Atomic Structure and the Periodic Table

Atoms are the smallest units of matter and constitute all forms of substances.
An atom is comprised of a nucleus at its centre, with protons (positively charged) and neutrons (no charge).
Orbiting the nucleus are electrons (negatively charged) which reside in energy levels, also known as electron shells.
The atomic number of an element, found on the periodic table, signifies the number of protons in the nucleus. The atomic number also equals the number of electrons in a neutral atom.
The mass number (also found on the periodic table) represents the total number of protons and neutrons in an atom.
The different number of neutrons in atoms of the same element results in isotopes. Isotopes of an element have identical chemical properties, however, their physical properties may vary due to the difference in mass.

An element is a pure substance constituted by atoms with the same number of protons.
A total of 118 elements are organised in the Periodic Table, with each element represented by a unique chemical symbol.
Elements are grouped into metals, nonmetals and semimetals (or metalloids) based on their properties.

The Periodic Table is a sorted arrangement of elements based on their atomic number, and categorised by their chemical and physical properties.
It is organised into rows (periods) and columns (groups).
Elements residing in the same group share similar chemical properties. This is because they have the same number of electrons in their outer shell.
The physical and chemical properties of elements show a periodic pattern when arranged by atomic number, which is depicted by the Periodic Table.
The Periodic Table is split into blocks based on the type of orbital occupying the outermost electron(s): the s-block, p-block, d-block, and f-block.

A compound is a substance formed when two or more elements chemically combine.
Compounds have distinct properties that are different from the individual elements that constitute them. For example, sodium is a reactive metal and chlorine is a toxic gas, but combined they form table salt (sodium chloride), which is edible.
Elements in a compound are combined in a specific ratio which can be visualised through the compound’s chemical formula.
A compound’s constituent elements are chemically bonded together, either through ionic or covalent bonding. Ionic bonding happens between metal and non-metal atoms, whereas covalent bonding happens between non-metal atoms.