Chapter 2:Unit 7. Rules for Determining Electron Configuration of Atom

Rules for Determining Electron Configuration of Atom

Rule #1: To write the ground state electronic configuration of an atom, electrons are added to the lowest energy orbitals, giving each orbital two electrons. This is called Aufbau Principle. The lowest energy arrangement of electrons is called Ground State.

According to Aufbau principle, electrons first occupy 1s orbital, then 2s and 2p. Next come, 3s and 3p orbitals. Electrons go to 4s before filling up 3d because 4s orbital energetically lower than 3d orbital. After 4s and 3d, electrons are filled up in 4p , 5s and then go back to 4d orbitals. The above figure helps us to find the order by which electrons are filled up in the orbitals.. Each electron is shown by an upward arrow.

Rule#2: Each orbital hold maximum of two electrons of opposite spin, this is called Pauli exclusion principle.

He 1s orbital

Rule#3: When orbitals are equal in energy, one electron is added to each orbital until the orbitals are half filled, before any orbital is completely filled. This rule is called Hund’s rule.

Since 2p orbitals are energetically degenerate, always they are half filled first then completely filled up.

The atomic number of the element tells us how many electrons must be placed in the electron configuration. Electron configuration is shown using superscripts to indicate how many electrons an orbital contains. For example, the electron configuration of the six electrons in a carbon atom is 1s²2s²2p². This is called spectroscopic notation.

Electron configurations are written as energy level followed by sublevel with a superscript that indicates the total number of electrons in that sublevel.

For example, Hydrogen has 1 electron; that should be placed in level 1 and sublevel that contains s orbital. So the configuration is 1s¹

Since s orbital can hold max. 2 electrons, in case of Li, the 3^rd electron goes to level2 s orbital giving configuration 1s²2s¹

Atomic number of Boron is 5, 5^th electron of B will go to the next sublevel p of energy level 2. Electronic configuration of Boron: 1s²2s¹2p¹

Orbital Diagram: A box is used to represent each orbital and arrows to represent electrons.

Single electron is called unpaired electron ↑, two electrons in an orbital have paired spin. ↑↓.

In case of Nitrogen, there are three electrons in 2p sublevel. Following the Hund’s rule each electron occupies one of the three 2p atomic orbitals, from the next element, oxygen, elctrons start pairing up with opposite spin following Pauli’s Exclusion Principle.

Here is the orbital box diagram for common elements.

Sometimes, for larger atomic number atoms, previous noble gas configuration is used as core electrons followed by configuration of outermost shell. It is called noble gas notation. For example, Sodium (Na) is written as [Ne] 3s¹

Ions are formed when neutral atom loses or gains electrons. When an element loses electrons, the element has positive charge and it is equal to the number of electrons lost. Similarly, when an element gains electrons electron, the element is negatively charged and charge of the element is equal to the electrons gained.

Below some of the configurations and orbital box diagram of period 2 elements are shown.

In period 4, Potassium is the first element with atomic number 19. Following the Aufbau principle, 4s orbitals are filled up before 3d orbitals.  Below is the electronic arrangement of Potassium atom.

For transition metals, following the Aufbau principle, d orbital s are filled up.  Some discrepancies are observed due to Hund’s rule. For example: Chromium( atomic number 24). Should have configuration: 1s²2s²2p⁶3s²3p⁶4s²3d⁴. But since half filled orbital sare more stable than full filled, Cr configuration: 1s²2s²2p⁶3s²3p⁶4s¹3d⁵_. Same trend follows for Copper (atomic number 29).

Below is the chart for transition metals.

Questions:

1. Draw an orbital diagram for each element (a) Magnesium, (b) Aluminum, (c) Bromine

2. What element(s) have each electronic configuration?

a) 1s²2s²2p⁶3s²3p^2
b) 1s²2s²2p⁶3s²3p⁶4s²3d^1
c) [Ne]3s²3p^4
d) [Ar]4s²3d¹⁰
e) How many electrons does Cl^1- have?  Can you write its electron configuration (1s²…)
f) Name some elements that have partially filled d orbitals

Ans: 1. See your instructor

2.a. Si
b) Sc
c) S
d) Zn
e) 18 electrons, 1s²2s²2p⁶3s²3p^6
f) Fe, Co, Ni