Magnesium (Mg) is an alkaline earth metal with the atomic number 12. Its neutral state has 12 protons and 12 electrons. However, magnesium commonly forms ions with a 2+ charge, denoted as Mg2+. Determining the number of electrons in Mg2+ is a straightforward matter of understanding electron configurations, ion charges, and the periodic table.
Quick Answer
Mg2+ has 10 electrons.
Electron Configuration of Magnesium
The electron configuration of an atom describes the distribution of electrons in electron shells and subshells. Electrons fill from the lowest to the highest energy levels, governed by principles such as the Aufbau principle, Hund’s rule, and the Pauli exclusion principle.
The electron configuration of magnesium is 1s22s22p63s2. This corresponds to:
1s2 – Two electrons in the 1s orbital
2s2 – Two electrons in the 2s orbital
2p6 – Six electrons in the three 2p orbitals
3s2 – Two electrons in the 3s orbital
So magnesium has two electrons in the lowest energy 1s orbital, eight electrons in the second energy level (two in the 2s orbital and six in the three 2p orbitals), and two electrons in the outermost 3s orbital, for a total of 12 electrons.
Ion Charges
When an atom forms an ion, it gains or loses electrons to achieve a full outer electron shell, attaining a noble gas configuration. Magnesium readily forms cations with a +2 charge.
A cation’s charge comes from having fewer electrons than protons. Since magnesium has 12 protons, for it to achieve a 2+ charge it must have two fewer electrons than protons, i.e. 10 electrons.
So Mg2+ has 10 electrons.
Finding Electrons from the Periodic Table
The periodic table can also help us deduce the electron configuration. Magnesium’s atomic number of 12 tells us it has 12 protons. An atom is electrically neutral when it has the same number of protons and electrons.
Looking at magnesium’s position in period 3, we can infer its electron configuration. Elements in period 3 have electrons filling the 3s and 3p orbitals. Magnesium is in group 2, meaning its two electrons in its highest occupied shell are in the 3s orbital. The preceding 10 electrons fill the lower energy 1s, 2s, 2p, and 3s orbitals.
Therefore, neutral magnesium has two electrons in the 3s orbital and 10 electrons in the lower orbitals, for 12 total electrons.
When forming Mg2+, magnesium loses two electrons from its 3s orbital, leaving it with 10 electrons.
Determining Ion Electrons from Charge
We can also deduce the electron count just from the ion charge:
– Magnesium has 12 protons (known from atomic number)
– Mg2+ has a charge of 2+ (two fewer electrons than protons)
– So Mg2+ must have 12 protons – 2 electrons = 10 electrons
Role of Valence Electrons
Why does magnesium readily give up two electrons to form the 2+ ion? The explanation lies in its valence electrons.
Valence electrons are the outermost electrons and largely determine an element’s chemical properties. Magnesium has two valence electrons in its 3s orbital.
Metals tend to lose valence electrons to attain a noble gas electron configuration. Magnesium’s nearest noble gas is neon, with 10 electrons. By losing two electrons from its 3s orbital, magnesium attains the same 10 electron configuration. This stable neon-like configuration is why magnesium forms a 2+ ion.
Electron Configuration of Mg2+
We can summarize magnesium’s electron configuration changes when forming Mg2+:
Neutral magnesium: 1s22s22p63s2
Mg2+ ion: 1s22s22p6
Magnesium has lost its two valence 3s electrons to form the Mg2+ ion with 10 electrons.
Cations vs. Anions
Looking at other ion charges in the periodic table reveals interesting trends:
– Main group metals tend to form cations by losing valence electrons. This gives cations like Na+, Ca2+, Mg2+.
– Main group nonmetals tend to form anions by gaining electrons. This gives anions like N3-, O2-, F-.
This relates to metals wanting to lose electrons to achieve stability, while nonmetals want to gain electrons to fill their valence shell.
So metals like magnesium tend to become cations by losing electrons, resulting in a positive charge and electron loss.
Example Problem
Let’s practice determining electron counts for ions, using sodium (Na) as an example.
– Sodium has atomic number 11
– So neutral sodium has 11 electrons
– The common ion formed is Na+
– This 1+ charge indicates sodium has lost 1 electron
– Therefore, Na+ has 11 – 1 = 10 electrons
This demonstrates that from the ion charge, we can deduce the number of electrons compared to the original atom.
Practice Problems
Determine the number of electrons in the following ions:
1) Fe3+
– Iron has atomic number 26, so neutral Fe has 26 electrons
– Fe3+ has a 3+ charge, indicating it has lost 3 electrons
– Therefore, Fe3+ has 26 – 3 = 23 electrons
2) Cu2+
– Copper has atomic number 29, so neutral Cu has 29 electrons
– Cu2+ has a 2+ charge, indicating it has lost 2 electrons
– Therefore, Cu2+ has 29 – 2 = 27 electrons
3) Cl-
– Chlorine has atomic number 17, so neutral Cl has 17 electrons
– Cl- has a 1- charge, indicating it has gained 1 electron
– Therefore, Cl- has 17 + 1 = 18 electrons
Electron Configuration of Other Ions
We can apply these principles to determine the electron configuration of any ionic form of an element. For example:
Sodium (Na): 1s22s22p63s1
Na+ ion: 1s22s22p6
Chlorine (Cl): 1s22s22p63s23p5
Cl- ion: 1s22s22p63s23p6
Oxygen (O): 1s22s22p4
O2- ion: 1s22s22p6
Looking at elements across the periodic table, metals tend to lose electrons to form cations, while nonmetals tend to gain electrons to form anions. By understanding the electron configuration and ion charges, we can determine the electron count in any ionic form.
