Electrons and the Periodic Table 3

Electrostatic Attraction

• exists between positive and negative charges

• holds individual atoms together

• holds atoms together in molecules

• holds molecules together in all forms of matter

Trends in Atomic Radius

• increasing going left ←

• increasing going down ↓

• atomic radius - measurement of an atom’s size

  • the distance from an atom’s nucleus to its outermost electron shell

Trends in Effective Nuclear Charge/Core Charge

• increases going right →

• stays the same going down ↓

• effective nuclear charge - amount of attraction to the nucleus experienced by valence electrons, while accounting for the shielding effect of all inner/non-valence electrons

  • i.e. the leftover positive attraction to the nucleus after the inner electrons have blocked most of it

• the ‘shielding’ effect occurs as a result of the inner electrons repelling the valence electrons outwards

• given as a number of protons minus the number of inner/non-valence electrons as an atom has

  • effective nuclear charge = no. of protons - no. of inner electrons

Trends in Metallic Character

• increasing going left ←

• increasing going down ↓

• describes how closely an element exhibits the properties commonly associated with metals → closely related to ionisation energy

• opposite of electronegativity

Trends in Electronegativity

• increasing going right →

• increasing going up ↑

• electronegativity - unitless measure of an atom’s ability to attract and ‘hog’ shared electrons towards itself (i.e. covalent bond)

• the greater the attraction of an atoms valence electrons to its nucleus, the greater its electronegativity value

• electronegativity tends to be greater when:

  • an atom has a higher effective nuclear charge (more positive attraction to the nucleus)

  • an atom has a lower atomic radius (reduced influence of distance on attraction to the nucleus)

Trends in Ionisation Energy

• increasing going right →

• decreasing going down ↓

• ionisation energy - describes the amount of energy required to dislodge a valence electron from an atom’s outer shell/energy level, to form a cation

  • measure of how easy/difficult it is to remove an electron from an atom’s outer shell

• ionisation energy depends on:

  • the distance of the valence electrons from the nucleus (atomic radius)

  • the level of attraction of valence electrons to the nucleus (effective nuclear charge & electronegativity)

• successive ionisation - describes the energy required each time another electron is removed from an atom

  • tends to refer to individual elements at a time

  • successive ionisation energy generally increases with each successive electron

  • big jump between some electrons

    • corresponds to the reduction in atomic radius occurring at those points → as the atom falls back to a previous, inner energy level/electron shell

    • is electrons are from the same energy level/electron shell → increase in successive ionisation energies are relatively consistent

Atomic Radius’ Effect on Ionisation Energy

• increasing atomic radius → increases the distance between an atom’s valence electrons and its nucleus

  • weakening the electrostatic attraction between them

• ‘loosens’ the valence electrons of atoms with a large atomic radius → reducing the amount of energy required to remove them

• ∴ increasing atomic radius decreases ionisation energy

Effective Nuclear Charge’s Effect on Ionisation Energy

• effective nuclear charge increases → attraction of electrons to the nucleus increases

• the greater electrostatic attraction makes it more difficult for valence electrons to be removed → increasing the amount of energy required to do so

• ∴ increasing effective nuclear charge increases ionisation energy

Electronegativity’s Effect on Ionisation Energy

• electronegativity increases → atom’s ability to attract electrons to itself increases

• higher electronegativity value indicates a stronger tendency for an atom to hold onto its electrons → making them harder to remove & increasing the amount of energy required to do so

• ∴ increasing electronegativity increases ionisation energy

Quantisation of Energy

• describes the idea that electrons exist and travel around a nucleus in distinct energy levels (often corresponding to electron shells)

• energy levels are denoted by n=1, n=2, n=3…

  • counted based on proximity from the nucleus (n=1 is closest)

• each energy level corresponds to a certain amount of energy that’s consistently identical between all electrons within that same level (energy values per level may differ between elements)

Ground State to Excited State

• ground state - atom with all its electrons in their default energy levels

• excited state - when an atom’s electrons have been (temporarily) promoted to higher energy levels

  • electrons are capable of jumping up into higher energy levels when energy is applied to the electrons and absorbed

  • can make jumps beyond the energy level right above it

  • size of the jump depends on the wavelength of energy absorbed

  • excited state is only temporary and will eventually return to its ground state through emission

• common energy input:

  • light

  • heat

  • electricity

Visible Light & Waves

• type of electromagnetic radiation

• visible light - form of electromagnetic radiation and travels in a repeating wave pattern

• wavelength determines what colour a particular instance of light will appear as

• wavelength - the distance between the crests of a wave’s repeating pattern

Electromagnetic (E.M) Spectrum

• outlines the full range of wavelengths corresponding to different kinds of E.M radiation (of which visible light is a part of)

• E.M. radiation at different wavelengths will have different levels of energy

Visible Light Spectrum

• different colours correspond to different wavelengths of light

  • ∴ different colours also correspond to different amounts of energy

• nm = nanometers (10^-9 m)

Absorption Spectra

• electrons in energy levels close to the nucleus have the lowest energies and experience the strongest attraction to the nucleus

• an electron can jump up to a higher energy level if it absorbs energy that corresponds exactly to the difference in energy between the lower energy level and higher energy level

• the result of shining a beam of visible light through the atoms of a vaporised sample of element, which absorb specific wavelengths from that light

• the ‘missing’ bands corresponds to the specific wavelengths of a light (∴ amounts of energy) being absorbed by the electrons in an atom to promote them to higher energy levels

• each missing band corresponds with a potential energy jump the electrons may make

Absorption

1. a photon of light travels towards a hydrogen atom in its ground state

2. the valence electron absorbs the photon and is promoted to a higher energy level

Emission Spectra

• ‘opposite side of the same coin’ from an absorption spectrum

• the emission that returns an excited atom to its ground state are initially absorbed wavelengths of light being emitted, as the electrons return to ground state, and are viewable to the naked eye as coloured bands

  • why coloured emission bands are 1:1 match with the missing bands on the absorption spectrum

Emission

1. the excited state is only temporary and the atom wants to return to its more stable ground state

2. the absorbed photo is re-emitted by the excited electron

3. the formerly excited electron returns to ground state

Spectroscopy

study of the interaction between matter and electromagnetic radiation

Flame Test

• different colours produced when metals are heated are caused by the different electron transitions occurring in the different atoms

• only some produce flame colours as the emission lines of some elements are not within the visible spectrum of light

• limitations:

  • qualitative test only

  • only a small range of metals are detectable by the flame test

  • metals in low concentrations may be difficult to observe

  • mixtures of metals will produce confusing results

Atomic Absorption Spectroscopy

• sample being analysed is vaporised in a flame to produce free atoms in their ground state

• cloud of ground-state atoms then passes through the optical path of a light source, which produces the emission spectrum of the particular chemical being examined

• as the light source produces light of the same energy required to raise ground-state atoms to an excited state, the cloud of gaseous atoms absorb some portion of the incident light

• the intensity of the absorbed light is measured, giving an indication of the concentration in the original sample

• addresses some of the limitations of flame tests

  • provides both a qualitative and quantitative test

  • more than 70 elements can be analysed

  • can detect elements in concentrations as low as micrograms per litre

  • highly selective and is regularly used with mixtures