How Ions Form · Intro Chemistry

Sodium + chlorine = table salt?

Sodium metal is so soft you can cut it with a knife, and it reacts violently with water. Chlorine is a toxic yellow-green gas. Yet sodium burning in chlorine produces the table salt you eat every day: sodium chloride. How can two dangerous substances turn into something harmless? The answer lies in the electrons on the outside of their atoms.

Electrons are arranged in shells

Borrowing a few results: Lesson 1.4.4 gives the complete rules for arranging electrons in shells, with diagrams. To explain ions, this lesson borrows a few of those results ahead of time, above all the number of outer-shell electrons. If you can read an arrangement like “2, 8, 1” and say how many electrons are in the outer shell, you have all you need here.

The electrons around a nucleus are arranged in layers, or shells. Electrons with less energy stay closer to the nucleus, and those with more energy are farther out. For the first 20 elements, the arrangement follows a simple pattern:

The first shell holds at most 2 electrons, and the second at most 8.
The outer shell holds at most 8 electrons (or 2, if there is only one shell).

For example, sodium (11 electrons) is 2, 8, 1; chlorine (17 electrons) is 2, 8, 7; and neon (10 electrons) is 2, 8. Lesson 1.4.4 gives you plenty of practice drawing these arrangements.

An outer shell holding 8 electrons (or 2 when there’s only one shell) is a stable arrangement相对稳定结构, often called an octet. Noble gas atoms already have one, which is why they hardly react with anything.

Atoms	Outer electrons	Tendency in reactions	Examples
Noble gases	8 (helium: 2)	Rarely gain or lose electrons; stable	He, Ne, Ar
Metals	Usually fewer than 4	Lose their outer electrons	Na, Mg, Al
Nonmetals	Usually 4 or more	Gain electrons to fill the outer shell to 8	O, S, Cl

So an atom’s number of outer electrons determines how its element behaves chemically. (In AP Chemistry these are called valence electrons.)

Ions

When an atom gains or loses electrons, its number of protons stays the same but its number of electrons changes. The positive and negative charges no longer balance, so the particle has a charge. A charged atom (or group of atoms) is an ion离子:

A positive ion is a cation阳离子, such as $\ce{Na+}$ or $\ce{Mg^2+}$ . Atoms form cations by losing electrons.
A negative ion is an anion阴离子, such as $\ce{Cl-}$ or $\ce{O^2-}$ . Atoms form anions by gaining electrons.

How to write an ion symbol: put the charge at the upper right of the element symbol, number first, then the sign. For a charge of 1, leave out the 1.

Take $\ce{2Mg^2+}$ : the 2 in front means two magnesium ions, and the 2 at the upper right means each ion has a charge of +2.

Simple cations keep the element’s name (sodium ion, magnesium ion). Simple anions change the ending to -ide: chlorine becomes chloride, oxygen becomes oxide, sulfur becomes sulfide.

Pick an atom and make it lose or gain electrons. When does the outer shell become stable, and how is the ion written?

Try it: pick an atom and make it lose or gain electrons

Metals

Nonmetals

How sodium chloride forms

Step-by-step solution

The sodium atom. Sodium is 2, 8, 1. With just 1 outer electron, it loses that electron easily.
The chlorine atom. Chlorine is 2, 8, 7. It is 1 electron short of 8, so it gains 1 electron easily.
Electron transfer. The electron lost by sodium goes to chlorine. Sodium becomes $\ce{Na+}$ (2, 8) and chlorine becomes $\ce{Cl-}$ (2, 8, 8). Both now have a stable outer shell of 8.
Attraction. The oppositely charged $\ce{Na+}$ and $\ce{Cl-}$ ions attract each other and form sodium chloride. Sodium chloride is made of sodium ions and chloride ions; there are no sodium chloride molecules.

So ions are another kind of particle that makes up substances: a substance can be made of molecules, atoms, or ions. An ion’s charge is closely related to the valence you’ll meet in Module 5, and lesson 1.5.2 compares how the two are written.

