1.5: Atomic Structure and Electron Configuration

Learning Targets/ Content Objectives:

I can represent the electron configuration of an element or ions of an element using the Aufbau principle.
I can apply my knowledge of coulombs law to compare ionization energies of electrons an element in different atomic subshells.
I can apply my knowledge of coulombs law to compare ionization energies of electrons from the same subshells in different atoms.

Vocabulary:

Electron configuration- shells (energy levels) - subshells (energy sublevels)- valence shell- valence electron- Aufbau principle- core electrons- Ionization energy- Electron affinity- coulomb's law- atomic radius- electric charge.

Aufbau Principle

According to the Aufbau Principle, electrons in the ground state of an atom, start filling the orbitals of the lowest energy level before filling in the orbitals of higher energy levels. For example, electrons start filling the 1s orbital before filling in the 2s orbital.

Electron Configuration of Atoms

Watch the following video for help on how to write the electron configuration for a given element.

Electron Configuration of Ions

Watch the video below to review how to write the electron configuration of a negatively charges ions and that of a positively charged ions

When ions are formed electrons are lost or added to the valence shell of the atom and not to any other shell.

Checking for Understanding

Write the electron configuration of Mercury, Hg

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10

then, you need to rearrange the orbitals by the order of the shells as follows

1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 6s2

2. Write the electron configuration of Aluminum ion, Al3+ ion

Al3+ has lost 3 electrons so it has a total of 10 electrons its electron configuration is as follows

1s2 2s2 2p6

3. Write the electron configuration of Bromide ion, Br-

1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6

Exceptions to Electron configuration

Exceptions are based on the fact that half-full or full shells or subshells are more stable than partially filled ones. When the difference in energy levels between two subshells is small, an electron may transfer to the higher level shell to fill or half-fill it.

There are two main exceptions to electron configuration: chromium and copper. In these cases, a completely full or half full d sub-level is more stable than a partially filled d sub-level, so an electron from the 4s orbital is excited and rises to a 3d orbital. Therefore the abbreviated electron configuration for Cr and Cu will be

Cr: [Ar] 4s1 3d5 and not ~~[Ar] 4s2 3d4~~

Cu: [Ar] 4s1 3d10 and not ~~[Ar] 4s2 3d9~~

Orbital Notation Diagram

Orbital notation is a way of writing an electron configuration to provide more specific information about the electrons in an atom of an element. In orbital notation boxes are used to represent orbitals and arrows are used to represent electrons. Each orbital has a capacity of 2 electrons and that's why each box can fit a maximum of two electrons. Arrows are drawn inside the boxes to represent electrons. Two electrons in the same orbital must have opposite spins so the arrows are drawn pointing in opposite directions. The following is an orbital diagram for Selenium.

In writing orbital diagram Follow the following steps:

Step 1: Determine the number of electrons in an atom. Normally this is the same as the number of protons which is known as the atomic number.

Step 2: Draw boxes for the orbitals. (1 box for s orbital, 3 boxes for p orbital, 5 boxes for d and 7 boxes for F orbital)

Step 3: Draw arrows in the boxes to represent electrons starting with the lowest energy sublevel and working up. This is known as the Aufbau rule. The Pauli exclusion principle requires that electrons in the same orbital have opposite spins and Hund's rule states that orbitals in a given sublevel are half-filled before they are completely filled.

Coulomb's Law

Coulomb's law gives the magnitude of the force between two charged particles.

According to Coulomb's law if two objects have electrical charges Q1 and Q2 and are separated by a distance "r", the electrical force between the two particles is given by Coulomb's law and is proportional to the charge of the particles and inversely proportional to the distance that separates them.

That means that the bigger the charge is the bigger the force will be. However, as the distance between the two charged particles increase the force between them will decrease.

Coulomb's Law and Ionization Energy

Ionization Energy:

Is the energy needed to remove an electron from an atom in the gaseous state.

The stronger the attraction between the nucleus and the electron the harder it is to extract an electron from the atom and therefore the higher the ionization energy will be.

Coulomb's law also explains the attraction between the nucleus that is positively charged and the electrons that are negatively charged. Coulomb's law is used to explain how strong the attraction is between the valence electrons of an atom and and its nucleus. Coulomb's law also is used to compare the ionization energies of different atoms.

First, second and third ionization energies of an atom

The energy required to remove the outermost valence electron from a neutral atom is the first ionization energy. The second ionization energy is that required to remove the next electron, and so on. The second ionization energy is always higher than the first ionization energy. Take, for example, an alkali metal atom. Removing the first electron is relatively easy because its loss gives the atom a stable electron shell. Removing the second electron involves a new electron shell that is closer and more tightly bound to the atomic nucleus.

The closer is the electron to the nucleus the more energy it needs to be removed.

Ionization Energy Trend in the periodic table

As we move across the periodic table from left to right, atoms gain more protons and therefore there is a larger force of attraction and it is harder to extract an electron. This means that as we move across the periodic table from left to right the ionization energy increases.
As we move down the periodic table ionization energies within the same group decrease and that is because as we move down the atom gains extra shell and electrons are farther away from the nucleus and therefore ionization energy decreases. According to Coulomb's law the bigger the distance between the electrically charged particle, the less is the force of attraction.

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