Physical 1 - Atomic Structure

Plum Pudding Model

Initially thought that atoms consisted of a sphere of positive charge, with small negative charges distributed evenly within it.

Electron Shell Model

The atom consists of a small, dense central nucleus surrounded by orbiting electrons in electron shells, discovered in the Rutherford scattering experiment in 1911.

Nucleus

Consists of protons and neutrons giving it an overall positive charge and contains almost the entire mass of the atom.

Neutral Atom

In a neutral atom, the number of electrons is equal to the number of protons due to the relative charges.

Maximum Electrons in Shell

The maximum number of orbiting electrons that can be held by any single shell is calculated using 2n², where n is the number of the shell.

Mass Number

Represented using A and can be calculated as the sum of protons and neutrons in an atom.

Atomic Number

Represented using Z and is equal to the number of protons in an atom.

Relative Atomic Mass (Ar)

Defined as the mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope.

Isotopes

Atoms of the same element with the same atomic number but with a different number of neutrons, resulting in a different mass number.

Chemical Reactivity of Isotopes

Neutral atoms of isotopes will react chemically in the same way as their proton number and electron configuration is the same.

Ions

Formed when an atom loses or gains electrons, meaning it is no longer neutral and will have an overall charge.

What is Mass Spectrometry and its purpose?

An analytical instrumental technique used to identify different isotopes and find the overall relative atomic mass of an element. Relative molecular mass can be found in substances made of molecules.

Time of Flight (TOF) Mass Spectrometry

Records the time it takes for ions of each isotope to reach a detector, producing spectra showing each isotope present. Tells you relative isotopic abundance and relative molecular mass of molecules.

What is electron impact ionisation?

A sample of an element is vapourised and high energy electrons are fired at it using an electron gun. This knocks off one electron from each particle forming a + ion. The ions are attracted to the negative plate where they are accelerated. The ion often breaks into smaller fragments. Used for lower formula mass.

Equation for electron impact?

X(g) -> X+(g) + e-

What is the method of electrospray ionisation?

The sample is dissolved in a volatile solvent and injected at high pressure through a fine hypodermic needle to give a mist.

Goes to the positive terminal of a high voltage power supply, and particles are ionised by gaining a proton from the solvent. Solvent evaporates, and the XH+ ions are attracted to negative plate.

Equation for electrospray ionisation:

X(g) + H+ -> XH+(g)

What happens at acceleration?

Positively charged ions are accelerated towards a negatively charged detection plate so they have the same kinetic energy.

What happens in the flight tube?

Ions travel through the flight tube, and time of flight depends on mass and velocity (as ions have the same KE). Lighter ions travel faster, heavier ions have a slower velocity.

How are ions detected?

When positive ions hit the negatively charged detection plate, they gain an electron generating a current. Size of current is proportional to relative isotopic abundance.

Analysis

Current values are used in combination with flight times to produce a spectra print-out with the relative abundance of each isotope displayed.

Mass to charge ratio (m/z)

The ratio that is halved in mass spectrometry, observable on spectra as a trace at half the expected m/z value.

Electron Orbitals definition?

Region of space where there is a high probability of finding an electron. They can be occupied by a maximum of 2 electrons and exist at specific energy levels.

Subshell definition?

Groups of atomic orbitals that correspond to the same principal quantum number, and will contain one or more orbitals.

What are the 4 types of orbitals and their capacities?

s-orbital = 2 electrons (1 orbital), p-orbital = 6 electrons (3 orbitals) , d-orbital = 10 electrons (5 orbitals), f orbital = 14 electrons (7 orbitals)

Energy of Orbitals

Increases from s to d, meaning orbitals are filled in the order of increasing energy.

What is the maximum number of electrons in each shell?

n=1: 2e

n=2: 8e

n=3: 18e

n=4: 32e

Electron Configuration Example for Na

For Sodium (Na) with 11 electrons: Na = 1s2 2s2 2p6 3s1.

What is spin and what are rules for pairing?

They either have an or down spin, and electrons with like charges repel. Electrons pair up with opposite spin within an orbital to stabilize the atom.

Rules for Electron Configurations

1. The lowest energy orbital is filled first. 2. Electrons with the same spin fill an orbital first before pairing begins. 3. No single orbital holds more than 2 electrons.

Electron Configuration Change Example

For an unstable 3p4 orbital, the configuration changes to 3p3 4s1 for stability.

Why is 4s filled before 3d?

3d is higher in energy than 4s

What are the electron configurations of Cu and Cr?

Cr: [Ar] 4s1 3d5

Cu: [Ar} 4s1 3d10

Why are the configurations of Cu and Cr different?

3d subshell is filled symmetrically to increase the stability, which outweighs the energy required to move the electrons from 4s to 3d.

What are the electron configuration rules for d block ions?

Filling 3d reduces its energy, so 4s becomes then becomes higher in energy, when electrons are lost and ions form, 4s empties first, so Cu+ is [Ar]3d10 and Cu2+ is [Ar}3d9

Ionisation Energy definition?

The minimum energy required to remove one mole of electrons from one mole of atoms in a gaseous state, measured in kJmol-1.

First Ionisation Energy Reaction (Na)

Na(g) → Na+(g) + e-.

What are factors affecting ionisation energies?

Nuclear charge (more electrostatic attraction), distance of electron from nucleus, shielding

What are the general trends of IE going across a period?

The shielding across a period remains similar, but more protons are in the nucleus so nuclear charge increases. Therefore electrostatic attraction increases so more energy is needed to remove outermost electron, so the first IE energies increases across periods. Atomic radius decreases too as the nuclear charge increases across the period.

What is the general trend of IE going down the group?

Number of shells increases so outer electron is further from nucleus, and greater shielding effect from inner electrons. Nuclear charge increases, but this is outweighed by greater distance from nucleus and shielding. Therefore there is a weaker electrostatic attraction on the outer electron, less energy is needed to remove the outer electron so the first IE decreases down the group.

What are the exceptions to the trend across a period from groups 2-3?

There is a drop in ionisation energy between groups 2 and 3 (Aluminium IE is lower than Mg). This is due to the electron in the 3p subshell having slightly more energy than the 3s subshell, so the 3p subshell is slightly further from the nucleus so has slightly more shielding than 3s, so electrostatic force of attraction is weaker and less energy needed to remove it.

Why does IE drop between groups 5 and 6?

IE of Sulfur is lower than Phosphorous, as P has no paired electrons but S has a pair of electrons in one of the 3p orbitals. The paired electrons repel each other, so less energy is needed to remove the paired electron than the unpaired.

2nd ionisation energy definition?

The energy required to remove an electron from a + ion in the gaseous state forming a 2+ ion.

What is the general trend of successive ionisation energies?

They increase because electrons are being removed from an increasingly positive ion, so as electrons are removed the electrostatic attraction between the nucleus and outer e- increase. Shielding may also decrease as the subshells go down.

What happens to successive IE within the same shell?

The ionisation energy increases by a similar amount each time because the distance from the nucleus remains the same. The values increase as the e- is being removed from an increasingly positive ion.

How does ionisation energy provide evidence for the atomic orbital theory?

Large energy increases in ionisation energy provide supporting evidence for the atomic orbital theory.