The universe is made up of millions of different substances. All of these substances are made up of building blocks known as atoms. Different types of atoms can combine with each other to form new substances. Understanding atoms help scientists create new materials for more advanced applications such as LCD screens, lasers, and solar cells.

All materials are made up of tiny building blocks known as atoms.

Atoms are so small that they cannot be seen with even the most powerful optical microscope. To see atoms, scientists must use a special type of microscope known as a scanning tunneling microscope or STM.

There are 118 known types of atoms and only 98 of these are found naturally on earth. The remaining 20 types of atoms must be made in a laboratory.

Each atom is given its own chemical symbol that is usually made up of one or two letters. In some special cases, the symbol may have 3 letters. The symbol refers to the name of the atom.

Atoms in elements and compounds

Atoms stick together to form all of the different substances that you see around you. When atoms stick together they can form either cluster of atoms known as molecules or large grid-like structures known as crystal lattices.

For example, water ($H_2O$) is made up of molecules. Every water molecule is identical and contains two hydrogen atoms (H) and one oxygen atom (O). On the other hand, a grain of beach sand is a crystal lattice of silicon (Si) and oxygen (O) atoms. The number of atoms in the lattice depends on the size of the grain of sand.

Elements

If a substance is made up of just one type of atom, then it is referred to as an element. Some types of elements like carbon can form very large molecules. A buckyball is made up of 60 carbon atoms (C60) in the shape of a soccer ball, while a carbon nanotube can have thousands of carbon atoms forming a long cylinder.

Carbon is a non-metallic element that can form crystal lattices. The diamonds found in jewelry and the graphite in pencil ‘leads’ are two forms of carbon crystal lattices. Metallic elements such as copper and gold always form crystal lattices.

Compounds

If a substance is made up of different types of atoms, then it is known as a compound. The molecules that makeup compounds range from small to very large. For example, a glucose sugar molecule is made up of just 24 atoms. In contrast, a single molecule of DNA inside one of your cells is made up of billions of atoms and can be stretched to over a meter in length.

Many compounds are crystal lattices. Common table salt is a lattice of sodium (Na) and chlorine (Cl) arranged into a three-dimensional grid.

Inside atoms

Atoms were once thought to be hard and unbreakable. Today, scientist know that atoms are made up of even smaller particles known as subatomic particles. Each atom is made up of three types of subatomic particles: protons, neutrons, and electrons.

The protons and neutrons form a cluster that sits at the center of the atom. This cluster is known as the nucleus. The electrons are much smaller and lighter and move very fast around the nucleus to form an electron cloud that surrounds the nucleus.

Protons and neutrons are similar in mass. However, protons have a positive electric charge while neutrons have no electric charge. Electrons are approximately 1800 times lighter than protons and neutrons and have a negative electric charge.

The negative charge of electrons causes them to be attracted to the positively charged protons in the nucleus. This is because opposite electric charges attract each other, a bit like the way opposite poles of a magnet attract each other. This attractive force is known as an electrostatic attraction.

💡 Approximately 98% of the atoms in the universe are either hydrogen (H) or helium (He) atoms. These atoms make up the Sun and the stars. The other types of atoms make up only 2% of all the atoms in the universe.

Atomic numbers and mass number

The number of protons in the nucleus determines the type of atom it is and what element it belongs to. For example, all gold atoms contain 79 protons while all oxygen atoms contain eight protons. The number of protons in an atom is called the atomic number.

The number of protons plus neutrons in an atom is called the mass number. These numbers are often written alongside the chemical symbol. For example, an atom of sodium, Na, can be shown as from this one symbol, you can calculate the number of protons, neutrons, and electrons in the sodium atom: 

• The number of protons in the atomic number is 11. So there are 11 protons in the nucleus.
• The number of electrons is equal to the number of protons. So there are 11 electrons spinning in a cloud around the nucleus.
• The number of neutrons is the mass number minus the atomic number: 23-11=12. So there are 12 neutrons in the nucleus.

Electrons and the nucleus

The number of electrons surrounding the nucleus of an atom is exactly equal to the number of protons in the nucleus. As a result, atoms and neutral. However, the electron can be 100 or even 1000 times wider than the nucleus. This means that most of an atom is empty space.

Electron shells

Even the electron cloud that surrounds the nucleus has structure. The electron cloud can be broken down into electron shells that surround the nucleus like the layers of an onion.

Each shell can only hold a certain number of electrons.

• 1st shell: 2 electrons
• 2nd shell: 8 electrons
• 3rd shell: 18 electrons

An aluminum atom has 13 electrons, the electron configuration of 2,8,3.

Ions

Atoms can lose or gain electrons to become electrically charged particles called ions.

• Cations (positive/cats are good): When an atom loses an electron, it has more protons than electrons. this gives the atom a positive charge.
• Anions (negative): When an atom gains an electron, then it has more electrons than protons. It is known negatively charged and is known as an anion.

The symbol for an ion is the same as the chemical symbol for the atom is was formed from, but with the charge of the ion added to it. For example, when a sodium atom (Na) forms a sodium cation, it loses one electron. this gives it a charge of +1 and so the symbol of the sodium ion is Na$^+$.

The same is true for anions. when a chlorine atom gains one electron, it forms an ion with a charge of -1. Its symbol is Cl$^-$. If an oxygen atom gains two electrons, then it becomes an ion with a charge of -2 and its symbol is written as O$^2$$^-$.

Ions can be formed in many situations but they are most commonly found when some substances are dissolved in water. Substances that form ions when dissolved will conduct electricity because the charged ions are free to move through the liquid, carrying the electric current with them.

For example, salt (sodium chloride, NaCl) dissolves because water molecules break up the lattice. This releases sodium ions (Na$^+$) and chloride ions (Cl$^-$)into the solution. The presence of these ions in the solution means that salt water is a conductor of electricity.