Yr 11 Chem ATAR; atomic structure, periodic trends, AAS

Isotope

different forms of the same element containing equal numbers of protons (atomic number) but different number of neutrons in nuclei (atomic mass)

relative atomic mass

the weighted average of the relative isotopic masses of an element, compared to 1/12 of a carbon-12 atom

core charge

the attractive force experienced by the outer-shell electrons towards the nucleus in an atom. a.k.a. effective nuclear charge

shielding electrons

inner shell electrons repelling the valence electrons, reducing the effective nuclear charge

valence electrons

electrons found in the outermost shell of an atom/ion, the valence shell

Ionisation energy

the energy required to remove one electron from an atom of an element in the gas phase

atomic radius

distance between the nucleus and outermost electron, measurement for the size of atom

electronegativity

the ability of an atom to attract electrons in a covalent bond towards itself

metallic character

how much an element exhibits properties associated with metals. mainly readily losing an electron to form a cation

non-metallic character

to what extent an element exhibits properties associated with non-metals. mainly gaining electrons to form anions

AAS

a quantative analytical technique that uses light absorption to measure the concentration of a metal in a sample, even trace amounts

AAS steps

1. solution of sample containing element of interest is sprayed onto flame, atomizing it into atomic vapour in ground state

2. hollow cathode lamp made of element of interest emits light of characteristic wavelengths targeting atom of interest

3. atom of interest element absorb the oncoming radiation

4. light beam passes through monochromator, selecting chosen wavelength of light

5. intensity is measured by a detector

AES

analytical technique that measures the concentration of a metal in a sample, analyzing the light emitted when the sample is excited

• characteristic line of the element are created

AES steps

1. prepare and introduce it into a source excitation

2. sample is vaporized + excited causing atoms emit light

3. the emitted light corresponding to element of interest is selected by a monochromator

4. measure the intensity of emitted light, determine the concentration of elements present

First ionisation energy

the amount of energy required to remove least tightly held electron from an atom in gaseous state

Electronegativity

ability of an atom to attract and form bonds with an electron

atomic radius

distance from the center of the nucleus to the boundary of the electron cloud

metallic character

tendency for an atom to lose electrons

first ionisation energy across the period

increases

• proton number increases, makes nucleus more positive

• creates stronger electrostatic attraction for valence electrons

• therefore more energy required to remove it

first ionisation energy down a group

decreases

• least tightly held electrons occupy higher energy level further from nucleus

• number of shells, therefore inner shell electrons increase

• increasing the shielding/repelling effect from the positivity of the nucleus to outermost electron

• therefore less energy required to remove

electronegativity across a period

increases

• increasing proton number creates stronger electrostatic attraction for valence electrons

• decreasing atomic radius

• thus electrons can get closer

electronegativity down a group

decreases

• atomic radius increases

• bonding electron isn’t able to get closer to the nucleus

• thus less strongly attracted

atomic radius across period

Decreases

• increasing proton number

• stronger electrostatic attraction for outer electrons all within same shell

• pulls them in closer

atomic radius down group

increases

• electrons occupy higher energy levels further away from nucleus

• thus increasing electron size

metallic character across period

decreases

• number of valence electrons in outer shell increases

• this decreases the tendency for atoms to lose electrons

metallic character down group

increases

• atomic radius increases, valence electrons have weaker attraction to nucleus

• additionally, shielding effect by inner shell electrons of the positivity from nucleus reduces the attraction

• therefore electrons more easily lost by atom

mass spectrometry

analytical technique used to measure the mass-to-charge ratio of ions, determines composition and structure of an element

steps of mass spectrometry

1. vaporisation- sample of chosen element is vaporised into gaseous form

2. ionisation- sample is passed through high-energy electron beam/intense flame to remove one or two valence electrons, forming cations that can react with electric and magnetic fields

3. acceleration- the cations are passed through an electric field to accelerate, also passed through a velocity selector to measure the velocity

4. deflection- the cations move into a magnetic field following a circular path due to deflection. the degree of deflection depends on the mass and charge of the particle

5. detection- a detector records the radius and intensity of the ions. the mass:charge ratio and abundance which is displayed on a graph

successive ionisation energy

the energy required to achieve the sequential removal of electrons from the atom

group 1 name

alkali metals

group 2 name

alkaline earth metals

group 17 name

halogens

group 18 name

noble gases

relative atomic mass (A_R)

the weighted average of the relative isotopic masses of an element, relative to 1/12 of a carbon-12 atom

isotopes

two or more forms of the same element containing equal numbers of protons (atomic number) but differing numbers of neutrons (atomic mass)

electrostatic attraction

the force of attraction between the positive and negative particles

the Bohr model

a theory of the atom which states that elections occupy fixed, circular orbits that correspond to specific energy levels

John Dalton atom model

• 1903

• billard ball model

• matter is made up of indivisible particles

• identical in size, mass and chemical properties

• wrong because atoms have sub-atomic particles

JJ Thomson atom model

• 1904

• plum pudding model

• small negatively charged electrons in overall positively charged sphere

• experiment: cathode ray tube, cathode rays go to positive plate therefore have some negative charge

• incorrect: no nucleus

Ernest Rutherford

• 1911

• planetary model

• atoms are mostly empty space, electrons orbiting small postive charged nucleus

• experiment: gold foil experiment, alpha particles delfecting instead of travelling straight, must be central positive charge and empty space

quantisation

energy levels only exsist as discreet levels in different amounts

James Chadwick

• 1932

• similar to quantum model, nucleus containing positively charged protons and neutrally charged neutrons