Atomic Structure and the Periodic Table


Mass Spectrometer

1. The sample is first atomised. To vapourise the sample.

2. The vapour is bombarded with high energy electrons to knock off electrons and form positive ions. 

3. The ions are accelerated by an electric field. 

4. They pass through a velocity sector to ensure they are all of the same velocity.

5. The enter a magnetic field. This deflects them depending on the mass of the atom. 

6. The detector then detects the ions and shows peaks on the spectre. 

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Energy Levels and Electron Shells

Electrons in atoms are arranged in a series of shells. - each shell is described by the principal quantum number, n

The larger the value n, the further from the nucleus you are likely to find the electron. 

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Ionisation Energies

Ionisation is the complete removal of an electron from an atom. 

Ionisation is an endothermic process. 

The amount of energy required to remove the atom is called ionisation energy. 

An atom in its ground state is at its lowest level. 

For an atom with more than one electron to remove the first electron is called the first ionisation energy, the energy to remove the second is called the second ionisation energy

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Shell 1 is closest to the nucleus to takes the most energy to remove. 

Within a shell, the subshells have different energies. s (lowest energy) < p < d.

Each subshell contains one or more orbitals. This is a region where electrons are most likely to be found. 

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Electron Spin

An electron in an atom behaves like a tiny atom. An electron can spin either clockwise or anticlockwise. 

Two electrons in the same orbital cannot have the same spin. 

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Hund's Rule

When electrons are placed in a set of orbitals with equal energy, they 'spread out' to maximise the numer of unpaired electrons. 

Electrons will fill the lowest-energy orbitals first, then the remaining orbitals in order of increasiing energy. 

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Periodic Table

Vertical columns in the periodic table are called groups.

All elements in groups have the same number of elements in their outer shells. 

Horizontal rows are periods. 

All elements in a period have the same number of electron shells.

The properties of the elements are a function of their atomic number. 

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Elements from group 1& 2. 

All outermost electrons are in the s subshell. These electrons are lost easily to form positive ions. 

The S block includes metals; sodium, potassium, calcium and magnesium, which are very reactive. Forming stable ionic compounds with non-metals.

Referred to as reactive metals. They have lower melting temperatures and boiling temperatures and lower densities than other metals.  

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Between groups 2 & 3.

Often called transition metals. 

Much less reactive than groups 1 & 2 because in d oribtals are being filled whilst outer s blocks are full. 

All conduct electricity and heat. 

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Successiv electrons are being added in the f subshell. 

The top row of 14 elements (the lanthanides) are all very similar metals. 

The second row (the actinides) are all radioactive. 

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Those in groups 3, 4, 5, 6, 7 and 8.

Electrons are being added to p orbitals in the outer shell. 

P-block contains all the non-metals and metalloids as well as some metals. 

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Periodic Table Trends

Moving across the periodic table, elements gain electrons. 

Moving down a group, elements gain electron shells.

Atomic radius generally decreases across a period. 

Atomic radius generally increases down a group. 

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Periodic Trends in Ionisation energy

Three main factors affecting:

  • The attraction between the nucleus and the outermost electron - reduces ionisation energy
  • The size of the positive nucear charge - higher ionisation energy
  • Inner shells of electrons repel the outer electron, screening or shielding it from the nucleus - lower the ionision energy
  • Ionisation increases across a period 
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