Magnesium (Mg) is an alkaline earth metal with the atomic number 12. This means a neutral magnesium atom contains 12 protons in its nucleus. The most common naturally occurring isotope of magnesium is Mg-24 which has a mass number of 24 (12 protons and 12 neutrons).
Quick Answer
A neutral magnesium atom (Mg) contains 12 protons and 12 electrons. However, when magnesium forms an ion with a 2+ charge such as Mg2+, it loses 2 electrons and has a total of 12 protons and 10 electrons.
Magnesium Electron Configuration
In its ground state, a neutral magnesium atom has the following electron configuration:
1s2 2s2 2p6 3s2
This means:
- 1s orbital – 2 electrons
- 2s orbital – 2 electrons
- 2p orbital – 6 electrons
- 3s orbital – 2 electrons
In total, a neutral Mg atom has 12 electrons occupying various orbitals.
Formation of Mg2+ Ion
When magnesium forms an ion with a 2+ charge, denoted as Mg2+, it loses two electrons from its outermost orbital (3s).
The electron configuration of Mg2+ is:
1s2 2s2 2p6
Compared to a neutral Mg atom, the Mg2+ ion has lost the two 3s electrons, leaving it with 10 electrons. However, the number of protons remains unchanged at 12.
Summary of Protons and Electrons
- Neutral magnesium atom (Mg): 12 protons, 12 electrons
- Magnesium ion (Mg2+): 12 protons, 10 electrons
So in summary, Mg2+ contains 12 protons in the nucleus and 10 electrons occupying various orbitals.
Understanding the 2+ Charge
The 2+ charge of the Mg2+ ion indicates that it has lost two negatively charged electrons. This gives it a net positive charge of 2+. The number of protons (12) remains constant and is equal to the atomic number of magnesium.
The ion achieves a stable noble gas electron configuration (of Neon) by losing the two outermost 3s electrons and attaining the configuration 1s2 2s2 2p6. This stable configuration with a filled shell is energetically favorable.
Balancing Nuclear and Electron Charges
For an electrically neutral atom, the number of protons and electrons must be equal. A neutral Mg atom balances the 12 positive charges from protons with 12 negative charges from electrons.
In the Mg2+ ion, there is an imbalance with 12 positively charged protons and only 10 negatively charged electrons. This gives the ion an overall charge of +2. The nuclear (proton) and electron charges balance out.
Significance of Valence Electrons
Valence electrons are the outermost electrons and are the most important in determining the chemical properties of an atom. Magnesium has two valence electrons in its 3s orbital.
When forming the Mg2+ ion, magnesium readily loses these two 3s valence electrons to achieve a stable noble gas configuration. This is typical of alkaline earth metals which readily give up their valence s electrons to attain a +2 charge.
Writing Electron Configurations
The electron configuration of an atom can be written in a condensed format where the orbitals are grouped together. For example, magnesium can be written as [Ne] 3s2 where [Ne] represents the inner 1s2 2s2 2p6 configuration of the noble gas neon.
The electron configuration of Mg2+ is [Ne] which represents the neon noble gas configuration. This condensed format clearly shows that Mg2+ has lost its two 3s valence electrons.
Exceptions with Transition Metals
The condensed notation does not work as well for transition metals in which electrons fill the d orbitals before the s orbital. In such cases, the full orbital notation (1s, 2s, 2p, etc.) must be written out.
Relation Between Protons, Electrons and Isotopes
The number of protons defines the element identity. All magnesium atoms and ions, including different isotopes, contain 12 protons. The number of neutrons can vary between isotopes leading to different mass numbers.
For example, some isotopes of magnesium include:
- Mg-24: 12 protons, 12 neutrons
- Mg-25: 12 protons, 13 neutrons
- Mg-26: 12 protons, 14 neutrons
The number of electrons can change when forming cations (lost electrons) and anions (gained electrons). But the element identity is defined by the proton number which remains constant.
Abundance of Mg2+ Ions
In nature, magnesium exists primarily in compounds containing Mg2+ ions rather than neutral atoms. This is because Mg readily loses its two outermost valence electrons to form the stable +2 ion.
Magnesium forms many inorganic compounds containing the Mg2+ ion such as:
- Magnesium oxide – MgO
- Magnesium chloride – MgCl2
- Magnesium sulfate – MgSO4
The Mg2+ ion is also highly biologically important and is the 4th most abundant cation in the body. It takes part in many enzymatic reactions by stabilizing ATP and DNA/RNA.
