atomic structure

relative atomic mass

weighted average atomic mass of its naturally occurring isotopes. determined by mass spectrometer.

isotopes

atoms with the same number of protons but different number of neutrons. have same chemical properties but different physical properties.

isoelectronic

atoms/ions with same number of electrons

isotonic

atoms/ions with same number of neutrons

in mass spectrometry, why is the mass/charge ratio numerically equal to mass?

because ions with single charge are much more abundant in the mass spectrometer (electron beam extracts one electron to form +1 radical cation)

how does mass spectrometer work?

• electron bean hits sample, extracts one electron

• sample becomes +1 radical cation

• when put into electrical field, deflected

• angle of deflection depends on mass/charge ratio

  • heavier: deflects less

  • lighter: deflects more

the electromagnetic spectrum

the arrangement of all electromagnetic radiations in increasing order of wavelengths/decreasing order of frequencies (note: reverse order from o levels)

bohr model

• atom is a small positively charged nucleus surrounded by electrons that move in circular orbits around the nucleus

• electrostatic forces of attraction

• electrons move in orbits of fixed size and energy (ie energy levels)

• energy of electron is quantized

energy of electron is quantized

cannot vary continuously but can change discontinuously to have specific values (ie not a spectrum)

hydrogen emission spectrum

• non-continuous spectrum, lines correspond to photons of particular wavelength

• lowest energy level is closest to nucleus

• within each series, as frequency increases, lines converge: electronic energy levels get closer to each other (smaller energy gap) as distance from nucleus increases

• when sample of GASEOUS hydrogen atoms at LOW PRESSURE subjected to HIGH ENERGY, atoms emit electromagnetic radiation

• electron absorbs a quantised amount of energy, undergo electronic transition to higher energy levels

• when electron returns to lower energy level, emits light energy.

ground state

electron occupuies lowest energy level. an atom can only have one ground state.

excited state

electron occupies a higher energy level. an atom can have many excited states. when electron returns to lower energy level, emits light energy.

1st ionisation energy

minimum energy required to remove one mole of valence electrons from one mole of gaseous atoms to form one mole it singly positively charged gaseous cation. endothermic since energy absorbed to remove electron.

2nd ionisation energy

minimum energy required to remove one mole of valence electrons from one mole of gaseous M+ ions in its ground state. endothermic since energy absorbed to remove electron.

convergence limit

frequency where the spectra lines begin to meet. n = infinite → the electron escapes the atom. therefore, the energy level at convergence limit = first IE

why ionisation energy increases (more exothermic) from 1st to 2nd to 3rd?

• ion becomes more positive

• electrostatic force of attration between nucleus and valence electrons increase

• more energy needed to remove electrons

energy of orbitals depends on

1. nuclear charge: attraction between electrons and nucleus

2. interelectronic repulsion

electron in atom stable when total attractive > repulsive

degenerate orbitals

• orbitals with same energy level, same distance for nucleus. represented by subscript. (eg for 2P orbital: 2Px 2Py 2Pz)

orbitals

• sub-levels contain a fixed no. of orbitals: regions of space with higher probability of finding an electron

  • size increase, probability decrease

  • p orbital directional (along axes), larger probability along axes

  • s non-directional, probability same along all axes

• spdf orbitals coexist to form electron cloud

• each orbital holds max 2 electrons

• s orbitals: spherical shape, non directional.

  • size of s orbital increases with principal quantum no.

• p orbitals: dumbbell shape, directional along axes. (Px Py Pz)

  • size of p orbitals increase with principal quantum no.

aufbau principle

• electrons added progressively to orbitals starting with lowest energy

• EXCEPTION: Cr/Cu

  • small energy difference between 3d and 4s

  • fully filled orbitals and half filled orbitals have more stability because symmetrical

  • to be more stable, one electron from 4s2 enters 3d orbital so that it becomes half filled 3d5 4s1 /fully filled 3d10 4s1

pauli exclusion principle

• each orbital holds max 2 electrons, which must have opposite spin states

  • magnetic attraction resulting from opposite spin directions cancels out the electronic repulsion

hund’s rule

• when filling a sub-level, each degenerate orbital must be filled singly before can pair

remove from 4s or 3d?

when 3d orbitals have electron, will repel 4s to higher energy level → take out from 4s first

describe the hydrogen emission spectrum

The hydrogen emission spectrum consists of four coloured lines separated by dark bands which are associated with the electron transitions from a higher energy level to n=2. lines get closer with increasing frequency and converge to a limit.