Common elements and their symbols (first 30): Exercises

Level 1 — Recognition

Can you recall the badge and the roll number?

L1.1

State the symbol and atomic number of: (a) Carbon, (b) Sodium, (c) Iron.

Recall Solution

Read straight off the ordered list of the first 30.

(a) Carbon → C, $Z = 6$ . English name, first letter.
(b) Sodium → Na, $Z = 11$ . Latin Natrium — an oddball, so the badge is "Na", not "So".
(c) Iron → Fe, $Z = 26$ . Latin Ferrum — badge "Fe", not "Ir".

L1.2

Name the elements with these symbols: (a) K, (b) Cl, (c) Zn, (d) N.

Recall Solution

(a) K = Potassium, $Z = 19$ (Latin Kalium, single letter).
(b) Cl = Chlorine, $Z = 17$ .
(c) Zn = Zinc, $Z = 30$ (the last of the first 30).
(d) N = Nitrogen, $Z = 7$ . Careful: N alone is nitrogen; Na is sodium.

Level 2 — Application

Use $Z$ and $A$ to compute.

L2.1

An atom is written $_{20}^{40} X$ . Identify X and find its number of neutrons.

Recall Solution

$Z = 20$ → from the list, element 20 is Calcium, so $X = Ca$ .
Neutrons $= A - Z = 40 - 20 = 20$ . Why subtract? $A$ counts all nucleons; remove the $Z$ protons and only neutrons remain.

L2.2

How many neutrons are in $_{26}^{56} Fe$ ? Confirm the symbol matches its $Z$ .

Recall Solution

$neutrons = A - Z = 56 - 26 = 30$ .
Fe = Iron, and iron's atomic number is $Z = 26$ ✓ — the subscript agrees with the badge.

L2.3

A cobalt atom has 27 protons and 32 neutrons. Write its full notation $_{Z}^{A} X$ .

Recall Solution

Symbol: cobalt → Co (capital C, lowercase o).
$Z = protons = 27$ .
$A = protons + neutrons = 27 + 32 = 59$ .
Full notation: $_{27}^{59} Co$ .

Level 3 — Analysis

Spot what's wrong, or reason from a subtle clue.

L3.1

A student writes the formula "CO" and calls it cobalt. Explain in one sentence what "CO" actually is and give the correct symbol for cobalt.

Recall Solution

"CO" = C (carbon) + O (oxygen) = carbon monoxide, a compound of two elements. Cobalt's symbol is Co (capital C, lowercase o) — one element. The lowercase second letter is what says "this is a single two-letter symbol, not two separate atoms."

L3.2

Two atoms are $_{17}^{35} Cl$ and $_{17}^{37} Cl$ . Are they the same element? How do their proton and neutron counts compare?

Recall Solution

Same $Z = 17$ → both are Chlorine, so yes, same element (see Isotopes).
Protons: both have $17$ (equal — $Z$ is fixed for an element).
Neutrons: $35 - 17 = 18$ vs $37 - 17 = 20$ . They differ by 2 neutrons.
This is exactly what an isotope is: same protons ( $Z$ ), different mass number ( $A$ ).

L3.3

Someone claims the element at $Z = 19$ is "Kr". Is that right? If not, correct it.

Recall Solution

Wrong. Kr = Krypton, which lies beyond the first 30 ( $Z = 36$ ). The element at $Z = 19$ is Potassium, K (single letter, Latin Kalium). Discriminator: K (one letter) = potassium; Kr (two letters) = krypton.

Level 4 — Synthesis

Build an answer from several clues at once.

L4.1

I am element number $6 + 2$ in the ordered list, I am a nonmetal, and $H_{2} me$ is water. Who am I? Give symbol and $Z$ .

Recall Solution

$6 + 2 = 8$ , so $Z = 8$ → Oxygen.
Symbol O, and indeed $H_{2} O$ is water ✓.
It is a nonmetal ✓ (see Metals Nonmetals and Metalloids).

L4.2

List, in order, the symbols of the transition metals from iron to zinc ( $Z = 26$ to $30$ ). Then state how many neutrons $_{30}^{64} Zn$ has.

Recall Solution

$26$ Fe, $27$ Co, $28$ Ni, $29$ Cu, $30$ Zn → Fe, Co, Ni, Cu, Zn.
Neutrons in $_{30}^{64} Zn = A - Z = 64 - 30 = 34$ .

L4.3

Decode the compound $NaCl$ : name each element, its $Z$ , and explain why it is not read as "N + a + C + l".

Recall Solution

Na (one capital + one lowercase) = Sodium, $Z = 11$ .
Cl (one capital + one lowercase) = Chlorine, $Z = 17$ .
Reading rule: scan left to right; a lowercase letter belongs to the capital before it. So "Na" groups together, then "Cl" groups together — never four separate letters. Hence $NaCl$ = sodium chloride (table salt), two elements, not four.

Level 5 — Mastery

Multi-step, exam-final level — every rule at once.

L5.1

An unknown atom has mass number $A = 39$ and contains $20$ neutrons. (a) Find $Z$ . (b) Identify the element and write its symbol. (c) Write the full notation. (d) Is it a metal or nonmetal?

Recall Solution

(a) $A = Z + neutrons \Rightarrow Z = A - neutrons = 39 - 20 = 19$ .
(b) $Z = 19$ → Potassium, symbol K (Latin Kalium, single letter).
(c) Full notation: $_{19}^{39} K$ .
(d) Potassium is a metal (an alkali metal). See Metals Nonmetals and Metalloids.

L5.2

Three atoms: $_{12}^{24} Mg$ , $_{12}^{25} Mg$ , $_{12}^{26} Mg$ . (a) Same element? (b) Neutron count of each. (c) What single term describes this trio? (d) Which has the most neutrons?

Recall Solution

(a) All have $Z = 12$ → all Magnesium, so yes, same element.
(b) Neutrons $= A - Z$ :
- $24 - 12 = 12$
- $25 - 12 = 13$
- $26 - 12 = 14$
(c) Same $Z$ , different $A$ → they are isotopes of magnesium.
(d) $_{12}^{26} Mg$ has the most neutrons ( $14$ ).

L5.3

Chain puzzle. Element P is at $Z = 15$ . Element Q sits three places after P in the list. Element R is the Latin-named metal at $Z = 26$ . (a) Name and give symbols of P, Q, R. (b) If R appears as $_{26}^{58} R$ , how many neutrons? (c) Which of P, Q, R is a nonmetal?

Recall Solution

P: $Z = 15$ → Phosphorus, P (yes, the symbol is literally P).
Q: three after $Z = 15$ is $Z = 18$ → Argon, Ar (a noble gas).
R: $Z = 26$ , Latin-named metal → Iron, Fe (Ferrum).
(b) Neutrons in $_{26}^{58} Fe = 58 - 26 = 32$ .
(c) Phosphorus is the nonmetal. (Argon is a noble gas / nonmetal too, but the clean "nonmetal solid" answer intended here is phosphorus; iron is a metal.)

Connections

Common elements and their symbols (first 30) (index 1.4.5) — the parent note these drills test.
Atomic Number and Mass Number — the $Z$ and $A$ used in every problem.
Structure of the Atom — why protons/neutrons/electrons underlie these counts.
Isotopes — L3.2 and L5.2 rest entirely on this idea.
Metals Nonmetals and Metalloids — classification steps in L4 and L5.
Valency and Electron Configuration — the next layer once symbols are fluent.