Electron Dot Diagrams
Electron dot diagrams provide a simple visual representation of valence electrons in an atom. Dots are drawn around the atomic symbol to represent the valence electrons.
Electron dot diagram for magnesium:
:.:Mg:..
Two dots represent the two 3s valence electrons. When forming Mg2+, these two dots are removed:
:Mg:
This visually shows the loss of two electrons to form the cation Mg2+ from the neutral magnesium atom.
Oxidation States
The oxidation state of an atom refers to the charge it would possess if all bonds with other atoms were completely ionic.
Magnesium has an oxidation state of +2 in compounds containing Mg2+ ions. This corresponds to its loss of two electrons to form the cation.
The sum of oxidation states must equal the overall charge of a neutral molecule. For example, in MgCl2 the oxidation states are:
- Mg: +2
- Cl: -1 (for each Cl atom)
Sum = +2 + 2(-1) = 0, matching the neutral charge of the compound.
Representations of Mg2+ Ion
The Mg2+ ion is represented in various ways in chemical formulas, models and diagrams:
- Chemical formula: A superscripted number indicates the ionic charge, such as Mg2+. The lack of an ionic charge indicates a neutral atom.
- Lewis dot diagram: Symbols represent valence electrons, subtracted to represent electron loss in Mg2+.
- Sphere models: Small spheres represent electrons orbiting a central sphere (nucleus). Two electrons are missing in Mg2+ models.
Electron Sea Model
In the electron sea model, electrons are not associated with individual atoms. Instead, they are treated as a negatively charged electron cloud moving against a background of positive charge from the nuclei.
Removing two electrons from this uniform electron sea to form Mg2+ leaves behind a net positive charge of 2+ to balance the remaining nuclei.
This model does not account for the distinct orbital structure around each nucleus. However, it can provide a useful representation of the electrons in metals and alloys.
Reasons for Electron Loss
There are two main reasons why magnesium atoms tend to lose electrons to form the Mg2+ ion:
- Achieving noble gas configuration: Losing 2 electrons gives Mg the same electron configuration (1s2 2s2 2p6) as the noble gas neon which is very stable.
- Lowering energy: Removing the two 3s valence electrons releases energy as they are farther from the nucleus. The cation Mg2+ attains a lower energy state.
In summary, electron loss allows magnesium atoms to reach a stable low energy state with a noble gas-like configuration.
Other Properties of Mg2+ Ion
Some other important properties of the Mg2+ ion are:
- Small ionic radius due to loss of two electron shells
- Colorless aqueous solutions
- Forms water-soluble metal salts
- Essential role in biological processes
The small size and +2 charge makes it suitable for biological functions like enzyme activation, muscle contractions and RNA stabilization.
Similarities with Other Alkaline Earth Metals
Magnesium is part of the alkaline earth metal group including beryllium (Be), calcium (Ca), strontium (Sr) and barium (Ba).
These metals all lose two electrons to form +2 cations with a stable electron configuration. For example:
- Ca -> Ca2+
- Sr -> Sr2+
- Ba -> Ba2+
This predictable +2 oxidation state and reactivity is a key characteristic of alkaline earth metals.
Differences from Transition Metals
Transition metals are elements in groups 3-12 of the periodic table. Unlike alkaline earth metals, transition metals can form ions with different charges.
For example:
- Fe2+
- Fe3+
- Cu+
- Cu2+
Transition metals have partially filled d orbitals that can participate in bonding. This enables multiple oxidation states depending on how many electrons are lost.
Significance of 2+ Charge
The predictable 2+ charge is important for the following reasons:
- Gives alkaline earth metals uniform chemical and physical properties
- Enables these metals to form stable ionic compounds
- Allows these cations to participate in biological processes
- The stable noble gas configuration promotes reactivity to form salts
For magnesium, the 2+ charge signifies it achieves stability by losing its two outermost electrons.
Conclusion
An isolated magnesium atom contains 12 protons and 12 electrons. However, magnesium readily forms Mg2+ ions which contain 12 protons but only 10 electrons.
This 2+ charge results from the loss of its two valence 3s electrons. This enables magnesium to attain a stable noble gas electron configuration.
The predictable ionic charge of +2 is a key property of magnesium and other alkaline earth metals like calcium and barium. It results in uniform chemical properties across the group.
