An atom of an element contains 29 electrons and 35 neutrons. Deduce (i) the number of protons and (ii) the ele

Exercises · 2.26

Q. An atom of an element contains 29 electrons and 35 neutrons. Deduce

(i) the number of protons and

(ii) the electronic configuration of the element.

Jammu Kashmir JkboseTextbookSubjective· 2mImportance★★★★★est

31% · 44/140 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Atomic Notation

Atomic Notation: The ID Card of an Atom

Imagine you're at a huge stadium filled with people. To identify any one person, you'd need more than just a name — you'd need their jersey number, their team, maybe their position. An atom is the same way. Just saying "carbon" or "oxygen" tells you the kind of atom, but not the full story. Atomic notation is the shorthand that gives you the complete identity.

The Intuition: Three Numbers, One Symbol

Every atom is built from three key particles:

Protons (positive charge) — these define what element it is.
Neutrons (no charge) — these add mass and stability.
Electrons (negative charge) — these orbit the nucleus and determine chemical behaviour.

Atomic notation packs all this information around the element's symbol. Think of it as a label with two numbers attached:

^{A}_{Z}\text{X}

Where:

X is the element symbol (e.g., C for carbon, O for oxygen).
Z is the atomic number (number of protons).
A is the mass number (protons + neutrons).

Note

The atomic number $Z$ is what makes an atom that element. Change $Z$ , and you change the element entirely. Carbon always has $Z=6$ ; if you change it to $7$ , it's nitrogen.

The Precise Statement

For any atom represented as $^{A}_{Z}\text{X}$ :

Atomic number $Z$ = number of protons = number of electrons (in a neutral atom).
Mass number $A$ = number of protons + number of neutrons.
Number of neutrons $N = A - Z$ .

A neutral atom has equal protons and electrons. If the atom gains or loses electrons, it becomes an ion — but the notation for the nucleus stays the same.

Example: Carbon-12

The most common carbon atom is written as:

^{12}_{6}\text{C}

From this:

$Z=6$ → 6 protons, and in a neutral atom, 6 electrons.
$A=12$ → total nucleons (protons + neutrons) = 12.
Neutrons $N = 12 - 6 = 6$ .

Why Two Numbers? Why Not Just One?

If you only knew the mass number $A$ , you wouldn't know the element — because different elements can have the same mass number (e.g., $^{14}\text{C}$ and $^{14}\text{N}$ both have $A=14$ ). If you only knew the atomic number $Z$ , you wouldn't know how many neutrons are present — and that matters for isotopes.

Important

Isotopes are atoms of the same element (same $Z$ ) with different numbers of neutrons (different $A$ ). For example:

$^{12}_{6}\text{C}$ (6 neutrons)
$^{13}_{6}\text{C}$ (7 neutrons)
$^{14}_{6}\text{C}$ (8 neutrons) All are carbon, but they have different masses and some are radioactive.

A Quick Table for Clarity

Notation	Element	Protons ( $Z$ )	Neutrons ( $N$ )	Mass Number ( $A$ )
$^{1}_{1}\text{H}$	Hydrogen	1	0	1
$^{4}_{2}\text{He}$	Helium	2	2	4
$^{23}_{11}\text{Na}$	Sodium	11	12	23

Start your 14-day free trial to unlock the full solution →

In a neutral atom, the number of protons equals the number of electrons. With 29 electrons, the element has 29 protons, which identifies it as copper (Cu). Its electronic configuration is $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$ .

The key to this problem lies in understanding the relationship between the three fundamental particles in an atom: protons, electrons, and neutrons. For a neutral atom — which is what we always assume unless told otherwise — the number of protons and electrons must be equal. This is because the positive charge of each proton balances the negative charge of each electron, making the atom electrically neutral.

The number of neutrons, on the other hand, is independent of the charge. It only affects the mass of the atom (the isotope), not its identity. So the first step is always: protons = electrons in a neutral atom.

Finding the number of protons

The atom has 29 electrons. Since it is neutral, the number of protons must also be 29.

This number is the atomic number (Z) of the element.

So $Z = 29$ .
Identifying the element

The atomic number 29 corresponds to copper (Cu) on the periodic table.

The mass number (protons + neutrons) would be $29 + 35 = 64$ , so this is the isotope $^{64}\text{Cu}$ , but the question only asks for the electronic configuration, not the isotope.
Writing the electronic configuration

For an atom with 29 electrons, we fill orbitals in order of increasing energy (the Aufbau principle). The order is:

$1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p$ , etc.

Let’s fill step by step:
- $1s^2$ → 2 electrons
- $2s^2$ → 2 more (total 4)
- $2p^6$ → 6 more (total 10)
- $3s^2$ → 2 more (total 12)
- $3p^6$ → 6 more (total 18)
- $4s^2$ → 2 more (total 20)
- Now we have 9 electrons left to place. The next orbital is $3d$ , which can hold up to 10 electrons. …

Start your 14-day free trial to unlock the full solution →

←Previous question · exercises2.25 — An electron is in one of the 3d orbitals. Give the possible values of …→Next question · exercises2.27 — Give the number of electrons in the species H₂^+, H₂ and O₂^+.

Q.An atom of an element contains 29 electrons and 35 neutrons. Deduce

🎓Unlock everything free for 14 days

Unlock everything free for 14 days