A chemical bond formed by the transfer of electrons from one atom to another, producing oppositely charged ions that attract each other. Typically between a metal and a non-metal.

A chemical bond formed when two atoms share one or more pairs of electrons. Usually occurs between non-metal atoms and forms molecules.

A group of two or more atoms joined by covalent bonds, acting as a single particle. Example: H2O, CO2, O2.

A shorthand way to show which elements and how many atoms of each are in a substance. Example: H2O has 2 hydrogen atoms and 1 oxygen atom.

A way of describing a reaction using words: reactants → products. Example: magnesium + oxygen → magnesium oxide.

A charged particle formed when an atom or group of atoms gains or loses one or more electrons.

A regular, repeating 3D arrangement of particles, such as ions in an ionic solid.

A group of atoms held together by covalent bonds, behaving as a single particle.

Substances formed by a chemical reaction, written on the right side of an equation.

How Atoms Stick: Chemical Bonding and Simple Reactions — SHS Science Foundations: Integrated Chemistry, Biology, and Physics

1. From Particles to Bonds: Why Atoms 'Stick'

Why Atoms 'Stick'

Atoms are more stable when their outer electron shell is full. To get this, they can transfer or share electrons with other atoms. This 'sticking together' is called chemical bonding.

Valence Electrons

The electrons in the outermost shell (valence electrons) are the ones involved in bonding. They can be moved or shared when atoms react.

Two Main Bond Types

If atoms transfer electrons, they form ions that attract each other: ionic bonding. If atoms share electrons, they form molecules: covalent bonding.

Chemical Reactions

When bonds form or break, a chemical reaction happens. New substances form, but the total number of each type of atom stays the same: the law of conservation of mass.

2. Ionic Bonding: Electron Transfer and Ions

What Is Ionic Bonding?

Ionic bonding happens when one atom loses electrons and another gains them, forming charged particles called ions. This usually occurs between a metal and a non-metal.

Forming Ions

Metals like sodium (Na) tend to lose electrons and form positive ions (cations). Non-metals like chlorine (Cl) tend to gain electrons and form negative ions (anions).

NaCl Example

Sodium loses one electron to become Na⁺. Chlorine gains one electron to become Cl⁻. The opposite charges attract, forming ionic bonds in sodium chloride, with formula NaCl.

Ionic Lattice Properties

In ionic solids, many ions pack into a giant lattice. Ionic compounds usually have high melting points and conduct electricity when molten or in solution, because ions can move.

3. Covalent Bonding: Sharing Electrons and Molecules

What Is Covalent Bonding?

Covalent bonding happens when atoms share pairs of electrons. This usually occurs between non-metal atoms and leads to the formation of molecules.

Hydrogen Molecule (H2)

Two hydrogen atoms each have 1 electron and want 2. They share a pair of electrons so each can count 2 in its outer shell, forming a hydrogen molecule, H2.

Water and Oxygen

In water, H2O, oxygen shares electrons with two hydrogen atoms. In oxygen gas, O2, two oxygen atoms share two pairs of electrons, forming a double bond.

Reading Formulas

In a chemical formula, subscripts show how many atoms of each element: H2O has 2 H and 1 O; CO2 has 1 C and 2 O. These are molecules held by covalent bonds.

4. Comparing Ionic and Covalent with Everyday Substances

Table Salt: Ionic

NaCl is made from sodium (metal) and chlorine (non-metal). It forms ions Na⁺ and Cl⁻ in a lattice. Salt is solid at room temperature and its solutions conduct electricity.

Water: Covalent

H2O is made from hydrogen and oxygen, both non-metals. Each water molecule has covalent bonds. Water is liquid at room temperature and pure water is a poor conductor.

Oxygen Gas: Covalent

O2 consists of only oxygen atoms, a non-metal. Two O atoms share electrons in a double covalent bond, forming O2 molecules, a gas needed for combustion.

Classify These

MgO, CO2, Na2O, Cl2. Metal + non-metal → ionic; non-metal + non-metal → covalent. So: MgO ionic, CO2 covalent, Na2O ionic, Cl2 covalent.

5. Word Equations: Describing Reactions in Words

Word Equations Basics

A chemical reaction rearranges atoms. We show this with word equations: reactants → products. Reactants are starting substances; products are new substances formed.

Magnesium Burning

When magnesium burns in oxygen, magnesium oxide forms. Word equation: magnesium + oxygen → magnesium oxide.

Making Water

Hydrogen gas reacts with oxygen gas to form water. Word equation: hydrogen + oxygen → water.

Acid + Metal Pattern

A metal reacting with dilute acid often forms a salt and hydrogen. Example: hydrochloric acid + magnesium → magnesium chloride + hydrogen.

6. Symbol Equations and Conservation of Mass (Qualitative)

Symbol Equations

Symbol equations use formulas instead of words. Example: magnesium + oxygen → magnesium oxide becomes Mg + O2 → MgO (unbalanced).

Conservation of Mass

In any reaction, atoms are not created or destroyed. The number of each type of atom is the same before and after. This is the law of conservation of mass.

Checking Atoms

For Mg + O2 → MgO: left has 1 Mg, 2 O; right has 1 Mg, 1 O. Oxygen does not match, so the equation is not balanced yet.

Balancing Idea

To match atoms, we change the number of molecules with big numbers in front (coefficients), not the subscripts. Example: 2Mg + O2 → 2MgO has 2 Mg and 2 O on both sides.

