Atomic Structure: Protons, Neutrons, Electrons, and Ions

What an Atom Is Made Of

Atomic structure explains how atoms are built from smaller particles and how those particles determine an element’s identity, mass, and electrical behavior. For beginners, the most useful model is a tiny, dense center called the nucleus surrounded by electrons that occupy regions of space around it. The nucleus contains protons and neutrons. Electrons are much lighter and move in the space outside the nucleus. Even though drawings often show electrons as little dots on circular paths, it is more accurate to think of them as occupying “cloud-like” regions where they are likely to be found.

Three subatomic particles matter most in introductory chemistry: protons, neutrons, and electrons. Each has a charge and a relative mass, and each plays a different role in chemical behavior. Understanding how to count them, and how they change when atoms become ions, is the foundation for later topics like bonding and reactions.

Protons

A proton has a positive charge of +1 (in elementary charge units). Protons are located in the nucleus. The number of protons in an atom is called the atomic number. This number is what defines the element. If you change the number of protons, you no longer have the same element.

• Hydrogen always has 1 proton.
• Carbon always has 6 protons.
• Oxygen always has 8 protons.

Because atomic number defines the element, it is the most important identifier on the periodic table. When you see an element symbol, you can find its atomic number and immediately know how many protons are in its atoms.

Neutrons

A neutron has no electrical charge (0). Neutrons are also located in the nucleus. Neutrons contribute to the mass of the atom and help stabilize the nucleus. The number of neutrons can vary among atoms of the same element, creating different isotopes. Isotopes are still the same element because they have the same number of protons, but they have different masses because they have different numbers of neutrons.

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Neutrons matter in chemistry mainly because they affect atomic mass and, in some cases, nuclear stability. For most chemical reactions, the number of neutrons does not change, but you often need to calculate neutron count when interpreting isotope notation or when estimating mass numbers.

Electrons

An electron has a negative charge of −1. Electrons are found outside the nucleus in regions often called electron shells or energy levels. Electrons have extremely small mass compared with protons and neutrons. In many introductory problems, electron mass is treated as negligible when calculating atomic mass.

Electrons are the main players in chemical change. When atoms form bonds or become ions, it is electrons that are shared, gained, or lost. The arrangement of electrons (especially the outermost electrons) controls how an atom interacts with other atoms.

Charge and Mass: A Simple Comparison

It helps to compare the three particles side by side. In chemistry, we often use relative masses rather than exact values.

• Proton: charge +1, relative mass about 1
• Neutron: charge 0, relative mass about 1
• Electron: charge −1, relative mass about 1/1836 (very small)

Because protons and neutrons have similar masses and both reside in the nucleus, nearly all the mass of an atom is concentrated in the nucleus. Electrons contribute very little to mass but strongly influence size and chemical behavior.

Atomic Number, Mass Number, and Isotopes

Two counting ideas appear often: atomic number and mass number.

• Atomic number (Z) = number of protons
• Mass number (A) = number of protons + number of neutrons

Mass number is a whole number because it counts particles in the nucleus. It is not the same as the atomic mass shown on the periodic table, which is usually a weighted average of isotopes and therefore not typically a whole number.

Step-by-step: Finding Neutrons from Atomic Number and Mass Number

If you know the element and the isotope’s mass number, you can calculate neutrons directly.

• Step 1: Identify the atomic number Z (this equals protons).
• Step 2: Identify the mass number A (protons + neutrons).
• Step 3: Compute neutrons = A − Z.

Example: Carbon-14

• Carbon has Z = 6, so it has 6 protons.
• Carbon-14 has A = 14.
• Neutrons = 14 − 6 = 8 neutrons.

Example: Sodium-23

• Sodium has Z = 11, so it has 11 protons.
• Sodium-23 has A = 23.
• Neutrons = 23 − 11 = 12 neutrons.

Common Isotope Notation

You may see isotopes written in several ways:

• Hyphen notation: carbon-14, uranium-235
• Nuclear symbol notation:

  ^A_Z X

  where X is the element symbol, A is mass number, and Z is atomic number.

For example, carbon-14 can be written as:

^14_6 C

This tells you immediately: 6 protons, 14 total nucleons (protons + neutrons), so 8 neutrons.

Neutral Atoms and the Meaning of “Equal Protons and Electrons”

An atom is electrically neutral when its total positive charge equals its total negative charge. Since protons carry +1 and electrons carry −1, neutrality occurs when:

• number of electrons = number of protons

Example: A neutral magnesium atom has 12 protons (because magnesium’s atomic number is 12). Therefore, a neutral magnesium atom has 12 electrons.

