Practice worksheet for writing and analyzing electron configurations of elements.

Let’s go step by step to solve each part of this worksheet. We’ll start with unabbreviated electron configurations, then abbreviated ones, then identify elements from given configs, and finally check which configs are invalid.

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Part 1: Unabbreviated Electron Configurations (Questions 1–5)

We write out the full configuration using the order:
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p...

Remember:
- s holds max 2 electrons
- p holds max 6
- d holds max 10
- f holds max 14

Also, follow the Aufbau principle (fill lowest energy first) and remember exceptions for some transition metals — but for these, we’ll stick to standard rules unless noted.

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1) Sodium (Na) – atomic number 11

Fill up to 11 electrons:

1s² (2)
2s² (4 total)
2p⁶ (10 total)
3s¹ (11 total)

→ 1s² 2s² 2p⁶ 3s¹

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2) Iron (Fe) – atomic number 26

1s² (2)
2s² (4)
2p⁶ (10)
3s² (12)
3p⁶ (18)
4s² (20) ← fills before 3d
3d⁶ (26)

→ 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

*(Note: Some sources write 3d before 4s in final notation, but when writing unabbreviated, we usually list by shell order: so 3d comes after 4s in filling, but in written form, sometimes grouped by n. However, for practice worksheets like this, they often expect the order as filled: 4s before 3d. But actually, standard convention is to write in order of increasing n, so 3d before 4s? Let me double-check.*

Actually, in most high school contexts, they accept either, but the *correct* way to write unabbreviated is by principal quantum number (n), so:

After argon core (18 e⁻): [Ar] = 1s²2s²2p⁶3s²3p⁶

Then for Fe: add 4s²3d⁶ → but when writing fully, group by n:

So: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s²

Wait — no! The actual ground state configuration of iron is written as [Ar] 4s² 3d⁶, meaning 4s fills before 3d, but when listing all orbitals, we typically list them in order of n, so 3d before 4s? Actually, no — the convention is to list in the order they are filled, or by n?

I think for “unabbreviated”, they just want all orbitals listed in order of increasing n, and within n, s,p,d,f.

So for Fe (Z=26):

n=1: 1s²
n=2: 2s² 2p⁶
n=3: 3s² 3p⁶ 3d⁶
n=4: 4s²

But wait — that would be 2+8+14+2 = 26? 3d⁶ is 6 electrons, so n=3 has 2+6+6=14? No:

3s² + 3p⁶ + 3d⁶ = 14 electrons in n=3? That’s correct.

Total: n1:2, n2:8, n3:14, n4:2 → 26. Yes.

But actually, the standard way to write it is:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ — because 4s fills before 3d, and many textbooks list it that way even though n=4 > n=3.

To avoid confusion, let’s check a reliable source mentally: Iron is commonly written as [Ar] 4s² 3d⁶, so unabbreviated would be:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

Yes, that’s what most worksheets expect.

So I’ll go with: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

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3) Bromine (Br) – atomic number 35

Up to Ar (18): 1s²2s²2p⁶3s²3p⁶

Then: 4s² (20), 3d¹⁰ (30), 4p⁵ (35)

So: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵

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4) Barium (Ba) – atomic number 56

After Xe (54): [Xe] = 1s²...5p⁶

Then Ba adds 6s²

So full config: up to Xe is 54 electrons, then 6s².

Xe is: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶

Then Ba: add 6s²

So: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s²

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5) Neptunium (Np) – atomic number 93

This is an actinide. After Rn (86), we have 7s², then 5f⁴, 6d¹? Wait, let's recall.

Radon (Rn) is 86: [Xe]4f¹⁴5d¹⁰6s²6p⁶? Better to build step by step.

Standard configuration for Np is [Rn] 5f⁴ 6d¹ 7s²

So unabbreviated: start from Rn, which is:

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

That’s 86 electrons.

Then Np (93) adds 7 more: 7s², 5f⁴, 6d¹ — but order of filling: 7s fills before 5f and 6d.

Actual ground state: [Rn] 5f⁴ 6d¹ 7s²

So full config:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹

But when writing, we should group by n? Or list in filling order? For consistency, since we did Fe as 4s before 3d, here we’ll list 7s before 5f and 6d.

However, standard notation lists subshells in order of n, so:

n=1 to 6 as above, then n=7: 7s², and n=5: 5f⁴, n=6: 6d¹ — but that mixes n.

Better to list in order of increasing n, and within n, s,p,d,f.

So:

After Rn core (which includes up to 6p⁶), we have:

For Np: additional electrons in 7s, 5f, 6d.

But 5f is n=5, 6d is n=6, 7s is n=7.

So in order of n:

n=5: already has 5s²5p⁶5d¹⁰ from Rn? Rn has 5s²5p⁶, and 4f¹⁴, 5d¹⁰? Let's count Rn properly.

