Atoms, Elements, Compounds and the Model of the Atom
What are Atoms?
Everything around us is made up of tiny particles called atoms. Atoms are incredibly small — an atom has a radius of about 0.1 nanometres, or \(1 \times 10^{-10}\) m. To put this in perspective, if an atom was the size of a marble, a football would be the size of Earth!
All substances are made of atoms. Atoms cannot be created or destroyed in chemical reactions.
Atoms are made up of three types of particles: protons, neutrons, and electrons. Understanding how these particles are arranged helps us understand how different elements behave and react with each other.
Elements and the Periodic Table
An element is a pure substance that contains only one type of atom. For example, oxygen contains only oxygen atoms, and carbon contains only carbon atoms. There are about 100 different elements, and they’re all organised in the periodic table.
Each element has a unique atomic number. This tells us how many protons are in each atom of that element. Hydrogen has atomic number 1 (1 proton), helium has atomic number 2 (2 protons), and so on.
Elements in the same column (group) of the periodic table have similar properties because they have the same number of electrons in their outer shell. This affects how they react with other elements.
Compounds and Chemical Formulas
When atoms of different elements join together, they form compounds. For example, water (\(H_2O\)) is a compound made from hydrogen and oxygen atoms. The chemical formula tells us exactly which atoms are present and in what ratio.
In chemical formulas, the small numbers (subscripts) tell us how many atoms of each element are present. \(H_2O\) means 2 hydrogen atoms and 1 oxygen atom.
Common compounds you should know include:
- Water: \(H_2O\) (hydrogen and oxygen)
- Carbon dioxide: \(CO_2\) (carbon and oxygen)
- Sodium chloride: \(NaCl\) (sodium and chlorine)
- Methane: \(CH_4\) (carbon and hydrogen)
The Structure of the Atom
Modern atomic theory tells us that atoms have a specific structure. At the centre is the nucleus, which contains protons and neutrons. Around the nucleus, electrons move in regions called electron shells or energy levels.
Protons: Positively charged particles in the nucleus. The number of protons determines what element the atom is.
Neutrons: Neutral particles (no charge) in the nucleus. They add mass to the atom but don’t affect its chemical properties.
Electrons: Negatively charged particles that move around the nucleus in shells. They’re involved in chemical bonding.
The relative masses are important to remember: protons and neutrons each have a relative mass of 1, while electrons have a relative mass of almost zero (about 1 over 1836 to be precise).
How the Atomic Model Developed
Our understanding of atomic structure has changed dramatically over time as scientists gathered new evidence. This shows how scientific knowledge develops through experimentation and discovery.
Dalton’s Atomic Theory
John Dalton proposed that all matter was made of tiny, indivisible particles called atoms. He thought atoms were like tiny solid spheres.
Discovery of the Electron
J.J. Thomson discovered electrons and proposed the ‘plum pudding’ model — atoms were positive spheres with negative electrons embedded in them.
Rutherford’s Gold Foil Experiment
Ernest Rutherford fired alpha particles at gold foil. Most passed through, but some bounced back, proving atoms have a tiny, dense nucleus.
Bohr’s Model
Niels Bohr proposed that electrons orbit the nucleus in specific energy levels or shells, like planets around the sun.
Each new discovery built on previous work and led to a better understanding of atomic structure. This is a great example of how scientific knowledge is provisional — it can change when new evidence is discovered.
Mass Number and Atomic Number
Every element can be described using two important numbers. The atomic number tells us how many protons are in the nucleus. The mass number tells us the total number of protons and neutrons.
We can use these numbers to work out the number of each type of particle in an atom:
- Number of protons = atomic number
- Number of neutrons = mass number - atomic number
- Number of electrons = number of protons (in a neutral atom)
In a neutral atom, the number of protons always equals the number of electrons. This means the positive and negative charges balance out.