Atomic structure

Matter made of atoms

Ordinary matter is made of atoms. An atom is the smallest unit of an element that still has the chemical identity of that element. A gold atom is gold; an oxygen atom is oxygen. Atoms are incredibly small, but their structure explains many properties of matter.

An atom has a tiny, dense nucleus surrounded by electrons. The nucleus contains protons and neutrons. Protons have positive electric charge. Neutrons have no electric charge. Electrons have negative electric charge and occupy regions around the nucleus.

Atomic number and identity

The number of protons in the nucleus determines the element. This number is called the atomic number. Hydrogen has 1 proton. Carbon has 6 protons. Oxygen has 8 protons. Gold has 79 protons. If the number of protons changes, the element changes.

A neutral atom has equal numbers of protons and electrons. The positive charge of the protons balances the negative charge of the electrons. If an atom gains or loses electrons, it becomes an ion.

Ions

An ion is an atom or group of atoms with net electric charge. If an atom loses electrons, it has more protons than electrons and becomes positively charged. If it gains electrons, it becomes negatively charged.

For example, sodium can lose one electron and become a positive ion. Chlorine can gain one electron and become a negative ion. Opposite charges attract, helping form ionic compounds such as table salt.

Neutrons and isotopes

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These different forms are called isotopes. Carbon-12 and carbon-14 are both carbon because both have 6 protons. But carbon-12 has 6 neutrons, while carbon-14 has 8 neutrons.

Some isotopes are stable. Others are radioactive, meaning their nuclei can change over time and emit radiation. Radioactive isotopes are used in medicine, archaeology, energy, and scientific research, but they must be handled carefully.

Atomic mass

Most of an atom's mass is in the nucleus because protons and neutrons are much more massive than electrons. The mass number of an atom is the total number of protons and neutrons. For example, an atom with 6 protons and 6 neutrons has mass number 12.

A simple relationship is:

$A = Z + N$

Here $A$ is mass number, $Z$ is atomic number, and $N$ is number of neutrons.

Models of the atom

Atomic models have changed over time. Early models imagined atoms as tiny solid particles. Later experiments showed that atoms contain smaller charged parts. Rutherford's gold foil experiment revealed that atoms have small, dense, positive nuclei. Modern quantum models describe electrons not as tiny planets in fixed orbits, but as occupying probability regions called orbitals.

For a foundations course, the key point is that models improve as evidence improves. The simple nucleus-electron picture is useful, but more advanced physics gives a deeper and stranger description.

The big idea

Atoms are made of protons, neutrons, and electrons. Protons determine the element. Electrons influence chemical behavior and electric charge. Neutrons affect isotope identity and nuclear stability. Atomic structure connects electricity, chemistry, materials, light, and modern physics.