Radon (86):

1s² (2)
2s²2p⁶ (8) → total 10
3s²3p⁶ (8) → 18
4s²3d¹⁰4p⁶ (18) → 36
5s²4d¹⁰5p⁶ (18) → 54
6s²4f¹⁴5d¹⁰6p⁶ (32) → 86? 54 + 2+14+10+6 = 86 yes.

So Rn: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶

Now Np (93) adds 7 electrons: typically 7s² 5f⁴ 6d¹

So full config: same as Rn plus 7s² 5f⁴ 6d¹

When writing unabbreviated, we can insert them in order of n:

So after 6p⁶, we have n=7: 7s², but also n=5: 5f⁴ (since 5f is part of n=5), and n=6: 6d¹.

But 5f should come after 4f in the list? In the sequence, 4f is listed after 6s in Rn, which is not by n.

To avoid confusion, for this level, we'll write in the order they are added: after Rn, add 7s², then 5f⁴, then 6d¹.

But standard is to list all subshells in order of n, so:

List all n=1 to n=7.

n=1: 1s²
n=2: 2s² 2p⁶
n=3: 3s² 3p⁶ 3d¹⁰
n=4: 4s² 4p⁶ 4d¹⁰ 4f¹⁴
n=5: 5s² 5p⁶ 5d¹⁰ 5f⁴
n=6: 6s² 6p⁶ 6d¹
n=7: 7s²

Now sum: n1:2, n2:8, n3:18 (3s2+3p6+3d10=18), n4: 4s2+4p6+4d10+4f14=32, n5:5s2+5p6+5d10+5f4=22, n6:6s2+6p6+6d1=9, n7:2 → total 2+8+18+32+22+9+2=93. Good.

But this is messy. Most worksheets expect the configuration written in the order of filling, not grouped by n.

For example, for uranium, it's often written as [Rn] 5f³ 6d¹ 7s², so unabbreviated would be Rn config plus those.

And Rn is written as 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p6

Then add 7s2 5f4 6d1 — but 5f4 should come before 6d1? In filling order, 7s fills first, then 5f, then 6d for early actinides.

For Np, it's [Rn] 5f4 6d1 7s2, so perhaps write as:

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹

I think that's acceptable.

Some might write 5f before 7s, but I'll go with filling order.

So: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹

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Part 2: Abbreviated Electron Configurations (Questions 6–10)

Use noble gas cores.

6) Cobalt (Co) – atomic number 27

Previous noble gas: Ar (18)

Then: 4s² 3d⁷

So: [Ar] 4s² 3d⁷

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7) Silver (Ag) – atomic number 47

Exception! Silver is [Kr] 5s¹ 4d¹⁰, not 5s² 4d⁹

Because half-filled or full d-subshell is stable.

Kr is 36, so Ag: 47 - 36 = 11 electrons: normally 5s² 4d⁹, but actual is 5s¹ 4d¹⁰

So: [Kr] 5s¹ 4d¹⁰

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8) Tellurium (Te) – atomic number 52

Kr is 36, so 52-36=16 electrons: 5s² 4d¹⁰ 5p⁴

So: [Kr] 5s² 4d¹⁰ 5p⁴

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9) Radium (Ra) – atomic number 88

Previous noble gas: Rn (86)

Then: 7s²

So: [Rn] 7s²

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10) Lawrencium (Lr) – atomic number 103

After Rn (86), we have 17 more electrons.

Lawrencium is [Rn] 7s² 5f¹⁴ 7p¹? Or 6d¹?

Actual configuration: [Rn] 5f¹⁴ 7s² 7p¹? I think it's [Rn] 5f¹⁴ 6d¹ 7s²

Yes, lawrencium is [Rn] 5f¹⁴ 6d¹ 7s²

So: [Rn] 5f¹⁴ 6d¹ 7s²

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Part 3: Identify Elements from Configurations (Questions 11–15)

Count total electrons.

11) 1s² 2s² 2p⁶ 3s² 3p⁵

Sum: 2+2+6+2+5 = 17 → Chlorine (Cl)

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12) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹

Sum: 2+2+6+2+6+2+10+6+1 = let's calculate:

Up to 4p⁶: that's Kr, 36 electrons

Then 5s¹ → 37 → Rubidium (Rb)

But Rb is [Kr]5s¹, yes.

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13) [Kr] 5s² 4d¹⁰ 5p³

Kr is 36, plus 2+10+3=15 → 51 → Antimony (Sb)

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14) [Xe] 6s² 4f¹⁴ 5d⁶

Xe is 54, plus 2+14+6=22 → 76 → Osmium (Os)

Check: Os is [Xe] 4f14 5d6 6s2, yes.

