Atoms and Molecules (G3 Lower Secondary + Extra)

Parts of an atom

Nucleus - Consists of protons and neutrons
Electrons - Travel around the nucleus in orbits

Proton

Mass number: 1
Charge: 1+

Atom has different proton number = Atom is of different element

Neutron

Mass number: 1
Charge: 0

Atom of same element but different neutron number = Atom of same element but different mass number = Isotope

Electron

Mass number: Close to 0
Charge: 1-

Same atom but different number of electrons = Ions (Net charge is not 0)
+ charge: Cation
- charge: Anion

Molecule

Consists of multiple atoms connected together by covalent bonds (sharing of electrons).

Carbon dioxide (CO₂)

Molecule which contains one carbon (C) atom and two oxygen (O) atoms.

Ionic compounds

Consists of ions - cations (+) and anions (-) - which are connected together by ionic bonds (attraction of opposite charges).

Cation

Positively charged ion. Metals form cations.

The name of the cation is the same as the name of the metal. E.g., Sodium (Na) forms the sodium ion (Na⁺).

How to remember: ‘t’ looks like ‘+’

Anion

Negatively charged ion. Non-metals form anions.

The name of the anion is derived from the name of the non-metal with the suffix -ide. E.g., Chloride (Cl) forms the chloride ion (Cl^-).

Polyatomic ion

Ion which contains more than one atom. Within the ion, the atoms are connected by covalent bonds, but the entire ion interacts with other oppositely charge ions with ionic bonds.

Examples: Ammonium, Nitrate, Sulfate, Carbonate, Hydroxide

Sodium chloride (NaCl)

Ionic compound which consists of sodium ions (Na⁺) and chloride ions (Cl^-).

Sodium hydroxide (NaOH)

Ionic compound which consists of sodium ions (Na⁺) and hydroxide ions (OH^-, a polyatomic ion).

Calcium carbonate (CaCO₃)

Ionic compound which consists of calcium ions (Ca²⁺) and carbonate ions (CO₃^2-, a polyatomic ion).