Science Double Award Co-Ordinated (0654) iGCSE 2026 Chemistry 2 (Atoms, Element and Compounds)

What is an element?

A pure substance made up of only one type of atom.

Give one example of an element.

Oxygen, copper, iron, or hydrogen (any correct example).

What is a compound?

A substance made up of two or more elements chemically bonded together.

Give one example of a compound.

Water (H₂O), carbon dioxide (CO₂), sodium chloride (NaCl), (Other examples accepted)

What is a mixture?

A combination of two or more substances that are not chemically bonded.

Give one example of a mixture.

Air, salt water, or sand and iron filings (any correct example).

How can the components of a mixture be separated?

By physical methods e.g. Filtration

How can the elements in a compound be separated?

Only by chemical reactions

What happens to the properties of elements when they form a compound?

The compound has different properties from the elements that form it.

Are elements and compounds pure substances?

Yes

Is a mixture a pure substance?

No

Why can mixtures be easily separated?

Because the substances are not chemically bonded together.

Describe the structure of an atom.

An atom has a nucleus containing protons and neutrons, surrounded by electrons in shells.

What particles are found in the nucleus?

Protons and neutrons.

Where are electrons found in an atom?

In shells around the nucleus.

What is the charge on a proton?

+1 (positive one).

What is the charge on a neutron?

0 - it is neutral.

What is the charge on an electron?

-1 (negative one).

Compare the relative masses of protons, neutrons and electrons.

Proton ≈ 1, Neutron ≈ 1, Electron ≈ 1/1840 (negligible).

Define the proton number (atomic number).

The number of protons in the nucleus of an atom.

Define the mass number (nucleon number).

The total number of protons and neutrons

How do you calculate the number of neutrons in an atom?

Neutrons = Mass number - Proton number.

Write the electronic configuration for sodium (proton number = 11).

2, 8, 1

Write the electronic configuration for aluminium (proton number = 13).

2, 8, 3

Write the electronic configuration for argon (proton number = 18).

2, 8, 8

What is the electronic configuration for calcium (proton number = 20)?

2, 8, 8, 2

What is special about the electron arrangement of Group VIII (noble gases)?

They have a full outer shell of electrons.

What does the group number of an element tell you?

The number of electrons in its outer shell (for Groups I-VII).

What does the period number of an element tell you?

The number of occupied electron shells.

How does the arrangement of electrons relate to the Periodic Table?

Elements in the same group have the same number of outer-shell electrons; elements in the same period have the same number of shells.

Define an isotope.

atoms of the same element that have the same number of protons but different numbers of neutrons.

What is the same in all isotopes of an element?

The number of protons (and electrons).

What is different in isotopes of the same element?

The number of neutrons

Why do isotopes of the same element have the same chemical properties?

Because they have the same electronic configuration.

Give an example of two isotopes of carbon.

Carbon-12 and Carbon-14.

How many protons and neutrons are there in the isotope ¹²₆C?

6 protons and 6 neutrons.

How many protons and neutrons are there in the isotope ¹⁴₆C?

6 protons and 8 neutrons.

How many electrons does a neutral chlorine atom (³⁵₁₇Cl) have?

17 electrons.

An atom has mass number 27 and 14 neutrons. Determine its proton number and identify the element.

Proton number = 13 → Aluminium (²⁷₁₃Al).

Why do simple molecular compounds have low melting and boiling points?

They have weak intermolecular forces, which require little energy to overcome.

Why are covalent bonds strong, but simple molecular substances have low melting points?

Covalent bonds within molecules are strong, but the forces between molecules (intermolecular forces) are weak.

Why do simple molecular substances not conduct electricity?

They do not have free electrons or charged particles to carry an electric current.

Why does CO₂ exist as a gas at room temperature?

Weak intermolecular forces mean CO₂ has a low boiling point, so it remains a gas at room temperature.

What is the structure of diamond?

Giant covalent lattice where each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement.

What is the structure of graphite?

Giant covalent layers of carbon atoms where each carbon is bonded to three others, forming hexagonal rings, with weak forces between layers.

Why is graphite used as a lubricant?

The layers slide over each other easily because of weak forces between layers.

Why is graphite used as an electrode?

It has delocalized electrons that allow it to conduct electricity.

Why is diamond used in cutting tools?

Diamond is extremely hard due to its strong covalent bonds in a rigid structure.

What is metallic bonding?

Electrostatic attraction between positive metal ions in a giant metallic lattice and a 'sea' of delocalized electrons.

Why do metals conduct electricity?

They have delocalized electrons that are free to move and carry charge.

Why are metals malleable and ductile?

The layers of metal ions can slide over each other while remaining held together by the delocalized electrons.