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15) [Rn] 7s² 5f¹¹

Rn is 86, plus 2+11=13 → 99 → Einsteinium (Es)

Es is [Rn] 5f11 7s2, yes.

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Part 4: Invalid Configurations (Questions 16–20)

Check if any subshell exceeds max electrons, or if order is wrong (but order might be ok, we care about validity).

Max per subshell: s:2, p:6, d:10, f:14

Also, no skipping without reason, but mainly check electron counts per subshell.

16) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 4d¹⁰ 4p⁵

Look at 4d¹⁰ — but 4d should come after 4p? In filling order, 4s, then 3d, then 4p, then 5s, then 4d.

Here, it has 4s², then 4d¹⁰, then 4p⁵ — that means 4d is filled before 4p, which is wrong because 4p has lower energy than 4d.

Moreover, after 4s², it should be 3d, not 4d.

The configuration lists 4d before 4p, but 4p should be filled before 4d.

Also, the subshells are out of order, but more importantly, is it possible? Total electrons: 2+2+6+2+6+2+10+5=35, which is bromine, but bromine is 1s2...4s2 3d10 4p5, not 4d10.

Here it has 4d10 instead of 3d10 — that's invalid because 3d must be filled before 4d.

Specifically, 4d cannot be occupied until after 5s, and certainly not before 4p.

So this is invalid.

Reason: 4d subshell is filled before 3d and 4p, which violates Aufbau principle. Also, 3d is missing entirely.

In this config, there is no 3d mentioned, but it jumps to 4d after 4s.

So definitely invalid.

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17) 1s² 2s² 2p⁶ 3s² 3d⁵

After 3s², it goes to 3d⁵, skipping 3p.

3p should be filled before 3d.

Total electrons: 2+2+6+2+5=17, but chlorine is 3p5, not 3d5.

3d starts after 4s.

So invalid: missing 3p subshell, and 3d filled too early.

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18) [Ra] 7s² 5f⁶

Ra is radium, atomic number 88, configuration [Rn]7s², so [Ra] means up to Ra.

Then adding 7s² 5f⁶ — but Ra already has 7s², so adding another 7s² would be duplicate.

[Ra] includes 7s², so [Ra] 7s² 5f⁶ implies 7s⁴, which is impossible since s subshell holds only 2.

So invalid: cannot have 7s² after [Ra] because [Ra] already has 7s².

If it were [Rn] 7s² 5f⁶, that would be curium or something, but [Ra] is wrong.

Ra is element 88, so [Ra] represents 88 electrons, all including 7s².

Adding more electrons to 7s is invalid.

So this is invalid.

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19) [Kr] 5s² 4d¹⁰ 5p⁵

Kr is 36, plus 2+10+5=17 → 53, which is iodine.

Iodine is [Kr] 5s² 4d¹⁰ 5p⁵ — yes, that's correct.

Is it valid? Subshells: 5s² (ok), 4d¹⁰ (ok), 5p⁵ (ok, less than 6).

Order: after Kr, 5s, then 4d, then 5p — that's standard.

So this is VALID. But the question is "which are not valid", so this one is valid, so not selected.

The task is to determine which are NOT valid, so we list the invalid ones.

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20) [Xe]

[Xe] alone means just xenon, which is a valid configuration for xenon atom.

But the question is "electron configurations", and [Xe] is the abbreviated config for Xe, so it is valid.

However, in context, probably they mean if it's written as is, it's fine.

But let's see the others.

For 20, it's just [Xe], which corresponds to 54 electrons, xenon, so valid.

But perhaps they consider it incomplete? No, it's a valid configuration.

Looking back at the worksheet, question 20 is "[Xe]", and we need to say if it's not valid.

It is valid.

But let's confirm all.

Earlier:

16: invalid (wrong order, missing 3d)

17: invalid (missing 3p, 3d too early)

18: invalid ([Ra] already has 7s², can't add more)

19: valid

20: valid

But 20 is just [Xe], which is fine.

Perhaps for 20, if it's meant to be an ion or something, but no, it's given as is.

Another thought: sometimes [Xe] alone might be considered not a full config, but in abbreviated form, it's acceptable for Xe.

I think it's valid.

But let's see the answer expected.

Perhaps I missed something.

For 18: [Ra] 7s² 5f⁶ — Ra is 88, config is [Rn]7s², so [Ra] implies 88 electrons with 7s² occupied. Adding 7s² again would require 7s⁴, impossible. So invalid.

For 20: [Xe] — this is the configuration for xenon, so it is valid. But perhaps the question is tricking us; maybe they think it's incomplete, but no.

Looking at the list, questions 16 to 20 are to determine which are not valid, so we list the numbers that are invalid.

From above: 16,17,18 are invalid; 19 and 20 are valid.

But let's double-check 19: [Kr] 5s² 4d¹⁰ 5p⁵ — yes, iodine, valid.

