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Atoms, elements and compounds

Lajoy Tucker

Teacher

Lajoy Tucker

Contents

Atoms – The Building Blocks of Matter

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Elements

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Diatomic Elements

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Compounds

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Molecules

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Check for Understanding

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Atoms – The Building Blocks of Matter

  • All substances are made of atoms.

  • An atom is the smallest part of an element that can exist on its own.

  • Atoms have a nucleus containing protons and neutrons, with electrons moving in shells around it.

The provided images illustrate both step-up and step-down transformers, which use electromagnetic induction across a laminated iron core to either increase or decrease voltage by varying the number of windings between the primary and secondary coils.

  • Atoms are extremely small, typically about 1 × 10⁻¹⁰ m in diameter.

Elements

  • An element is a pure substance made up of only one type of atom.

  • Each element is represented by a chemical symbol, for example:

    • O – oxygen

    • Na – sodium

    • Cl – chlorine

  • There are about 100 known elements, all arranged in the periodic table.

This is a periodic table that organizes chemical elements by their atomic number and chemical properties, detailing each element's mass, symbol, and name.

  • Elements cannot be broken down into simpler substances by chemical means.

Attention! All element symbols must start with a capital letter. If there is more than one letter, the second letter is lowercase

e.g. magnesium is Mg, NOT mg or MG

No answer provided.

Diatomic Elements

  • Some elements exist naturally as molecules made up of two atoms joined together.

  • These are called diatomic elements.

  • Each molecule contains two atoms of the same element bonded together.

This is a ball-and-stick model of a diatomic chlorine molecule, showing two chlorine atoms connected by a single covalent bond.

Diatomic Element

Symbol / Formula

Hydrogen

H₂

Nitrogen

N₂

Oxygen

O₂

Fluorine

F₂

Chlorine

Cl₂

Bromine

Br₂

Iodine

I₂

This is a periodic table of elements highlighting specific nonmetals, including nitrogen, oxygen, fluorine, and the halogens, categorized by their atomic numbers and grouped chemical properties.

Compounds

  • A compound forms when two or more elements are chemically combined in fixed proportions.

  • The compound’s properties are different from the elements it contains.

  • Compounds are represented by chemical formulae that show the symbols and ratios of atoms present.

This is a ball-and-stick model of a water molecule ($H_2O$), depicting two hydrogen atoms bonded to a single central oxygen atom in a bent geometric shape.

This is a three-dimensional lattice model of sodium chloride ($NaCl$), illustrating the alternating arrangement of sodium ions ($Na^+$) and chloride ions ($Cl^-$) in a giant ionic crystal structure.

Examples:

  • – water (2 hydrogen atoms, 1 oxygen atom)

  • – carbon dioxide (1 carbon atom, 2 oxygen atoms)

  • – sodium chloride (1 sodium atom, 1 chlorine atom)

  • Compounds can only be separated into elements by chemical reactions, not by physical methods.

Compounds are typically named based on the elements they contain:

Type of Elements

Naming Pattern

Example

Metal + Non-metal

Name ends in -ide

Sodium + chlorine → sodium chloride

Metal + Non-metal + Oxygen

Name ends in -ate or -ite

Copper + sulfur + oxygen → copper sulfate

Molecules

A molecule includes a few atoms covalently bonded together.

Molecules can contain one type of atom and be an element OR contain more than one type of atom and be a compound.

This diagram contrasts a chlorine molecule ($Cl_2$) as an element with a water molecule ($H_2O$) as a compound to show the difference between single-element molecules and those formed from different elements.

Check for Understanding

Recall

a) What is an atom?

Answer:

a) An atom is the smallest part of an element that can exist.

b) What is an element?

Answer:

b) An element is a pure substance made of only one type of atom.

c) What is a compound?

Answer:

c) A compound is a substance formed when two or more different elements are chemically combined in fixed proportions.

Application

d) Explain why O₂ is considered an element.

Answer:

d) O₂ is considered an element because it contains only one type of atom, which is oxygen.

e) Which of the following is the correct chemical symbol for iron?

Fe

FE

fe

fE

Answer:

e) Fe

Challenge (HT)

f) A substance has the formula CO₂. Explain why this substance is a compound rather than an element.

Answer:

f) CO₂ is a compound because it contains more than one type of element. It contains carbon and oxygen atoms that are chemically combined.

g) Explain why a substance can be called a molecule and an element at the same time.

Answer:

g) A substance can be called both a molecule and an element because:

  • A molecule is two or more atoms chemically bonded together.

  • An element is a substance made of only one type of atom.

So, if a molecule is made of two or more atoms of the same element, it fits both definitions.

Practice Question Videos

Mixtures