Atomic structure

Important terms: quantum shells, principle quantum number, energy levels,…

Calculate the number of protons, electrons and neutrons in…

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IsotopesDefinitions?

Remember - isotopes have exactly the same chemical properties. Why?

Questions p11

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Relative massesand mass spectrometry

Relative isotopic mass = mass of one particular atom of an element/one twelfth mass of one carbon-12 atom

RELATIVE ATOMIC MASS Ar

The average mass of the atoms of an element depending on the ABUNDANCE of possible ISOTOPES (again measured on a scale where 1 unit is one twelfth mass of one carbon-12 atom)

NB. The former is used to calculate the latter!

What problem did you have with Chlorine when you tried to determine the number of protons and neutrons?

Chlorine has 2 isotopes: 35Cl (75% abundance) and 37Cl (25%)

Calculate Ar for Chlorine:

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More calculations

Problems:

Calculate the relative atomic mass of the following elements given the isotopes and their abundances.

32S (95%), 33S (0.67%) and 34S (4.2%).

12C (98.9%), 13C (1.1%) and 14C (trace).

10B : 11B = 1 : 4

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The Mass Spectrometer

This instrument is used to determine the Ar, Mr, information about isotopes and information about the structure of the molecule in question.

The following diagram shows the principles of the low resolution mass spectrometer:

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Principles of the mass spectrometer:

1. The tungsten filament emits electrons, they are accelerated and they collide with the gaseous sample of the element or compound at low pressure.

2. Some atoms or molecules have electrons knocked off them by the bombarding electrons, forming positive particles (extra electrons are attracted by the positive plate).

3. The positive particles (ions) are accelerated by the electric field (NB. Very low pressure, to avoid collisions).

4. The ions are deflected by the magnetic field in a circular path. The radius of this path depends on the mass/charge (m/e) ratio and the strength of the magnetic field, which is altered so that a range of ions can be detected. The lighter the ion, the more the deflection. The charge is assumed to be +1.

5. The ions are detected here and the relative amounts are calculated automatically by the instrument.

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And we get something like this….

What is it and what is its Ar?

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Mass Spectra of Diatomic molecules

Chlorine

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Mass Spectra of Diatomic molecules

Bromine

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Mass spectra of molecules The peak furthest to the right (ignoring small peaks) corresponds to the molecular ion, M+ (m/z = 46), where one electron has been lost without fragmentation of the molecule.

This is sometimes the largest peak, however, this is not always the case as you can see from the spectra for ethanol.

The molecular ion peak is sometimes called the “parent ion” peak.

When isotopes are present, there will be more that one parent ion.

The M+1 peak is due to C-13 atoms

Draw this molecule and suggest how it has fragmented.

Questions p15

Applications: Radioactive dating Space research Drugs testing Identification of synthesised compounds

https://www.youtube.com/watch?v=J-wao0O0_qM

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Where the electrons are (or aren’t) …We have said that electrons are found in quantum shells around the nucleus, but the Bohr model is far from the truth.

Electrons within a quantum shell are found in different configurations and have different energy levels. These are called SUBSHELLS.

SUBSHELLS are made up of ORBITALS, each of which can hold 2 electrons.

These orbitals have distinctive shapes and characteristics. These

are sometimes referred to as electron density plots (or maps):

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- The s orbitals are spherical with the nucleus at the centre. They can hold a maximum of 2e.

- The p orbitals (px py pz) are a dumbell shape with the nucleus at the midpoint of the axis (p – principal). They each hold a maximum of 2e.

- There are 3 p orbitals in each shell after the first shell, so the p orbitals in a shell are made up of a maximum of 6e

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The d orbitals (d – diffuse) exist after the 2nd shell (maximum 10e) and the f – orbitals (f – fundamental) after the 3rd shell (maximum 14e).

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http://winter.group.shef.ac.uk/orbitron/AOs/2s/index.html

f - orbitals

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Electronic Configuration

This is a system for showing the electrons that make up the orbitals of an atom.

The possible orbitals that exist in each shell are:

1st shell 1s 2nd shell 2s, 2p 3rd shell 3s, 3p, 3d 4th shell 4s, 4p, 4d, 4f etc.

but they are not filled up in this order!!

The key question is one of ENERGY.

When an electron is added to an atom, it will go into the next available orbital with the lowest energy (which does not necessarily mean the closest orbital to the nucleus).

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NB. Electrons spin. 2 electrons in the same orbital spin in opposite directions producing 2 magnetic fields, clockwise and anti-clockwise, effectively canceling each other out resulting in the lowest possible energy state. We say that the spins are “paired” and we use the arrows to represent this.

Hund's Rule. Hund's rule: every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.

The Aufbau principle states that, hypothetically, electrons orbiting one or more atoms fill the lowest available energy levels before filling higher levels (e.g., 1s before 2s). In this way, the electrons of an atom, molecule, or ion harmonize into the most stable electron configuration possible.

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We classify the elements in the periodic table as s-block, p-block, d – block and f-block, according to where their valence electrons are (or really, according to which orbital the last electron went into):

Give examples of elements in each of these blocks and write their electronic configuration as electrons in boxes:

s-block elements:

p-block elements:

d-block elements: Questions p19

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Ionisation EnergiesEmission spectra gave evidence that energy

levels existed…

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First and second ionisation energies

First ionisation energy: the energy required to remove one mole of electrons from one mole of the gaseous atoms of an element to form 1 mole of gaseous mono-positive ions.

Eg. Na(g) Na+(g) + e- H = +496 kJ mol-1

Eg. He(g) He+(g) + e- H = +2372 kJ mol-1

Eg. F(g) F+(g) + e- H = +1681 kJ mol-1

Second ionisation energy: the energy required to remove one mole of electrons from one mole of gaseous monopositive

ions of an element to form 1 mole of gaseous dipositive ions.

Eg. Na+(g) Na2+ (g) + e- H = +520 kJ mol-1

NB. The increase from the 1st IE to the 2nd IE is due to the increased difficulty in removing an electron from a positively

charged ion.21

IE values depend upon 3 factors:

1. Nuclear charge 2. The distance between the nucleus and the electron in question. 3. Shielding by the other electrons in lower energy shells.

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Successive Ionisation Energies

How do we know where the electrons are in an atom?

The successive IEs* of an element give us evidence for the idea of electrons in SHELLS (sometimes called Quantum Shells or Orbitals)

The graph below shows the successive ionisation energies of the element chlorine (atomic number 17).

*Not to be confused with 1st IEs

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OK, remember that the IE depend upon 3 factors:

1. Nuclear charge

2. The distance between the nucleus and the electron in question (the quantum shell and the orbital in which the electron finds itself

3. Shielding by the other electrons in lower energy shells.

EXPLAINING TRENDS IN IEs

Explain the trend across the period

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Explain the trend down the group

Questions p23 25

The Periodic Table

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Periodicitythe quality or character of being periodic; the tendency to recur at intervals.

Mendeleev’s Holes….

Periodicity is about trends, but trends in what?Melting and boiling points

Atomic and ionic radii Ionisation energies Electron affinities

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Atomic radiiHard to measure - why?

atomic radius - half the distance between the centers of two atoms that are touching each other

TASK: Define….Van der Waals radius

Covalent radius

Metallic radius

Why does the covalent radius of an atom increase down a group (usually)?

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Melting and boiling points

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Ionisation Energies

We need to be able to explain these trends

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Explain:General increase across a period

The slight drop after Be

The slight drop after N

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Read pp24-29 and Answer Questions

Explain the trends in covalent radii

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