7. Mini Practice: From Words to Symbols and Atoms

Try these short tasks. Say your answers out loud or jot them down.

Identify bonding type and particles

For each substance, decide if it is mainly ionic or covalent and what particles it contains.

a) NaCl
b) H2O
c) CO2

Think:

Na (metal) + Cl (non-metal) → ionic, particles are ions.
H and O (non-metals) → covalent, particles are molecules.

Check yourself:

a) NaCl: ionic, Na⁺ and Cl⁻ ions in a lattice.
b) H2O: covalent, H2O molecules.
c) CO2: covalent, CO2 molecules.

Write a word equation

A strip of magnesium reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas.

Write the word equation:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

Count atoms in a formula

For each formula, state how many atoms of each element are in one molecule or formula unit.

a) H2O
b) CO2
c) Na2O

Answers:

a) H2O: 2 H, 1 O
b) CO2: 1 C, 2 O
c) Na2O: 2 Na, 1 O

Conservation thinking

In the reaction: carbon + oxygen → carbon dioxide

Word equation: carbon + oxygen → carbon dioxide

Symbol equation: C + O2 → CO2

Question: Does the number of each type of atom match on both sides?

Left: 1 C, 2 O
Right: 1 C, 2 O

Yes, so this simple equation is already balanced and obeys conservation of mass.

8. Quick Check: Bonds and Equations

Answer the multiple-choice question, then read the explanation.

Which statement best describes the reaction represented by the symbol equation: 2H2 + O2 → 2H2O ?

Hydrogen and oxygen atoms are destroyed to make water molecules, so mass is not conserved.
Hydrogen and oxygen molecules rearrange to form water molecules, and the total number of H and O atoms stays the same.
Hydrogen ions and oxide ions form an ionic lattice of water, so the reaction is ionic.
Hydrogen and oxygen mix physically without forming any new bonds, so no chemical change occurs.

Show Answer

Answer: B) Hydrogen and oxygen molecules rearrange to form water molecules, and the total number of H and O atoms stays the same.

In 2H2 + O2 → 2H2O, each H2 and O2 is a molecule with covalent bonds. During the reaction, bonds in H2 and O2 break and new covalent bonds form in H2O. Count atoms: left has 4 H and 2 O; right has 4 H and 2 O. No atoms are created or destroyed, so mass is conserved. Water is a covalent molecular substance, not an ionic lattice.

9. Evidence of Chemical Change

Chemical vs Physical Change

Chemical reactions make new substances by rearranging atoms. Physical changes, like melting or dissolving, do not create new substances.

Signs of Chemical Change

Common clues: color change, gas produced (bubbles not from boiling), temperature change, formation of a solid precipitate, and sometimes light or sound.

Examples

Burning magnesium: silver-grey metal to white powder (color change, heat, light). Acid + metal: bubbles of hydrogen gas. Mixing solutions: cloudy solid precipitate appears.

Mass Still Conserved

Even when you see these changes, atoms are just rearranged. Total mass stays the same, though some gas may escape or energy may be released.

10. Flashcard Review: Key Terms

Use these flashcards to review the main ideas from this module.

Ionic bond: A chemical bond formed by the transfer of electrons from one atom to another, producing oppositely charged ions that attract each other. Typically between a metal and a non-metal.
Covalent bond: A chemical bond formed when two atoms share one or more pairs of electrons. Usually occurs between non-metal atoms and forms molecules.
Molecule: A group of two or more atoms joined by covalent bonds, acting as a single particle. Example: H2O, CO2, O2.
Chemical formula: A shorthand way to show which elements and how many atoms of each are in a substance. Example: H2O has 2 hydrogen atoms and 1 oxygen atom.
Word equation: A way of describing a reaction using words: reactants → products. Example: magnesium + oxygen → magnesium oxide.
Symbol equation: A chemical equation using formulas instead of words. Example: 2H2 + O2 → 2H2O.
Law of conservation of mass: In a chemical reaction, atoms are not created or destroyed, so the total mass and the number of each type of atom stay the same before and after.
Precipitate: A solid that forms and appears in a liquid mixture during a chemical reaction, often making the solution cloudy.
Reactants: The starting substances in a chemical reaction, written on the left side of a word or symbol equation.
Products: The new substances formed in a chemical reaction, written on the right side of a word or symbol equation.

Key Terms

ion: A charged particle formed when an atom or group of atoms gains or loses one or more electrons.
lattice: A regular, repeating 3D arrangement of particles, such as ions in an ionic solid.
molecule: A group of atoms held together by covalent bonds, behaving as a single particle.
products: Substances formed by a chemical reaction, written on the right side of an equation.
reactants: Substances present at the start of a chemical reaction, written on the left side of an equation.
ionic bond: A bond formed by electron transfer between atoms, creating ions that attract each other, usually between a metal and a non-metal.
precipitate: A solid that forms from a solution during a chemical reaction and separates out, often making the mixture cloudy.
covalent bond: A bond where atoms share pairs of electrons, usually between non-metal atoms, forming molecules.
word equation: A description of a chemical reaction using the names of reactants and products with an arrow between them.
symbol equation: A chemical equation that uses formulas and symbols to show a reaction, such as 2H2 + O2 → 2H2O.
chemical formula: Notation that shows the elements in a compound and the number of atoms of each, using symbols and subscripts.
law of conservation of mass: Principle stating that mass is neither created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products.