For most elements, valence electrons are the ones in the outermost shell. Transition metals also use electrons in the inner level just below it. So the practical rule is to count the outer s electrons plus the d electrons, which for a neutral atom usually equals its group number. These two sets of electrons are so close in energy that a transition metal can lend out different numbers of them, which is why one element can form several different ions.1Mr Khemistry. Why Transition Elements Have Variable Oxidation States
The Group Number Method for Neutral Atoms
In the first row, scandium sits in group 3 and zinc in group 12. Iron, in group 8, has 8 valence electrons: 2 in 4s and 6 in 3d. Nickel, in group 10, has 10.
You can check the count with the electron configuration. A configuration lists electrons by energy level, and the d orbitals are a set of five slots that each hold two electrons, ten in total. For iron the configuration is [Ar] 3d6 4s2, so adding 6 and 2 gives 8. The 4s is written after the argon core, and the 3d is listed with it, because in the usual filling order taught in class, the 4s level fills before 3d.2CK-12. How to Find Valence Electrons for Transition Metals
Manganese, in group 7, is [Ar] 3d5 4s2. Adding the outer s and d electrons, 2 plus 5, gives 7. The argon core is ignored, because those inner electrons don’t take part in bonding.
That bracket is the shortcut for writing configurations quickly. Start from the previous noble gas, which is argon for the first row, then add the 4s and 3d electrons needed to reach the element’s atomic number. Only the electrons written after the bracket count as valence electrons. Counting only the outermost shell for a transition metal gives 2 for almost every element, which is wrong, because it leaves out the d electrons.
Tests sometimes name an element like cobalt or titanium without giving you a periodic table. You can still find the group by counting across the d-block from the left edge: the first-row transition metals run in order from scandium (3) through zinc (12). That puts cobalt in group 9 and titanium in group 4. Then split the group number into s and d. The s level usually holds 2, so d holds the group number minus 2, as in titanium’s 2 plus 2.
The number you find is how many electrons the atom could potentially use in bonding, not how many it will always use in a given compound. Check whether your textbook or teacher defines valence for transition metals as s plus d, since some courses only ask for the outer s electrons, and the definition decides the right answer.
Which Electrons Leave First When Ions Form
Even though 4s fills first, the outer s electrons are always the first ones removed when a transition metal forms a positive ion.3CK-12. How to Find Valence Electrons for Transition Metals So the removal order is not the reverse of the filling order. Once d electrons are present, the 3d orbitals are slightly lower in energy than 4s, so the 4s electrons are the highest-energy ones and leave first.4Chemistry LibreTexts. How to Find Valence Electrons for Transition Metals Fe2+ is [Ar] 3d6, because it loses both 4s electrons and keeps all 6 d electrons. Fe3+ loses one more electron, this time from the d level, leaving 3d5.
Take copper and cobalt. Copper is [Ar] 3d10 4s1, so Cu2+ loses the single 4s electron and then one d electron, leaving 3d9. Cobalt is 3d7 4s2, so Co2+ loses both 4s electrons and becomes 3d7.
A common exam mistake is removing the last electrons added, which are the 3d ones. Doing that for Fe2+ gives 3d4 4s2, which is wrong. Always empty 4s before touching 3d.
For the d count of an ion, there is a shortcut: take the group number and subtract the ion’s charge. Fe3+ is 8 minus 3, giving d5. This works for the common charges of first-row ions, though for copper and chromium you should confirm with the full configuration.
When you need the full configuration, work in steps. First write the neutral atom’s configuration with the noble gas shorthand, so you know exactly what is available to remove. Subtract electrons from 4s first, one for each unit of positive charge, until 4s is empty. If the charge is bigger than the number of 4s electrons, take the remaining electrons from 3d. Finish by checking that the electron total equals the atomic number minus the charge, which catches slips.
Why Chromium and Copper Break the Pattern
Chromium is [Ar] 3d5 4s1, not the predicted 3d4 4s2. This arrangement gives a half-filled d level and six unpaired electrons in total.5Revision Village. What are exceptions to electron configurations? A half-filled d level is a particularly stable arrangement. Electrons in separate orbitals repel each other less, and matching spins lower the energy further, which outweighs the small gap between 4s and 3d.6Revision Village. What are exceptions to electron configurations? Following the filling order you would expect 3d4 4s2, but moving one electron from 4s to 3d is cheap because the two levels are close in energy, so the stability gained wins out.
A full d10 level has every orbital doubly occupied and is symmetrical, which is also a particularly stable arrangement. Copper follows the same idea. It is [Ar] 3d10 4s1, which gives it a completely full d level. The total count still matches the group number, 6 for chromium and 11 for copper.
The exception doesn’t change the valence count; only the split between 4s and 3d changes. For ions it hardly matters, because the 4s electron leaves first anyway: Cr3+ ends up as d3 whichever starting configuration you use.
To remember the exceptions, expect them just before a half-full d level (d5) or a full one (d10). Chromium and copper are the two in the first row. Heavier rows contain more exceptions, and those usually have to be memorized.
