Chemistry 1

What is an ion?

An atom that has gained or lost electrons and has an overall charge.

Why do atoms form ions?

To achieve a full outer electron shell and become more stable.

What are valence electrons?

Electrons in the outermost shell of an atom.

Which electrons are lost or gained?

Valence (outer shell) electrons.

Why are valence electrons lost first?

They are furthest from the nucleus and are held least strongly.

What is a cation?

A positively charged ion.

How are cations formed?

By losing electrons.

Which elements usually form cations?

Metals.

Why are cations positive?

They have more protons than electrons.

Give three examples of cations.

Na⁺, Mg²⁺, Al³⁺.

What is an anion?

A negatively charged ion.

How are anions formed?

By gaining electrons.

Which elements usually form anions?

Non-metals.

Why are anions negative?

They have more electrons than protons.

Give three examples of anions.

Cl⁻, O²⁻, N³⁻.

What does the '+' sign mean?

Electrons have been lost.

What does the '−' sign mean?

Electrons have been gained.

What does the '2' in Mg²⁺ mean?

Two electrons were lost.

What does the '3' in N³⁻ mean?

Three electrons were gained.

What is ionic bonding?

The transfer of electrons between a metal and a non-metal.

What happens after electrons are transferred?

Positive and negative ions attract each other.

What force holds ionic compounds together?

Electrostatic attraction.

Why are ionic compounds neutral?

The total positive and negative charges equal zero.

Group 1 ions?

+1

Group 2 ions?

+2

Group 13 ions?

+3

Group 15 ions?

-3

Group 16 ions?

-2

Group 17 ions?

-1

Why don't Group 18 elements usually form ions?

They already have full outer shells.

Why can transition metals have different charges?

They can lose different numbers of electrons.

Explain how NaCl forms.

Sodium loses one electron to chlorine. Na⁺ and Cl⁻ attract to form NaCl.

Why is NaCl a 1:1 ratio?

+1 and -1 charges balance.

Explain how Na₂S forms.

Sulfur gains two electrons. Two sodium atoms each lose one electron.

Why are two sodium atoms needed?

Sulfur requires two electrons.

Which ion is written first?

The cation.

Which ion is named second?

The anion.

How do simple anions end?

'-ide'

NaCl

Sodium chloride

MgO

Magnesium oxide

MgCl₂

Magnesium chloride

CaO

Calcium oxide

Al₂O₃

Aluminium oxide

KBr

Potassium bromide

Li₂S

Lithium sulfide

AlCl₃

Aluminium chloride

Formula for calcium chloride.

CaCl₂

Formula for magnesium oxide.

MgO

Formula for sodium oxide.

Na₂O

Formula for aluminium oxide.

Al₂O₃

Formula for calcium nitride.

Ca₃N₂

Formula for aluminium nitride.

AlN

Formula for potassium sulfide.

K₂S

Step 1?

Identify the ions and their charges.

Step 2?

Balance the charges.

Step 3?

Write the correct formula.

Step 4?

Name the compound.

What is the cross method?

Swap ion charges to become subscripts.

When do you NOT write a subscript of 1?

Always omit it.

What is a polyatomic ion?

A charged group of covalently bonded atoms.

Hydroxide

OH⁻

Nitrate

NO₃⁻

Sulfate

SO₄²⁻

Carbonate

CO₃²⁻

Formula for magnesium hydroxide.

Mg(OH)₂

Formula for calcium hydroxide.

Ca(OH)₂

Formula for sodium nitrate.

NaNO₃

Formula for calcium nitrate.

Ca(NO₃)₂

Formula for aluminium hydroxide.

Al(OH)₃

Why are brackets used?

When more than one polyatomic ion is needed.

Why doesn't NaOH need brackets?

There is only one hydroxide ion.

What is percentage composition?

The percentage of a compound's mass made up by one element.

Percentage composition formula?

Mass of element ÷ Mass of compound × 100

Step 1?

Calculate the molar mass of the compound.

Step 2?

Calculate the mass of the chosen element.

Step 3?

Divide and multiply by 100.

Molar mass of Al₂O₃?

102 g/mol

Mass of oxygen in Al₂O₃?

48 g

Percentage oxygen?

47.1%

Hydrogen

1

Carbon

12

Nitrogen

14

Oxygen

16

Sodium

23

Magnesium

24

Aluminium

27

Sulfur

32

Chlorine

35.5

Why do metals lose electrons?

To achieve a full outer shell.

Why do non-metals gain electrons?

To achieve a full outer shell.

Why can't two cations bond together?

They repel because they have the same positive charge.

Why can't two anions bond together?

They repel because they have the same negative charge.

What is the overall charge of every ionic compound?

Zero.

Why are ionic compounds stable?

They have full outer shells and strong electrostatic attractions.

What is an electrovalence table?

A table showing the common charges of ions.

Do you need to memorise every ion charge?

No, an electrovalence table is usually provided in tests.