Atoms, Elements and Compounds

An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are incredibly tiny — about $1 \times 10^{-10}$ metres in diameter.

Structure of an Atom

Every atom consists of three subatomic particles:

The nucleus is at the centre of the atom. It contains protons and neutrons and is where almost all the mass of the atom is concentrated. The nucleus has an overall positive charge because of the protons.

Electrons orbit the nucleus in energy levels (shells). They have negligible mass but carry a negative charge that balances the positive charge of the protons, making atoms electrically neutral.

Atomic Number and Mass Number

• Atomic number (Z) = number of protons in the nucleus
• Mass number (A) = number of protons + number of neutrons

$\text{Number of neutrons} = A - Z$

For example, carbon has an atomic number of 6 and a mass number of 12:

• Protons = 6
• Neutrons = $12 - 6 = 6$
• Electrons = 6 (in a neutral atom)

Isotopes

Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. For example:

• Carbon-12: 6 protons, 6 neutrons
• Carbon-14: 6 protons, 8 neutrons

Isotopes have identical chemical properties (same electron configuration) but different physical properties (different masses).

An element is a substance made of only one type of atom. All atoms in an element have the same atomic number (same number of protons).

There are about 118 known elements, organised in the periodic table. Examples include:

Elements cannot be broken down into simpler substances by chemical methods.

A compound is a substance formed when two or more elements are chemically combined in fixed proportions. The properties of a compound are usually very different from the properties of the elements it contains.

For example:

• Water ($\text{H}_2\text{O}$): made from hydrogen (a flammable gas) and oxygen (supports combustion), yet water puts out fires
• Sodium chloride ($\text{NaCl}$): made from sodium (a reactive metal) and chlorine (a toxic gas), yet table salt is safe to eat

Chemical Formulae

Compounds are represented by chemical formulae that show:

• Which elements are present (using symbols)
• How many atoms of each element are in one unit (using subscript numbers)

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

A mixture contains two or more elements or compounds that are not chemically combined. The components retain their own properties and can be separated by physical methods.

Key Differences: Compounds vs Mixtures

Separation Techniques

1. Filtration — separates an insoluble solid from a liquid
2. Evaporation / Crystallisation — separates a dissolved solid from a solution
3. Simple distillation — separates a solvent from a solution (e.g. water from salt water)
4. Fractional distillation — separates miscible liquids with different boiling points
5. Chromatography — separates dissolved substances based on how they move through a stationary phase

Chemical reactions are described using word equations and balanced symbol equations.

Word Equation

$\text{magnesium} + \text{oxygen} \rightarrow \text{magnesium oxide}$

Symbol Equation

$2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$

In a balanced equation, the number of atoms of each element is the same on both sides. This follows the law of conservation of mass — atoms are not created or destroyed in a chemical reaction.

Steps to Balance an Equation

1. Write the unbalanced equation with correct formulae
2. Count atoms of each element on both sides
3. Add coefficients (big numbers in front) to balance
4. Check all elements are balanced

Example: Balance the equation for the reaction of iron with oxygen: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$

Step by step: $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$

Check: Fe — 4 on each side ✓; O — 6 on each side ✓

• Always state whether a substance is an element, compound, or mixture and give a reason
• When balancing equations, never change the small (subscript) numbers — only add coefficients
• Remember: atomic number = protons = electrons (for neutral atoms)
• Learn the symbols of the first 20 elements and common transition metals

• Atoms are the building blocks of matter, made of protons, neutrons, and electrons
• Elements contain only one type of atom
• Compounds form when elements chemically bond in fixed ratios
• Mixtures are physically combined and can be separated by physical methods
• Chemical equations must be balanced to conserve mass
• Isotopes are atoms with the same protons but different neutrons