Electrons in Atoms

Electrons are small, negatively charged particles that orbit the nucleus of an atom in specified energy levels or shells.
The first energy level (closest to the nucleus) can hold a maximum of 2 electrons. The second and third energy levels can hold up to 8 electrons each.
Electrons determine an atom’s chemical properties and its reactivity. Atoms aim to have a full outermost electron shell in order to be stable. This typically means having 8 electrons in its outermost shell.
The distribution or arrangement of electrons in an atom is called its electronic configuration. For example, an atom of oxygen (element 8) has an electronic configuration of 2,6.
If the outermost shell of an atom is not full, the atom can interact with other atoms to gain, lose or share electrons to become stable. This is how chemical bonds are formed.
Ions are formed when atoms lose or gain electrons. If an atom loses electrons, it becomes a positively charged ion (cation). If an atom gains electrons, it becomes a negatively charged ion (anion).
Elements in the same group of the periodic table have the same number of electrons in their outer shell, and thus have similar chemical properties.
The number of electrons in an atom is equal to the number of protons in its nucleus. This is what makes an atom neutral in charge.
In a diagram of an atomic structure, electrons are typically represented as dots orbiting concentric circles around the nucleus.
Quantum mechanics is the branch of physics that describes the behaviours of electrons within atoms. However, this is beyond the scope of your syllabus, and at this level, the simpler Bohr model of the atom is used.