3.1.1 Atomic structure - Chemistry Alevel

Atoms are made up of three fundamental particles - _________, _________ and _________.

Protons, neutrons, electrons.

The _________ and _________ form the _________.

Protons, neutrons, nucleus.

The _________ orbit the _________ in _________.

Electrons, nucleus, shells.

Mass of a proton.

1

Mass of a neutron.

1

Mass of an electron.

1/1836 or 1/1840

Charge of a proton.

+1

Charge of a neutron.

0

Charge of an electron.

-1

Location of a proton.

Nucleus.

Location of a neutron.

Nucleus.

Location of an electron.

Orbiting nucleus in shells.

What type of force attracts the electrons towards the nucleus?

An electrostatic force of attraction.

What happens to the electrostatic force of attraction as the distance between the electron and the nucleus increases & why?

The force decreases as the electrons are further from the nucleus that they are attracted to and shielding occurs.

What is shielding?

Electrons which are two or more shells away from the nucleus are "shielded" by the electrons in the shells closer to the nucleus

Compare the forces attracting the nucleus to the electron and the electron to the nucleus.

The force attracting the nucleus to the electron is the same size as the force attracting the electron to the nucleus. Therefore the electron doesn't get closer or further from the nucleus but travels at a constant velocity.

Similarities between the solar system and the atom.

-Both have smaller masses that orbit a larger mass.

-The forces of attraction are stronger with the closer small masses to larger.

-Circular orbits.

Differences between the solar system and the atom.

-Different forces of attraction.

-Bigger forces of attraction in the atom.

-The atom has multiple masses orbiting on the same orbit.

-Solar system orbiting forces attract whereas those in an atom repel.

What does the mass number (A) consist of?

Number of protons + number of neutrons

What is the atomic number (Z)

Number of protons

Why do isotopes have the same chemical properties?

As they have the same number and arrangement of electrons in the outer shell.

What does the relative atomic mass take into account?

-The amount of one isotope present in a naturally occurring sample of the element, compared to the other isotopes.

-The relative isotopic mass of each isotope.

Electron impact method

-sample is vaporised and then high energy electrons are fired at it from an electron gun.

- an electron is knocked off from each particle forming a 1+ ion (a molecular ion)

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

-1+ ions are attracted to a negative electric plate where they are accelerated.

Time-of x̅

Electron impact used

For species with low formula mass as the molecular ion can break down into smaller fragments which are also detected.

Electron spray ionisation method

-The sample is dissolved in a volatile solvent and then injected through a very fine needed to create a fine mist (aerosol).

-The tip of the needle is attached to a high-voltage power supply and so the particles are ionised by gaining a proton (H^+ ion) as they leave the needle.

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

-The ions are attracted towards a negative plate where they are accelerated.

Electro spray ionisation used

For many substances with higher molecular mass (e.g proteins) as fragmentation rarely takes place.

(NOTE: everything gets heavier by 1 so remember to subtract)

How are the molecules accelerated down the flight tube?

They are accelerated using an electric field so that they all have the same kinetic energy. Therefore the velocity of each particle depends on its mass and so the lighter the particle the faster the velocity.

Velocity (from kinetic energy equation) =

√(2KE/m)

What happens in the flight tube spray

The positive ions travel through a hole in the negatively charged plate into this tube. The time of flight depends on it's velocity and therefore it's mass.

Time of flight=

Length of flight tube/velocity [ Length of flight tube / √(m/2KE)]

Detection spray

The positive ions hit a charged electric plate and are discharged by gaining electrons.

This generates a movement in electrons and a current.

This current gives a measure of the number of ions hitting the plate as the bigger the current the higher the abundance.

Relative atomic mass=

∑mass of isotopes x abundance/100

Problems with electron spray ionisation

-Some ions may get a 2^+ charge

-One of the carbons could be carbon-13

-One of the hydrogen so could be hydrogen-2

Which is the molecular ion from a graph after ionisation?

The signal with the greatest m/z value.

Uses of mass spectrometer

-Monitoring breath of patients under anaesthesia

-Analysing molecules in space

-Gain further info about structures and chemical properties of molecules

-Determine the abundance of each isotope of an element

-Detecting banned substances such as steroids in athletes

-Identifying unknown compounds

-Detecting traces of toxic chemicals in contaminate marine life

-Detecting pesticides in/on foods

Factors effecting ionisation energy

-Attraction of the nucleus (more protons in the nucleus the greater the attraction for the outer electrons)

-Distance of the electrons from the nucleus

-Shielding of the attraction of the nucleus (electrons orbiting closer shield the outer electrons and weaken the attraction)

Successive ionisation energies - when & why big jumps

An electron is being removed from a shell that is closer to the nucleus, where the forces of attraction are stronger (& less shielding...)

Successive ionisation energies - general increasing trend - explanation

Electrons are being removed but there is the same number of protons meaning that the remaining electrons are attracted more strongly as the nuclear force is spread among fewer electrons and the electrons are closer to the nucleus.

First ionisation energies across the periodic table - peaks

Nobel gases are the hardest to ionise. They occupy peaks at the end of each period.

First ionisation energies across the periodic table - throughs

Alkali metals (group 1) are easiest to ionise so they occupy throughs at the start of each period.

First ionisation energies - general decrease - where and why

General decrease down a group as:

-The outer electron is further from the nucleus

-The outer electron experiences greater shielding

First ionisation energies - large drop (normal) where & why

Large drop from one period to another as a new shell is being occupied where the outer electron:

- is further away from the nucleus

- experiences greater shielding

First ionisation energies across the periodic table - general increase - explanation

Across a period as:

- nuclear charge increases

-electrons are added to the same shell and with the same shielding

S sub shell

1 orbital, max number of 2 electrons

P sub shell

3 orbitals, max number of 6 electrons

D sub shell

5 orbitals, max number of 10 electrons

F sub shell

7 orbitals, max number of 14 electrons

Sub shell order

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p,

First ionisation energies - irregular 1 (Mg Al)

X is starting to fill up a 3p sub shell, whereas Y has its outer electrons in the 3s sub shell. The electrons in the 3p sub shell are slightly easier to remove as 3p electrons are higher in energy.

First ionisation energies across the periodic table - Irregular 2 (P S)

When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove. Eg in X there are 4 p electrons and the 4th is starting to double fill the 3p orbital.

Gap between 2nd & 3rd ionisation energy vs 4th and 5th

2nd & 3rd gap is bigger than that between 4th and 5th. The 3rd ionisation should be around 8000.