Why One Metal Can Have Several Charges
Transition metals can take on a range of charges in compounds mainly because d electrons can be removed as well as s electrons.7Chemistry LibreTexts. How to Find Valence Electrons for Transition Metals For iron, the energy needed to remove each of the first four electrons rises gradually, with no sudden large jump.8Mr Khemistry. Why Transition Elements Have Variable Oxidation States The extra energy is paid back when the ion bonds or dissolves, which makes several states stable.9Mr Khemistry. Why Transition Elements Have Variable Oxidation States
Iron is commonly found as both +2 and +3. Rust contains Fe3+, while some iron supplements contain Fe2+. The charge depends on what the metal is bonded to. Each is a stable option because the cost of losing the next electron is not much bigger than the last one.
To find the charge from a formula, assign typical charges to the other atoms, such as -2 for oxygen and -1 for chlorine. The metal’s charge must make the whole compound neutral. Iron in FeCl3 is +3, which makes it d5.
Once a metal has more than five d electrons, the electrons pair up and are less available for bonding.10EURASIA Journal of Mathematics, Science and Technology Education. Addressing Alternative Conceptions about Transition Metals among Form Six Students using Information and Communication Technology based Instruction So maximum charges generally increase up to manganese, which can reach +7, and then decrease toward the end of the row. For early metals the maximum charge usually matches the group number, while late metals can’t use all of their d electrons.
Zinc, Cadmium and Mercury
Zinc, cadmium and mercury have a complete d10 level and keep it in every known oxidation state.11Wikipedia. Transition metal So they almost always form +2 ions using only their two s electrons. Some chemists classify them as main-group or post-transition elements rather than transition metals.12Journal of Chemical Education. The Place of Zinc, Cadmium, and Mercury in the Periodic Table Removing an electron from a full d10 level costs far more energy than the bonding returns, so zinc stops after its two s electrons and never forms a series of charges like iron.
On a test, check which definition your course uses. Under the group-number method, zinc has 12 electrons. For bonding purposes, zinc acts as if it has 2 valence electrons.
Counting Electrons in Metal Complexes
A ligand is a molecule or ion attached to a metal that shares electrons with it. Complexes with 18 valence electrons tend to be stable, because they match the configuration of the next noble gas.13Wikipedia. 18-electron rule The number 18 comes from 2 slots in s, 6 in p and 10 in d.
There are two methods for counting electrons in complexes, the neutral method and the ionic method.14ilpi.com. How to Find Valence Electrons for Transition Metals Both give the same total, so you can use one to check the other.15Wikipedia. Electron counting Some ligands, such as chloride, count as 1 electron in the neutral method but 2 in the ionic method.16Chemistry LibreTexts. How to Find Valence Electrons for Transition Metals
For the neutral method, start with the metal’s group number, its electron count as a neutral atom. Add the electrons given by each ligand, using 2 for a typical two-electron donor such as carbon monoxide and 1 for chloride. If the whole complex has a charge, add one for each negative charge or subtract one for each positive charge. Compare the final total with 18.
Chromium hexacarbonyl, Cr(CO)6, shows it at work. Neutral chromium contributes 6 electrons. Each carbon monoxide donates 2, so six of them give 12. The total of 18 explains why this compound is stable.
For the ionic method, first find the metal’s charge by treating each ligand as the ion it would be if it left, such as chloride as 1-. Next write the metal ion’s d count, the group number minus that charge. Then add 2 electrons for each ligand, since every ligand is treated as donating a pair. The answer should match the neutral method, so doing both catches mistakes.
The rule works best for compounds with carbon-metal bonds. Many simple salts in water break it, so treat it as a rule of thumb rather than a law.
What the d Electron Count Predicts
A compound’s magnetic behavior depends on how many unpaired d electrons the metal ion has.17King Saud University. Chemistry of Transition Metals d0 and d10 ions are usually colorless and not magnetic, while many other ions are colored.18King Saud University. Chemistry of Transition Metals Ligands split the five d orbitals into a higher-energy group and a lower-energy group.
Color comes from that gap. Electrons absorb light energy to jump across it, and the color we see is the light left over after absorption. A d10 ion has no room for such a jump, so it absorbs no visible light and looks colorless; Cu+, also d10, tends to form colorless compounds.
When the gap between the groups is small, electrons spread out one per orbital before pairing up, which is called high spin.19Wikipedia. Spin states (d electrons) When the gap is large, the lower orbitals fill completely first, which is called low spin.20Wikipedia. Spin states (d electrons) The number of unpaired electrons, and so the magnetic behavior, depends on which arrangement the ion takes.
Fe3+ is d5. In the high-spin arrangement it has 5 unpaired electrons, and in the low-spin arrangement only 1. So the same d count can give different magnetic strength depending on the arrangement.
To count unpaired electrons, first find the d count of the ion, then decide whether the arrangement is high spin or low spin. In high spin, place one electron in each of the five orbitals before pairing any. In low spin, fill the lower group completely before using the upper group. Then count the electrons left alone in an orbital, since each one adds to the magnetic effect.