This neutrality is important because many chemical processes involve moving electrons. When electrons are gained or lost, the balance between protons and electrons changes, and the atom becomes an ion.

Ions: Atoms (or Groups) with Charge

An ion is a particle with a net electric charge. Ions form when electrons are gained or lost. Protons in the nucleus do not change during ordinary chemical processes, so ion formation is almost always about electrons.

There are two main types:

• Cation: positively charged ion (formed by losing electrons)
• Anion: negatively charged ion (formed by gaining electrons)

Because electrons are negative, losing electrons removes negative charge and leaves a net positive charge. Gaining electrons adds negative charge and creates a net negative charge.

Step-by-step: Determining the Charge of an Ion from Protons and Electrons

To find the ion’s charge, compare protons and electrons.

• Step 1: Count protons (equals atomic number).
• Step 2: Count electrons (given or inferred from ion charge).
• Step 3: Net charge = (protons) − (electrons).

Example: An atom has 13 protons and 10 electrons.

• Net charge = 13 − 10 = +3
• This is a 3+ cation (commonly written as Al³⁺ because 13 protons corresponds to aluminum).

Example: An atom has 17 protons and 18 electrons.

• Net charge = 17 − 18 = −1
• This is a 1− anion (Cl⁻ because 17 protons corresponds to chlorine).

Step-by-step: Finding Electrons When You Know the Ion Charge

Often you know the element and the ion charge and need the electron count.

• Step 1: Find protons = atomic number Z.
• Step 2: Use the charge to adjust electron count.
• If the ion is positive (cation), it has fewer electrons than protons.
• If the ion is negative (anion), it has more electrons than protons.

Example: Ca²⁺

• Calcium has Z = 20, so 20 protons.
• 2+ means it has lost 2 electrons.
• Electrons = 20 − 2 = 18 electrons.

Example: O²⁻

• Oxygen has Z = 8, so 8 protons.
• 2− means it has gained 2 electrons.
• Electrons = 8 + 2 = 10 electrons.

Why Atoms Form Ions (A Practical View)

Atoms form ions because certain electron arrangements are more stable than others. In many cases, atoms tend to gain or lose electrons to reach a more stable outer electron configuration. While a full explanation involves electron energy levels, a practical beginner rule is:

• Many metals tend to lose electrons and form cations.
• Many nonmetals tend to gain electrons and form anions.

Example: Sodium (a metal) commonly forms Na⁺ by losing one electron. Chlorine (a nonmetal) commonly forms Cl⁻ by gaining one electron. The opposite charges attract, which is one reason ionic compounds form readily.

Writing and Interpreting Ion Symbols

Ion notation uses the element symbol followed by a superscript charge.

• Na⁺ means sodium with a +1 charge.
• Mg²⁺ means magnesium with a +2 charge.
• Cl⁻ means chloride with a −1 charge.
• N³⁻ means nitride with a −3 charge.

The charge tells you how many electrons were lost or gained compared with the neutral atom.

• +1: lost 1 electron
• +2: lost 2 electrons
• −1: gained 1 electron
• −2: gained 2 electrons

Common Ion Charges You Will See Often

While you will later learn patterns on the periodic table, you can start by recognizing a few common ions that appear in many chemical formulas.

• Group 1 metals (like Li, Na, K) often form +1 ions.
• Group 2 metals (like Mg, Ca) often form +2 ions.
• Aluminum often forms Al³⁺.
• Halogens (like F, Cl, Br, I) often form −1 ions.
• Oxygen often forms O²⁻ and sulfur often forms S²⁻.
• Nitrogen often forms N³⁻.

These are not arbitrary: they reflect how many electrons are typically lost or gained to reach a stable outer arrangement.

Atoms vs. Ions: What Changes and What Stays the Same

When an atom becomes an ion during a chemical process:

• The number of protons stays the same (element identity does not change).
• The number of neutrons stays the same (isotope does not change in ordinary chemistry).
• The number of electrons changes (this creates the charge).

This is a powerful idea because it lets you track chemical changes without confusion. If you see Na and Na⁺, both are sodium (11 protons). The difference is electron count: Na has 11 electrons, Na⁺ has 10 electrons.

