Structure of Atoms

Atoms are the smallest unit of a chemical element, consisting of a central positively-charged nucleus surrounded by negatively-charged electrons.
The nucleus, at the centre of the atom, contains protons and neutrons.
Protons possess a positive charge while neutrons carry no charge. Together, these are known as nucleons.
Surrounding the nucleus are electrons which carry a negative charge. They exist in different energy levels or shells positioned around the nucleus.
The atomic number, also known as the proton number, represents the number of protons in the nucleus of an atom. This gives each atom its unique identity as a particular element.
The mass number, on the other hand, is the total number of protons and neutrons in an atom’s nucleus.
Isotopes are atoms of the same element (same atomic number) but with differing numbers of neutrons (different mass numbers). As such, they possess the same chemical properties but may exhibit different physical properties.
The ‘relative atomic mass’ of an element takes into consideration the average mass of its isotopes relative to the mass of a carbon-12 atom.
Electrons occupy energy levels, also called electron shells, orbitals or quantum shells. The lower energy levels closest to the nucleus fill first.
Electron configuration explains how the electrons are distributed in these energy levels/orbitals. The first energy level can hold a maximum of two electrons, the second can hold up to eight electrons, and the third can hold eight as well.
The Periodic Table organises elements based on their atomic numbers. Going across a period from left to right, each element has one more proton and is less metallic than its predecessor.