Chemistry: Atoms First 2e · Atoms, Molecules, and Ions

Atomic Structure and Symbolism

7 min read

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

An atom's identity and behavior are set by three subatomic particles: protons, neutrons, and electrons. This topic explains how to count them using two key numbers — the atomic number Z, which is the number of protons, and the mass number A, which is the total number of protons plus neutrons. It introduces isotopes (atoms of the same element with different neutron counts), the standard notation such as ³⁵₁₇Cl, ions (atoms that have lost or gained electrons), and the reported on the periodic table, which is a weighted average over naturally occurring isotopes.

Why this matters

The atomic number is the single most important number in chemistry: it defines which element an atom is, and it cannot change without the atom becoming a different element. Isotopes explain why atomic masses on the periodic table are not whole numbers, and they are used in medicine (for example, in imaging and treatment) and in dating ancient materials. Ions are everywhere in biology and chemistry — sodium, potassium, calcium, and chloride ions carry nerve signals, and ionic compounds make up much of the mineral world. Nuclear symbols appear again in nuclear equations, so learning to read and write them now pays off throughout the course.

The college version

Core Concepts

The three subatomic particles

Particle	Charge	Mass (u)	Location
proton	1+	1.0073	nucleus
neutron	0	1.0087	nucleus
electron	1−	0.00055	outside nucleus

The proton and neutron are roughly 1,800 times more massive than the electron, which is why nearly all of an atom's mass is in its nucleus.

Atomic number, mass number, and neutron number

The atomic number Z is the number of protons. The mass number A is the number of protons plus neutrons. The neutron number N follows by subtraction:

$A = Z + N N = A - Z$

In a neutral atom, the number of electrons equals Z because the total positive and negative charges must balance.

Isotopes

Atoms of the same element can differ in their number of neutrons. Such atoms are called isotopes of that element. They share the same Z (same chemical identity) but have different A (different mass). Hydrogen has three natural isotopes: protium ¹₁H (one proton, no neutrons), deuterium ²₁H (one proton, one neutron), and tritium ³₁H (one proton, two neutrons).

Nuclear symbolism

The complete symbol for an atom or places the mass number as a left superscript and the atomic number as a left subscript:

$A Z X$

For example, ³⁵₁₇Cl is chlorine-35: 17 protons and 35 - 17 = 18 neutrons. The same can also be written chlorine-35 or Cl-35.

Ions

An ion is an atom (or group of atoms) with a net charge because it has lost or gained electrons. The charge is the number of protons minus the number of electrons:

$charge = Z - n e$

A cation has a positive charge (lost electrons); an anion has a negative charge (gained electrons). For example, ⁵⁶₂₆Fe²⁺ is an iron atom that lost two electrons.

Average atomic mass

Most elements occur as a mixture of isotopes, so the atomic mass on the periodic table is a weighted average:

$Ā = \sum i f i \cdot m i$

where f_i is the fractional abundance (the decimal form of the percent abundance) and m_i is the mass of isotope i. The is defined so that one atom of carbon-12 has a mass of exactly 12 u; one u equals about 1.6605 × 10^-27 kg.

How It Works / Step-by-Step Process

Read the symbol or problem statement and identify Z, A, and the charge.
Find N = A - Z for the neutron count.
Find the electron count from the charge: n_e = Z - charge.
For average atomic mass, convert each percent abundance to a fraction, multiply each isotopic mass by its fraction, and sum.
Check that the average lies between the lightest and heaviest isotopic masses and sits closer to the most abundant isotope.

Common Confusions

Do Not Confuse	With	Difference
Atomic number Z	Mass number A	Z is protons only (identity); A is protons + neutrons (mass). Changing Z changes the element; changing A alone makes an isotope.
Average atomic mass	Simple average of isotopic masses	The periodic table value is weighted by natural abundance; a simple mean ignores abundance and gives the wrong number.
Protons	Neutrons	Protons set the element and charge balance; neutrons only affect mass and isotope identity.
Cation	Anion	Cations lost electrons (positive); anions gained electrons (negative).
Isotope symbol	Ion symbol	Isotope notation shows mass number; ion notation shows charge. An atom can be both, as in ⁵⁶₂₆Fe²⁺.
Neutrons in an atom	Neutrons changing the element	Adding or removing neutrons makes an isotope of the same element; it never changes Z.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Every atom has a tiny center packed with protons and neutrons, and a cloud of much lighter electrons whizzing around it. The number of protons is like an atom's ID card: 6 protons always means carbon, and 8 protons always means oxygen. Some atoms of the same element carry extra neutrons, like twins with different weights — those are isotopes.

