Chemistry: Atomic Structure

Structure of Atoms

An atom is the smallest unit of matter that retains all of the chemical properties of an element.
Atoms consist of three types of particles: protons, neutrons, and electrons.
The nucleus of an atom contains protons and neutrons. Protons carry a positive charge while neutrons have no charge.
Electrons, which carry a negative charge, are located in energy levels outside the nucleus, known as electron shells.

The atomic number, or proton number, of an atom is the number of protons found in the nucleus. It identifies the element.
The atomic mass, or mass number, is the sum of protons and neutrons in the nucleus.
Elements are uniquely defined by their atomic number.

Electrons within an atom are arranged in energy levels, also known as electron shells.
The first shell can hold up to two electrons, and each subsequent shell can hold up to eight electrons.
The electron configuration of an atom can be written using numbers and letters that represent the energy levels and orbitals.
For instance, the configuration 2,8,8 means there are 2 electrons in the first shell, 8 in the second, and 8 in the third.

Isotopes are variants of a chemical element that have the same number of protons and electrons, but different numbers of neutrons.
Isotopes of an element have identical chemical properties but different physical properties, such as atomic mass.

Atoms can gain or lose electrons to form charged particles called ions.
An atom that gains one or more electrons becomes a negatively charged ion, or anion.
An atom that loses one or more electrons forms a positively charged ion, or cation.

The relative atomic mass (Ar) of an element is the average mass of its atoms, compared to 1/12th the mass of a carbon-12 atom. This takes into account the different isotopes of the element.

The Periodic Table is a chart that organises all known elements by their properties.
Elements are arranged in order of increasing atomic number, from left to right and top to bottom.
The rows of the table are called periods and the columns are called groups. Elements in the same group have similar properties.
There are different blocks in the Periodic Table: s-block, p-block, d-block, and f-block. Each block is associated with a specific type of atomic orbital: s, p, d, or f.

The arrangement of electrons in an atom determines many of the element’s chemical properties, including its reactivity and the types of bonds it can form.
The electron configuration of an atom can generally be predicted by the atom’s position in the periodic table.
Each period in the table corresponds to the filling of a distinct electron shell.