Atomic Structure and The Periodic Table

Structure of an Atom

Protons and neutrons are collectively called nucleons
An electron has a mass of 1/1840 units
1 unit of charge equals 1.602 x 10^-19 coulombs
An atom is named after the number of protons in its nucleus
Atoms with the same atomic number but different mass numbers (number of neutrons) are called isotopes
Ions with a positive charge are called cations
Ions with a negative charge are caled anions
The elative atomic/molecular mass of an atom is the ratio of the average mass of one atom/molecule of that element to 1/12th the mass of one atom of carbon-12

1 of 8

Four stages
- Electrospray ionisation - sample is dissolved in polar solvent, pushed through a nozzle at high pressure, high voltage is applied causing the sample to lose an electron, ionised particles are seperated from solved leaving a gaseous sample of ions
- Acceleration - positive ions are accelerated by an electric field, smaller ions have higher speed
- Ion drift - ions leave the electric field at different speeds depending on their mass/charge ratio
- Detection - ions create a current when they reach the detector, detector records time taken and current, the greater the abundance the larger the curren
The RAM can be calculated by calculating the sum of percentage abundance of each isotope, multiplying by the mass of each isotope, then dividing by 100
E.g. (90 x 20) + (10 x 22) / 100 = 20.2
Fragmentation is when molecules are broken up into smaller pieces
The molecular ion peak is the peak with the largest mass/charge ratio - this is the relative molecular mass of the molecule

2 of 8

Electrons occupy space known as orbitals
Every energy level contains one s-orbital - 1s, 2s, 3s, etc
Every energy level except the first contains three p-orbitals - 2p (2p1, 2p2, 2p3)
The third and following energy levels contain five d-orbitals - 3d (3d1, 3d2, 3d3, 3d4, 3d5)
Each orbital can contain 2 electrons
Hence, 4s can hold 2 electrons, 4p can hold 6 electrons, 4d can hold 10 electrons
Subshells are arranged as 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s...
Rules of electron arrangement
- Aufbau principle - electrons always fill the lowest energy orbitals first
- Hund's rule - electrons don't pair up in the same orbital unless all other orbitals of the same energy are occupied
- Pauli exclusion principle - only two electrons may occupy the same orbital, and do so with opposite spin
The electron arrangment of ions can be deduced by removing the outer shell electrons first, remove the p, then s, then d, remove paired electrons before unpaired in the same sub-level

3 of 8

Elements with one shell are placed in the first row, elements with two shells are placed in the second row
A row of elements is called a period
Elements in columns share a similar outer-shell electronic configuration
A column of elements is called a group
S-block elements are all those with only s electrons in the outer shell
P-block elements are all those with at least one p-electron in the outershell
D-block elements are all those with at least one f-electron and s-electron but no d or p electrons in the outershell

4 of 8

The first ionisation energy of an element is the energy required to remove one electron from each of a mole of free gaseous atoms of that element
The energy required depends on the number of protons and the electronic configuration of that atom
The greater the number of protons the greater the attraction of the electrons to the nucleus, therefore making it harder to remove electrons
The number of protons in the nucleus is known as the nuclear charge
Each inner shell and inner sub-shell effectively cancels out one unit of charge from the nucleus, this is known as shielding
The outermost electrons only feel the residual positive charge after this cancelling out, this is known as the effective nuclear charge

5 of 8

There are 4 factors to consider:
- Nuclear charge
- Shielding
- Effective nuclear charge
- Electron repulsion
Trend across period 1 - He has a higher first ionisation energy than H due to it having more protons and both having a lack of shielding as they do not have inner shells
The first ionisation energy increases across a period because the nuclear charge increases but the shielding remains the same
The first ionisation energy between decreases down a group as the effctive nuclear charge stays the same but the number of inner shells increase, pushing the electrons further away from the nucleus due to shielding

6 of 8

The second ionisation energy of an atom is the energy required to remove one electron from each of a mole of free gaseous unipositive ions (+1 ions)
The third ionisation energy of an atom is the energy required to remove one electron from each of a mole of free gaseous bipositive ions (+2 ions)
It becomes progressively more difficult to remove electrons as the more electrons removed from an atom, the number of electrons remaining in the atom decreases, therefore the repulsion between electrons decreases as the number of protons remains the same
The largest jumps between successive ionisation energies come when the electron is removed from an inner shell, this causes a drop in shielding, a large increase in effective nuclear charge, and a large increase in ionisation energy

7 of 8

Across a period the nuclear charge increases but the shielding stays the same, the attraction of outer electrons to the nucleus therefore increases and outer electrons are pulled in closer
This causes the size of the atom to decrease, e.g. Na is the largest atom in Period 3 and Ar is the smallest
Moving down a group the nuclear charge increases but so does the shielding, therefore cancelling eachother out, the outermost electrons are therefore held less closely and drift out
Therefore the size of atoms down a group increase, e.g. Be is the smallest atom in Group 2 and Ra is the largest
When an electron is removed from the outer shell the electron repulsion decreases and electrons move closer to the nucleus
This means cations are always smaller than corresponding atoms of the same element
When you add an electron to the outer shell the repulsion increases and pushes electrons further away from the nucleus
Anions are always larger than the corresponding atoms of the same element

8 of 8