TOPIC 2: ATOMIC THEORY

• Atoms may be divisible into sub-atomic particles

• All atoms (besides hydrogen) consist of protons, neutrons, and electrons

• The mass of one neutron is virtually identical to that of the hydrogen atom (1 proton, 1 electron)

• The mass of one electron is 1/1840th of that mass• Their mass therefore doesn’t really figure into the

mass of a complete atom

TOPIC 2: ATOMIC THEORYComposition of Atoms

• Although they differ greatly in mass, protonsand electrons have the same, opposite, charge

• All atoms are neutral, but by losing or gainingelectrons, they may become positive ornegative ions

• Nucleons (protons and neutrons) are found in the nucleus (radius 10-14 m)

• Electrons found in different energy levelsaround the nucleus

Notation of Atoms or Ions

• The Mass Number, A, is the total of protons and neutrons in the nucleus

• The Atomic Number, Z, is equal to the total number of protons in the nucleus/ the number of electrons

• Atomic number will define the element

• Ions have a charge, indicated to the right of the symbol

YA

Z

n+/n-

TOPIC 2: ISOTOPES

• Electrons are the particles in atoms that determine chemical properties (e.g. reactivity)

• Same element means same number of protons and electrons

• Number of neutrons may vary

• Atoms with the same atomic number but different mass number are isotopes

• Different number of neutrons means different physical properties (e.g. boiling point)

TOPIC 2: RELATIVE ATOMIC MASS

• Atomic masses are all relative to Carbon

• In nature, ratios of an element’s isotope exist

• 75% of naturally occurring chlorine has a mass number of 35

• 25% of naturally occurring chlorine has a mass number of 37

• The molar mass of chlorine is 35.5 g mol-1 Cl35

17

Cl37

17

EMISSION SPECTRA:The Electromagnetic spectrum

• Electromagnetic waves travel through: space, time, possibly matter

• c = λ x f (velocity = wavelength x frequency) (ms-1, m, s-1)

• Smaller the wavelength, the higher the frequency

• Visible light can be found in a rather narrow spectrum of electromagnetic waves

EMISSION SPECTRA:The Electromagnetic spectrum

Red light has the longest wavelength of visible light, violet the shortest

Shorter wavelengths have more energy

ATOMIC EMISSION SPECTRA

• White light consists of all colors found in the visible spectrum

• A prism will show a complete continuous spectrum

• Energy given to single elements causes them to emit wavelengths particular to that element• Each element has a unique

spectrum: line spectrum

ATOMIC EMISSION SPECTRA

• The image on the right shows the emission spectrum for hydrogen

• A convergence of lines at the high energy (short wavelength, violet) end can be observed

• Lines also converge (you see more of them) at one point in the ultraviolet region

ATOMIC EMISSION SPECTRA:an explanation

• Electrons become excited when atoms are supplied with energy

• Electrons can only be found at certain energy levels

• The excited electrons will jump from a lower (ground) state to a higher (excited) state

• Energy cannot be created or destroyed; it only changes form

• In order to return to original energy level, the excited electrons must release energy in some form: light

ATOMIC EMISSION SPECTRA:an explanation

• Hydrogen can absob or emit energy at certain frequencies; this means they can only absorb certain radiation

• This suggests a limited amount of energy levels within the hydrogen atom

• Electrons falling back to the n= 2 level constitute visible light

• Convergence happens because the energy levels themselves converge

THE ARRANGEMENT OF ELECTRONS

• First ionization energy: The energy required to remove one electron from an atom in its gaseous state

• Measured in kJ mol-1

• What is the noticeable trend occuring here?

Ana Sayfa

THE ARRANGEMENT OF ELECTRONS

• Energy levels can become full of electrons

• When they are full, the first ionization energy is greater

• Level 1: 2 electrons, Level 2: 8 electrons, Level 3: 8 electrons

Ana Sayfa

THE ARRANGEMENT OF ELECTRONS

• How electrons are arranged (in what level) is called electron configuration

• Those electrons in the highest energy level are termed valence electrons

• Be able to determine electron configuration for up to Z= 36!

Ana Sayfa

Electron Configuration: Important Vocabulary

First ionization energy

Second ionization energy

S, p, d, f orbitals

Aufbau principle

Hund’s rules

Pauli exclusion principle

Electron configuration (not arrangement)

Heisenberg’s uncertainty principle

Electron configuration

• Electrons are arranged in energy levels which contain sub-levels, which contain different orbitals of different shapes.• Distribution of electrons:

• Each orbital may contain a maximum of 2 electrons

• These electrons will have opposite spin

The shapes of electron orbitals: s is spherical, p is a figure 8, and d

a series of more complicatedshapes (Source: ChemicalComputing Group 2018)

Electron configuration

Electrons in each orbital will have opposite spin (Pauli exclusion principle) and will not occupy the same space at the same time

The shapes of electron orbitals: s is spherical, p is a figure 8, and d

a series of more complicatedshapes (Source: ChemicalComputing Group 2018)

Electron configuration

•1st energy level: 1 s orbital

•Second energy level: 1 s orbital and p sub-level with 3 p orbitals

•Third energy level: s, p, and d sub-levels

•Fourth energy level: s, p, d, and f sub-levels

Source: Chemogenesis webbook (2018)

Electron configuration: How the electrons arrange themselves

•Rule of thumb:• Lowest energy

configuration possible• Orbitals in sub-levels are

filled singly first (Hund’s rule)

• 4s level is lower than 3d?

Source: Chemogenesis webbook (2018)