Atomic number Z, mass number A, isotopes, isobars, isotones: Question bank

Reminder of the three counters before you start:

$Z$ = protons (the element's fingerprint).
$A$ = protons + neutrons = nucleons.
$N = A - Z$ = neutrons.

True or false — justify

An isotope of an element has a different atomic number from the parent element.

False — isotopes share the same

Z

(that is what keeps them the same element); only

A

(hence

N

) differs. Changing

Z

would make it a different element entirely.

Two atoms with the same mass number

A

must have the same number of neutrons.

False — same

A

is the isobar condition, but

N = A - Z

depends on

Z

too. Different elements can share

A

yet split it differently between protons and neutrons.

If two atoms have the same number of neutrons they must be isotones of each other.

True by definition — isotones are precisely "same

N

." But note they will differ in

Z

and

A

(otherwise they'd be the identical nuclide), so "same

N

, different element" is the full picture.

When a neutral atom loses an electron to become a cation, its mass number

A

decreases.

False —

A

counts only nucleons (protons + neutrons), and electrons are not nucleons. Removing an electron leaves the nucleus untouched, so

A

and

Z

are both unchanged; only the electron count drops.

All isotopes of one element behave identically in chemical reactions.

Essentially true — chemistry is decided by electrons, and same

Z

means same electron count and configuration. Real kinetic isotope effects make reaction rates differ slightly (heavier isotopes move more slowly), but this is tiny next to the difference between elements.

Isobars are always atoms of the same element.

False — isobars have the same

A

but different

Z

, so they are always different elements. Same-element/different-

A

would be isotopes, not isobars.

A single atom can be simultaneously an isotope of one atom and an isobar of another.

True — "isotope/isobar/isotone" describe relationships between pairs, not fixed labels.

_{18}^{40} Ar

is an isobar of

_{20}^{40} Ca

and also an isotope of

_{18}^{36} Ar

The number of electrons in a neutral atom equals its mass number.

False — for a neutral atom electrons equal

Z

(proton count), not

A

A

includes neutrons, which carry no charge and are never balanced by electrons.

Spot the error

_{6}^{14} C

and

_{7}^{14} N

have the same mass, so they are isotones."

The error is confusing isobars with isotones. Same

A = 14

makes them isobars; their neutron counts are

N = 8

and

N = 7

— different, so they are not isotones.

_{17}^{35} Cl

and

_{17}^{37} Cl

are isobars because they are the same element."

Wrong on two counts: same element means same

Z

, which is the isotope condition, and their

A

values (35 vs 37) actually differ, so they cannot be isobars anyway.

"An

O^{2 -}

ion has atomic number 10 because it has 10 electrons."

Atomic number is the proton count, not the electron count. Oxygen keeps

Z = 8

as an ion; gaining 2 electrons gives 10 electrons but leaves

Z = 8

unchanged.

"Neutrons in

_{20}^{40} Ca

A + Z = 40 + 20 = 60

The formula is reversed. Neutrons

= A - Z = 40 - 20 = 20

; you subtract protons from nucleons, never add.

"Since protons and neutrons both live in the nucleus,

Z

counts them both."

No —

Z

counts only protons. The count of all nucleons (protons + neutrons) is

A

, and neutrons alone are

N = A - Z

"Two nuclides with the same

Z

and the same

N

are isotopes."

Same

Z

and same

N

forces same

A

too — they are the identical nuclide, not a pair of isotopes. Isotopes must differ in

N

(hence

A

Why questions

Why does changing the number of neutrons not change which element an atom is?

Element identity is set by

Z

(proton count), which fixes the electron count and therefore all chemistry. Neutrons only add mass, so altering

N

moves you between isotopes of the same element.

Why must isobars be different elements even though the word "iso" means "same"?

"Iso" here refers to same mass number

A

, not same element. To have the same

A

with a different identity you must redistribute nucleons — different

Z

— which by definition means a different element.

Why is almost the entire mass of an atom concentrated in the nucleus?

Protons and neutrons each weigh about 1 unified mass unit, while an electron is ~1836× lighter. So the nucleons (counted by

A

) dominate the mass and electrons contribute almost nothing.

Why do we compute the trio

(Z, A, N)

first before classifying a pair?

Each family is defined by one matching quantity — isotopes match

Z

, isobars match

A

, isotones match

N

. Laying out all three lets you see at a glance which one coincides instead of guessing.

Why can an ion never be classified as a different element from its parent atom?

Ionization only adds or removes electrons; the nucleus (and thus

Z

) is never touched. Since the element is defined by

Z

, the ion is the same element carrying a charge.

Why does "same mass number" not guarantee "same nucleon split"?

A

fixes only the total nucleon count. That total can be partitioned into protons and neutrons in different ways (

A = Z + N

), so two nuclides with equal

A

can still have different

Z

and

N

Edge cases

Can two atoms be isotones with

N = 0

In principle yes for the extreme case — ordinary hydrogen

_{1}^{1} H

has

N = A - Z = 1 - 1 = 0

, so any two neutron-free nuclides would share

N = 0

. In practice

^{1}

H is the only common stable one, so such pairs are more a definitional point than a real classroom example.

_{1}^{1} H

(no neutrons at all) still a valid atom, and does

A = Z + N

still hold?

Yes — it has one proton, zero neutrons, so

A = 1 = Z + N = 1 + 0

. The formula holds perfectly;

N

is simply allowed to be zero.

For the bare nucleus

H^{+}

(a lone proton, zero electrons), does

Z

still equal the electron count?

No — the rule "electrons

= Z

" holds only for a neutral atom. Here electrons

= Z - q = 1 - 1 = 0

(charge

q = + 1

), so it has zero electrons while

Z

remains 1.

Can a nuclide be an isotope, isobar, and isotone all at once relative to three different partners?

Yes — because these are pairwise relationships, one nuclide can play all three roles with three different partners simultaneously. For instance

_{18}^{40} Ar

(

N = 22

) is an isotope of

_{18}^{36} Ar

, an isobar of

_{20}^{40} Ca

, and an isotone of

_{19}^{41} K

(both have

N = 22

Two nuclides have identical

Z

A

, and

N

— what relationship do they have?

None of the three "iso" relationships — they are the same nuclide, not a distinct pair. The families require some difference (in

A

, or

Z

N

), so identical numbers means identical atoms.

If an anion

X^{3 -}

has 18 electrons, what is

Z

, and does the charge affect

A

With charge

q = - 3

, electrons

= Z - q

, so

18 = Z - (- 3) = Z + 3

, giving

Z = 15

(phosphorus). The charge tells you nothing about

A

— mass number depends only on the untouched nucleus.

Connections

Discovery of Protons, Neutrons and Electrons — why electrons are so light that $A$ ignores them.
Average Atomic Mass and Isotopic Abundance — how isotope mixtures give non-integer atomic masses.
Radioactivity and Nuclear Stability — why the $N$ vs $Z$ balance matters for real nuclei.
Periodic Table and Atomic Number — elements ordered by $Z$ , the same fingerprint used here.