20: [Xe] — xenon, valid.

But perhaps for 20, since it's just the noble gas, and no additional electrons, it's still valid.

Maybe the worksheet expects us to say it's valid, so not in the "not valid" list.

Another point: in question 18, [Ra] 7s² 5f⁶ — even if we ignore the duplication, 5f⁶ after Ra: Ra is period 7, group 2, so next elements fill 5f, but [Ra] includes 7s², so adding 5f⁶ is ok, but adding another 7s² is not.

The config says [Ra] 7s² 5f⁶, which explicitly adds 7s², so it's wrong.

If it were [Rn] 7s² 5f⁶, that would be Cm, curium, atomic number 96.

But here it's [Ra], so invalid.

Now for 20, [Xe] is fine.

But let's see if there's a trick.

Perhaps [Xe] alone is not a configuration for an element in the sense that it's abbreviated, but it is valid for Xe.

I think it's valid.

Maybe the problem is that for neutral atoms, [Xe] represents Xe, so it's ok.

So invalid ones are 16,17,18.

But let's confirm 17: 1s² 2s² 2p⁶ 3s² 3d⁵ — this has no 3p, and 3d before 4s, which is wrong. Total electrons 17, but should be 1s2 2s2 2p6 3s2 3p5 for Cl, not 3d5. So invalid.

Similarly, 16 has 4d10 before 4p5 and no 3d, invalid.

18 invalid as explained.

19 and 20 valid.

But question 20 is "[Xe]", and in the list, it's item 20, while 19 is [Kr]5s2 4d10 5p5, which is valid.

Perhaps for 20, since it's just the symbol, and no electrons specified beyond, but it's understood as the config of Xe.

I think it's valid.

Maybe the worksheet has a typo, but based on standard, 20 is valid.

Another thought: in some contexts, writing just [Xe] might be considered incomplete if they expect full config, but the section is "determine which are not valid", and [Xe] is a valid abbreviated configuration.

Moreover, in part 2, they used [Kr], [Xe], etc., so it's acceptable.

So I'll go with 16,17,18 being invalid.

But let's see the numbering: questions 16,17,18,19,20.

In the user input, it's:

16) ...

17) ...

18) [Ra] 7s² 5f⁶

19) [Kr] 5s² 4d¹⁰ 5p⁵

20) [Xe]

And 18 is listed twice? No, in the original:

"16) 1s²2s²2p⁶3s²3p⁶4s²4d¹⁰4p⁵

17) 1s²2s²2p⁶3s²3d⁵

18) [Ra] 7s²5f⁶

19) [Kr] 5s²4d¹⁰5p⁵

20) [Xe]"

Yes.

So for 18: [Ra] 7s² 5f⁶ — invalid because [Ra] already contains 7s², so adding another 7s² makes 7s⁴, impossible.

For 20: [Xe] — valid.

But perhaps they consider that [Xe] alone doesn't specify the element, but it does imply Xe.

I think it's fine.

Maybe for 20, since it's just the noble gas, and no valence electrons, but it's still a valid configuration.

So the not valid ones are 16,17,18.

But let's make sure about 19: [Kr] 5s² 4d¹⁰ 5p⁵ — this is correct for iodine.

Is there any issue? 4d¹⁰ is full, 5p⁵ is almost full, all good.

So yes.

Therefore, for the last part, the invalid configurations are 16, 17, and 18.

Now, to summarize all answers.

Final Answer:

1) 1s² 2s² 2p⁶ 3s¹
2) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
3) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵
4) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s²
5) 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁴ 6d¹
6) [Ar] 4s² 3d⁷
7) [Kr] 5s¹ 4d¹⁰
8) [Kr] 5s² 4d¹⁰ 5p⁴
9) [Rn] 7s²
10) [Rn] 5f¹⁴ 6d¹ 7s²
11) Chlorine (Cl)
12) Rubidium (Rb)
13) Antimony (Sb)
14) Osmium (Os)
15) Einsteinium (Es)
16) Not valid
17) Not valid
18) Not valid
19) Valid (so not selected)
20) Valid (so not selected)

For the last part, since the question is "determine which are not valid", we list the numbers: 16, 17, 18.

In the answer, probably write the numbers or state which ones.

Since it's a worksheet, likely fill in the blank with "not valid" or something, but in our case, for the final answer, we can list them.

Looking at the format, for 16-20, it says "__________", so probably write "invalid" or the reason, but the instruction is to provide final answer, so perhaps list the invalid ones.

To be precise, for the response, I'll write the answers as per the worksheet.

Since the user will copy, I'll provide all.

But in Final Answer section, put all.

Now, for neptunium, I recall that the actual configuration might be different, but for this level, [Rn] 5f4 6d1 7s2 is accepted.

Similarly for lawrencium.

I think it's fine.

So compiling.