Worked example

Worked Example 1: Decoding a nuclear symbol

The ion ⁵⁶₂₆Fe²⁺ appears in a problem. How many protons, neutrons, and electrons does it contain?

Read the symbol directly: the left subscript is Z = 26, so the element is iron with 26 protons. The left superscript is A = 56, so

$N = A - Z = 56 - 26 = 30 neutrons$

The charge is 2+, so the atom has two more protons than electrons:

$n e = Z - charge = 26 - (+2) = 24 electrons$

Answer: 26 protons, 30 neutrons, 24 electrons. Check: 26 - 24 = +2, matching the charge.

Worked Example 2: Building a symbol from particle counts

An atom has 15 protons, 16 neutrons, and 18 electrons. Write its complete symbol.

Since Z = 15, the element is phosphorus (P). The mass number is

$A = Z + N = 15 + 16 = 31$

The charge is

$Z - n e = 15 - 18 = -3$

So the species is the phosphide ion, ³¹₁₅P^3-. It is an anion because it has three more electrons than protons.

Worked Example 3: Average atomic mass of chlorine

Naturally occurring chlorine is 75.76% chlorine-35 (mass 34.969 u) and 24.24% chlorine-37 (mass 36.966 u). Calculate the average atomic mass and compare it with the periodic table value.

Write the weighted-average formula, then substitute:

$Ā = f 35 \cdot m 35 + f 37 \cdot m 37$

$Ā = (0.7576)(34.969 u) + (0.2424)(36.966 u) = 26.50 u + 8.96 u = 35.45 u$

The result, 35.45 u, matches the periodic table value. Notice it is not the simple average (34.969 + 36.966)/2 = 35.97 u; the weighted value sits closer to 34.969 because chlorine-35 is more abundant. That difference is a common exam trap.

Key takeaways

Z = number of protons = element identity; A = Z + N; N = A - Z.
In a neutral atom, electrons = protons = Z.
Isotopes: same Z, different N and A; chemical behavior is nearly identical.
Nuclear symbol: mass number is the left superscript, atomic number the left subscript: ^A_ZX.
Ion charge = Z - n_e; cations are positive (fewer electrons), anions negative (more electrons).
Average atomic mass is a weighted average over isotopic masses and abundances, not a simple mean.
The periodic table value is the average, so most elements show non-integer atomic masses.

Check yourself

5 review questions from the chapter. Try each one, then open the answer.

An atom has 11 protons, 12 neutrons, and 11 electrons. Give Z, A, N, and the element's identity.
Show answer
Z = 11, A = 11 + 12 = 23, N = 12. Eleven protons means sodium, so this is sodium-23, ²³₁₁Na.
How many protons, neutrons, and electrons are in ²⁴₁₂Mg²⁺?
Show answer
Z = 12 gives 12 protons; N = 24 - 12 = 12 neutrons; charge 2+ gives n_e = 12 - 2 = 10 electrons.
Write the complete symbol for the ion with 8 protons, 8 neutrons, and 10 electrons.
Show answer
Z = 8 is oxygen; A = 8 + 8 = 16; charge = 8 - 10 = -2, so ¹⁶₈O^2-.
Why is the atomic mass of chlorine 35.45 u rather than a whole number?
Show answer
Because natural chlorine is a mixture of isotopes — mostly Cl-35 with a smaller amount of Cl-37 — and the periodic table value is their weighted average.
Does removing a neutron from an atom of carbon change it into a different element? Explain.
Show answer
No. The number of protons (6) is unchanged, so it is still carbon; only the mass number changes, making a different isotope.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

atomic number (Z): Number of protons in a nucleus
mass number (A): Total number of protons plus neutrons
isotope: Atoms of one element with different neutron counts
nuclear symbol: Notation ^A_ZX giving mass number, atomic number, and element
ion: Atom or group of atoms with a net charge
cation / anion: Positively / negatively charged ion
average atomic mass: Weighted average of isotopic masses by abundance
atomic mass unit (u): 1/12 the mass of one carbon-12 atom
atomic number Z: Number of protons; defines the element

Sources & references

openstax.org — Chemistry Atoms First 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.