Module 11: Atomic Structure

Timeline

Erwin Schrödinger & Werner Heisenberg (1926)

John Dalton (1808)

Ernest Rutherford (1911)

• Proposed the "nuclear model".
• Describes the atom as consisting of a positively charged dense nucleus made up of protons & neutrons.

• Proposed the "electron cloud model".
• They proposed the idea of orbital clouds.
• The exact position of electron is cannot be determined exactly, but there is probability where it can be found.

• His model was a condensed sphere.
• Later known as the father of the modern atomic theory.
• He published a book entitled "A New System of Chemical Philosophy".

Timeline

5th Century BC

Niels Bohr (1915)

Greek philosopher Democritus believed that matter consists of very small, "indivisible" particles which he named "atomos".

• Because his proposal was a modification of Rutherford's model, it is called Rutherford-Bohr, or simply Bohr's model.
• A "planetary model" in which the negatively-charged electrons orbit a small, positively-charged nucleus.
• He believed each electron orbit could only hold a set number of electrons.

Joseph John Thomson (1897)

• Proposed the "raisin bread model of the atom".
• According to him, an atom is a positively charged sphere with loosely embedded electrons.

Electrons

Electrons

• Negative charged particle found in all atoms.
• Discovered by Sir Joseph John Thompson.
• His cathode ray experiments led to the discovery of electrons & the proposal of the "raisin bread model".

Protons

• Positively charged particles in nucleus of the atom.
• A particle that is 1,867 times more massive than the electron.
• Eugene Goldstein discovered it in 1886.

Protons

Neutrons

• It almost has the same mass as proton.
• Sir James Chadwick discovered it in 1932.

Neutrons

Quarks

• Amongst the three sub-atomic particles, only the electron is fundamental; henceforth, indivisible.
• Protons & neutrons are consisted of smaller particles called quarks.

Quarks

Neutral Atoms

• The number of the protons and electrons are the same.
• Positive protons + negative electrons = 0

Neutral atoms

Isotopes

Isotopes

• Atoms of an element that have the same atomic number but differ in mass.
• Since they have different numbers of neutrons, they are identified using their mass number (sum of number of protons & neutrons).
• Atomic mass unit (amu) - mass of the particles, equivalent to 1.66054 x 10 raised to the power of - 24

Ions

Ions

• A charged particle of particle of the same element due to the removal or addition of electrons
• Cation - ion with positive charge, an electron is removed
• Anion - ion with negative charge, an electron is added

An atom can be at a ground state or an excited state.

Electrons inside an atom possess different energies.

• Ground state - state of atom which has the lowest energy arrangement of electrons.
• Excited state - an atom that absorbs energy through either heat or electrical charge.

The maximum number of electrons in an energy level is given by the following formula:

Each energy level of an atom can only accommodate a certain number of electrons, & these electrons orbit in their respective energy levels.

Principal Quantum Number (n)

Principal Quantum Number

• Specifies the energy of an electron and the size of the orbital (distance from the nucleus).
• The larger the value of n, the father the electron is from the nucleus).
• It is synonymous with the energy level by Niels Bohr.

Angular Momentum Quantum Number (l)

Angular Momentum Quantum Number

• Specifies the shape of an orbital within a particular principal quantum numbers.
• Groups of orbitals in the same shape are called subshells or sublevels.

Magnetic Quantum Number (m)

Magnetic Quantum Number

• Specifies the orientation in space of an orbital of a given energy (n) and shape (l).
• Divides the subshell into individual orbitals that holds the electrons.

Spin Quantum Number (s)

Spin Quantum Number

• Describes the spin of an electron in an orbital.
• The spin can be in two directions (clockwise or counterclockwise).

Aufbau principle

Aufbau Principle

Electrons fill the orbitals, one at a time, starting with the lowest energy orbital then proceeding to the one with higher energy.

Pauli Exclusion Principle

Only two electrons may occupy an orbital, and they must have opposite spins.

Pauli Exclusion Principle

Hund's rule

In filling the orbitals of the same energy level, an orbital must be singly filled up first before pairing the electrons.

Hund's Rule