nanometers.

Nanotechnology

The science and technology of building devices, such as electronic circuits, from single atoms and molecules. Materials of nanoparticle size have special properties that can be manipulated in order to perform specialised tasks.

Applications of nanotechnology

medicine, materials fabrications, electronics, energy production. carbon nanotubes (electronics and optics) and gold (used in electron microscopy, electronics and materials science)

Carbon Nanotubes (CNTs)

Tiny, hollow tubes made by rolling up sheets of graphene with a diameter as low as 1nm. In this form, carbon has the highest strength-to-weight ratio of any known material

Nanosilver

unknown environmental and health risks

Colloidal Gold

- At nanoparticle size, gold looks and behaves very different to normal gold.
- the most important property of colloidal gold is its ability to affect light in a predictable, measurable manner. Scientists can look at how light behaves around the particles and use this information in a number of ways.

Allotropes

two or more different molecular forms of the same element in the same physical state. e.g diamond is a tetrahedral structure while graphite has a flat-graphene sheet hexagonal structure. both are made up of covalently bonded carbon atoms.

Bohr’s Contribution

Due to the Rutherford Model accounting poorly for emission spectra of atoms, and also physics wise meant that atoms would be unstable (it showed orbitting electrons would constantly emit radiation and collapse into the nuclei). Therefore, Bohr proposed a new model to the atom (Bohr’s model) which accounted for the line spectra of elements and unstable electron orbits.

Dalton’s Contribution

He proposed an atomic theory:

* elements are composed of extremely small particles called atoms,
* all atoms of a given element are identical (same properties and size)
* Atoms are not created nor destroyed or changed during chemical reaction
* Chemical reaction only involves separation, combination or rearrangement of atoms
* Compounds are formed when atoms of more then one element combine in a specific ratio

Addition Reaction Hydrocarbons

Ethene + Br2 makes Dibromoethane

• occurs fast

Substitution Reactions

• Benzene + Cl2 makes Chlorobenzene + Hydrogen Chloride

• Methane + Cl2 makes Chloromethane + HCl