From Matter to Molecules: Atom, Proton, Neutron, Electron, Nucleus, Periodic Table

Ordinary matter is made of atoms. Each atom has a nucleus containing protons and usually neutrons, with electrons occupying regions around the nucleus. The number of protons identifies the element. Atoms can bond with other atoms to form molecules or larger structures.

The particles in an atom

A proton has a positive electric charge and is found in the nucleus. A neutron has no electric charge and is also found in the nucleus. An electron has a negative charge and occupies regions around the nucleus.

Protons and neutrons have similar masses; electrons are much lighter. Nearly all the mass of an atom is in its nucleus, though the nucleus takes up only a small part of the atom’s volume Encyclopaedia Britannica.

In a neutral atom, the number of electrons equals the number of protons, so the positive and negative charges balance Jefferson Lab. An atom that gains or loses electrons becomes an ion. Its element remains the same because its proton count has not changed.

How the nucleus identifies an element

An element’s atomic number is its number of protons. Changing that number changes the element. The mass number is the total number of protons and neutrons in a particular atom.

Atoms of the same element can have different numbers of neutrons. These forms are called isotopes. They share the same atomic number but have different mass numbers. For example, carbon-12 has 6 protons and 6 neutrons; carbon-13 has 6 protons and 7 neutrons. Both are carbon because each has 6 protons.

Example: carbon-12

A neutral carbon-12 atom has 6 protons, 6 neutrons, and 6 electrons. Its atomic number is 6, and its mass number is 12. The electrons occupy regions around the nucleus; a simple shell model places 2 in the first shell and 4 in the second.

How to read the periodic table

The periodic table arranges elements by atomic number, which increases from left to right across each row, called a period. Elements in the same column, called a group, often have similar chemical behavior because their outer electrons follow similar patterns IUPAC Encyclopaedia Britannica.

The table can help explain similarities, but it does not make every element in a group behave identically. An element’s properties also depend on its electron arrangement and the conditions in which it reacts.

For example, sodium and potassium are in the same group and each has one outer electron. Both can lose that electron in chemical reactions, which helps explain their similar behavior. They are in different periods, so their electron arrangements differ beyond the outermost electron.

Electrons and chemical bonds

Chemical bonding involves electrons, especially those in an atom’s outer region. Atoms can form bonds by sharing or transferring electrons. A simple shell model can help introduce these patterns, but the familiar idea that atoms always seek eight outer electrons is a rule of thumb, not a rule for every atom or bond.

In a covalent bond, atoms share electrons. Water is a molecule with two hydrogen atoms bonded to one oxygen atom, written as H₂O. In an ionic bond, charged ions attract after electrons transfer from one atom to another. Sodium chloride, or table salt, is made of sodium and chloride ions arranged in a solid structure. It is commonly called an ionic compound, not a collection of separate sodium chloride molecules.

A molecule is a group of two or more atoms held together by covalent bonds. Some substances, including many ionic solids, form extended structures instead of discrete molecules. This distinction matters when describing how atoms combine.

Key Takeaways

• An atom contains a nucleus of protons and neutrons, with electrons occupying regions around it.
• The number of protons, called the atomic number, identifies the element.
• Isotopes have the same number of protons and different numbers of neutrons. Their mass numbers differ.
• A neutral atom has equal numbers of protons and electrons. Gaining or losing electrons makes an ion without changing the element.
• The periodic table orders elements by atomic number. Elements in the same group often share chemical patterns.
• Covalent bonds involve shared electrons; ionic bonds involve attraction between oppositely charged ions. A molecule is a bonded group of atoms, while some compounds form larger structures.