Electron Configurations and Oxidation States
The charge on an ion is directly related to its oxidation state. Oxidation states give the apparent charge on an atom when it forms a compound. While electron configurations dictate the charge, the oxidation state specifically refers to that charge in compound formation.
Some key connections:
– Metals have positive oxidation states when forming cations
– Nonmetals have negative oxidation states when forming anions
– The oxidation state equals the ionic charge
For example, Na+ has an oxidation state of +1, matching its 1+ ionic charge. Mg2+ has an oxidation state of +2, matching its 2+ ionic charge.
Oxidation states can help confirm the expected ion charges and electron configurations.
Nuclear Charge and Ions
The nuclear charge is the positive charge in an atom’s nucleus, equal to the proton number. This helps attract negative electrons.
Ions have the same nuclear charge as the neutral atom. Only the electron count changes when an atom forms an ion.
For example:
– Magnesium has 12 protons, so its nuclear charge is +12.
– Mg2+ has 12 protons and 10 electrons
– Its nuclear charge remains +12, but it now has only 10 electrons.
Nuclear charge remains constant while electrons are gained/lost to form ions. This nuclear attraction helps stabilize ion formation.
Electron Distribution in Mg2+
We can visualize the electron distribution in the Mg2+ ion:
– 1s orbital – 2 electrons
– 2s orbital – 2 electrons
– 2p orbitals – 6 electrons
The inner electron shells remain unchanged. Only the valence 3s electrons are lost to form the positive 2+ ion.
This helps illustrate why successive ionization energies increase down a group. It becomes harder to remove inner shell electrons that experience greater nuclear attraction.
Successive Ionization Energies
Successive ionization energies measure how much energy is required to remove additional electrons from an atom or ion.
Magnesium’s successive ionization energies are:
1st ionization energy – removes 1 electron from neutral Mg
2nd ionization energy – removes 1 electron from Mg+
3rd ionization energy – removes 1 electron from Mg2+
Each successive ionization energy increases due to the nuclear attraction on the remaining electrons. The 3rd ionization energy of magnesium to form Mg3+ is extremely high, so magnesium rarely forms ions beyond Mg2+.
This trend also applies to other elements down their respective groups. Understanding ionization energies helps explain the common ionic forms.
Electron Shielding
Electrons further from the nucleus experience reduced nuclear attraction due to electron shielding by inner electrons. This determines the order in which electrons fill orbitals and are removed during ionization.
For magnesium:
– The 3s2 electrons feel shielded by inner shells and are removed first to form Mg2+
– The strongly attracted 1s2 electrons are closest to the nucleus and most difficult to remove
Electron shielding explains why electrons fill from the inside out, and why valence electrons are most easily removed when forming cations.
Common Uses for Mg2+
Some applications that utilize compounds containing Mg2+ ions include:
– Dietary supplements – Magnesium is an essential mineral for human health, hence its inclusion in multivitamins. Magnesium citrate and magnesium oxide are common magnesium supplements.
– Antacids – Magnesium hydroxide, known as milk of magnesia, is commonly used as an antacid to neutralize stomach acid.
– Fertilizer – Magnesium sulfate, also known as Epsom salt, provides essential magnesium ions for plant growth and is used as a fertilizer.
– Fire retardant – Magnesium carbonate releases carbon dioxide when heated, smothering flames. It is useful in fire extinguishers and smoke detectors.
– Refractory material – Magnesium oxide has a high heat capacity and chemical stability at high temperatures. It is utilized as a refractory material in furnace linings.
Biological Role of Mg2+
Magnesium ions play many key roles in biological systems:
– Enzyme cofactor – Magnesium is essential for the proper functioning of hundreds of enzymatic reactions, including those involved in ATP energy production.
– Muscle function – Magnesium allows proper muscle relaxation and contraction. Deficiency can lead to cramps and spasms.
– Bone formation – Magnesium constitutes around 1% of human bone mineral and assists in calcium deposition in bone crystal growth.
– Nerve transmission – Magnesium aids the conduction of nerve impulses along neurons.
– Protein synthesis – Magnesium assists in the binding of mRNA to ribosomes to begin translation.
– DNA stability – The negative charges on the phosphate backbone of DNA are stabilized through magnesium ion interactions.
Magnesium deficiency can therefore lead to many issues with energy, muscles, nerves, and protein synthesis. A balanced intake of magnesium ions helps maintain good health.
Comparison of Mg2+ to Other Ions
It is instructive to compare Mg2+ to other metallic ions:
– Mg2+ has a smaller ionic radius than Ca2+. Magnesium’s 2+ charge is spread over a smaller area, giving it a higher charge density.
– Mg2+ is more polarizing than Na+ due to its higher charge. Mg2+ distorts the electron clouds of neighboring ions more.
– Mg2+ is colorless, unlike transition metals with partially filled d orbitals like Cu2+ and Fe3+.
– Mg2+ has higher charge density and polarizing power than larger monovalent ions like K+ and Na+.
– Divalent Mg2+ can form more complex ionic lattices than monovalent Na+ and Li+.
Understanding the properties of Mg2+ vs. other metal cations helps elucidate its chemical behavior.
Conclusion
In summary, magnesium readily forms a 2+ ion by losing its two outer 3s electrons. This leaves Mg2+ with 10 electrons in a 1s22s22p6 configuration, the same as the nearest noble gas neon.
Determining the electron count in ions is straightforward once we know the parent element’s proton number, the ion charge, and electron configurations. Magnesium’s properties make sense when considering its small, highly charged Mg2+ cation and its full inner electron shells.