Practice-Style Walkthroughs (Step-by-step)

1) Determine Protons, Neutrons, and Electrons for a Neutral Atom

Problem: Find protons, neutrons, and electrons for neutral potassium-39.

• Step 1: Potassium has atomic number Z = 19, so protons = 19.
• Step 2: Neutral atom means electrons = protons = 19.
• Step 3: Mass number A = 39, so neutrons = 39 − 19 = 20.

Answer: 19 protons, 20 neutrons, 19 electrons.

2) Determine Protons, Neutrons, and Electrons for an Ion

Problem: Find protons, neutrons, and electrons for chloride-37, Cl⁻.

• Step 1: Chlorine has Z = 17, so protons = 17.
• Step 2: Mass number A = 37, so neutrons = 37 − 17 = 20.
• Step 3: The ion is Cl⁻, meaning it has gained 1 electron compared with neutral chlorine.
• Neutral chlorine would have 17 electrons, so Cl⁻ has 18 electrons.

Answer: 17 protons, 20 neutrons, 18 electrons.

3) Identify the Element from Proton Count, Then Find Charge

Problem: A particle has 9 protons and 10 electrons. What is it?

• Step 1: 9 protons means atomic number 9, which is fluorine (F).
• Step 2: Net charge = protons − electrons = 9 − 10 = −1.

Answer: F⁻ (fluoride ion).

4) Work Backward from Ion Symbol to Electron Count

Problem: How many electrons are in Fe²⁺?

• Step 1: Iron (Fe) has atomic number 26, so protons = 26.
• Step 2: 2+ means it has lost 2 electrons.
• Step 3: Electrons = 26 − 2 = 24.

Answer: 24 electrons.

Note: Some elements, especially transition metals like iron, can form more than one common ion charge (such as Fe²⁺ and Fe³⁺). The symbol’s charge tells you exactly which one you have, so you can always compute electrons reliably.

Electron Arrangement: Shells as a Beginner-Friendly Map

Electrons occupy energy levels around the nucleus. A simple way to start is to think in shells that fill in order. For the first few elements, a common beginner pattern is:

• 1st shell holds up to 2 electrons.
• 2nd shell holds up to 8 electrons.
• 3rd shell holds up to 8 electrons for many basic examples (a simplified rule useful early on).

This shell picture helps you understand why certain ions form. Atoms often gain or lose electrons to end up with a filled outer shell in this simplified model.

Step-by-step: Shell Counting for Simple Ions

Example: Sodium forming Na⁺

• Neutral sodium has 11 electrons.
• Fill shells: 2 in the first shell, 8 in the second shell, leaving 1 in the third shell (2, 8, 1).
• Na⁺ means sodium loses 1 electron, leaving 10 electrons.
• 10 electrons fill as (2, 8), which is a filled outer shell in this simplified model.

Example: Oxygen forming O²⁻

• Neutral oxygen has 8 electrons: (2, 6).
• O²⁻ means oxygen gains 2 electrons, giving 10 electrons total.
• 10 electrons fill as (2, 8), a filled outer shell in this simplified model.

This is not the full quantum-mechanical picture, but it is a practical stepping stone for predicting common ion charges and understanding why ions form.

Key Vocabulary Used Correctly in Problems

• Nucleus: dense center of the atom containing protons and neutrons.
• Atomic number (Z): number of protons; identifies the element.
• Mass number (A): protons + neutrons for a specific isotope.
• Isotope: atoms of the same element (same protons) with different neutrons.
• Ion: charged atom or charged group formed by gaining or losing electrons.
• Cation: positive ion (lost electrons).
• Anion: negative ion (gained electrons).

Mini Practice Set (With Answers)

1) Find neutrons

How many neutrons are in

^31_15 P

• Protons Z = 15
• Mass number A = 31
• Neutrons = 31 − 15 = 16

2) Find electrons in an ion

How many electrons are in Al³⁺?

• Al has Z = 13
• 3+ means lost 3 electrons
• Electrons = 13 − 3 = 10

3) Identify charge from counts

A particle has 35 protons and 36 electrons. Write its symbol with charge.

• Z = 35 is bromine (Br)
• Charge = 35 − 36 = −1
• Symbol: Br⁻

4) Protons, neutrons, electrons together

For Mg-24 as Mg²⁺, find protons, neutrons, electrons.

• Mg has Z = 12, so protons = 12
• A = 24, so neutrons = 24 − 12 = 12
• 2+ means electrons = 12 − 2 = 10