Example 1: From magnesium atom to magnesium ion

Magnesium (12 protons) has the arrangement 2, 8, 2. Does it tend to gain or lose electrons in reactions? How is its ion written?

Step-by-step solution

Look at the outer shell. It has 2 outer electrons, fewer than 4, so it’s a metal and tends to lose those 2 electrons.
Work out the charge. After losing 2 electrons: 12 protons, 10 electrons, so the charge is 12 − 10 = +2.
Write the symbol. $\ce{Mg^2+}$ . Its arrangement is now 2, 8, with a stable outer shell of 8.

Example 2: From sulfur atom to sulfide ion

Sulfur (16 protons) has the arrangement 2, 8, 6. How many electrons are in its ion, and what is the charge?

Step-by-step solution

Look at the outer shell. It has 6 outer electrons, so it’s a nonmetal and tends to gain 2 electrons to reach 8.
Count electrons. 16 + 2 = 18 electrons, arranged 2, 8, 8.
Work out the charge. 16 − 18 = −2, so the ion is written $\ce{S^2-}$ : the sulfide ion.

Practice: count the particles in each ion

Particle	Protons	Neutrons	Electrons	Mass number
$\ce{^{23}_{11}Na+}$
$\ce{^{24}_{12}Mg^2+}$
$\ce{^{27}_{13}Al^3+}$
$\ce{^{16}_{8}O^2-}$
$\ce{^{32}_{16}S^2-}$
$\ce{^{35}_{17}Cl-}$

Practice: from the numbers of protons and electrons, is it an atom, a cation, or an anion?

11 protons, 10 electrons
17 protons, 18 electrons
18 protons, 18 electrons
12 protons, 10 electrons
8 protons, 10 electrons
10 protons, 10 electrons
13 protons, 10 electrons
16 protons, 18 electrons

Common mistakes

Mistake 1: Writing the charge backward or including the 1. The magnesium ion is $\ce{Mg^2+}$ , not Mg⁺² or Mg2+. The sodium ion is $\ce{Na+}$ , not Na¹⁺.

Mistake 2: “Gaining electrons makes an ion positive.” Electrons are negative. Gaining electrons adds negative charge, giving an anion; losing electrons gives a cation.

Mistake 3: “The number of protons changes when an ion forms.” Only the electrons change. The protons stay the same, so it’s still the same element: $\ce{Na}$ and $\ce{Na+}$ are both sodium.

Quick check

How many electrons does a chloride ion, $\ce{Cl-}$ , have? (Chlorine's atomic number is 17.)

Which of these means two magnesium ions?

Summary

Electrons are arranged in shells: at most 2 in the first, at most 8 in the second, and at most 8 in the outer shell. A full outer shell of 8 (or 2 with only one shell) is stable.
Metal atoms usually have fewer than 4 outer electrons and lose them to form cations; nonmetal atoms usually have 4 or more and gain electrons to form anions.
Ion symbols: the charge goes at the upper right, number first, then the sign, with a 1 left out.
Only electrons change when an ion forms. Sodium chloride is made of $\ce{Na+}$ and $\ce{Cl-}$ ions, and substances can be made of molecules, atoms, or ions.

On to AP (for later)

In topic 1.8, AP predicts ion charges from an element’s place on the periodic table: Group 1 forms +1 ions, Group 2 forms +2, Group 17 forms −1, and Group 16 forms −2, all because of the outer (valence) electrons you looked at today. Topics 2.1 and 2.3 use the Coulombic attraction between cations and anions to explain the properties of ionic compounds.

How Ions Form (Gaining and Losing Electrons)离子的形成（得失电子初步）

Learning goals

Before you start

Sodium + chlorine = table salt?

Electrons are arranged in shells

Ions

How sodium chloride forms

Example 1: From magnesium atom to magnesium ion

Example 2: From sulfur atom to sulfide ion

Common mistakes

Quick check

Summary

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