Valence Electrons and Chemical Behavior

Valence electrons are those found in the outermost shell (valence shell).
The chemical behavior of an atom is mostly determined by its valence electrons, not the electrons in the inner shells.

Hydrogen (H), Lithium (Li), and Sodium (Na) each have only one electron in their valence shell.
Hydrogen has 1 electron in its first shell, which is its valence shell.
Lithium has 3 electrons: 2 in the first shell and 1 in the second (valence) shell.
Sodium has 11 electrons: 2 in the first shell, 8 in the second shell, and 1 in the third (valence) shell.
These elements behave similarly chemically due to the single electron in their outermost shell.

Helium (He) has a filled outermost shell with 2 electrons.
Neon (Ne) has a filled outermost shell (second shell) with 8 electrons and a total of 10 electrons (2 in the first shell).
Elements with a full valence shell are chemically inert and do not readily participate in chemical reactions; these are called noble gases (Helium, Neon, Argon).
Noble gases are very stable and have no tendency to gain, lose, or share electrons.

Atoms are grouped based on the number of electrons in their outermost shell, not the number of protons.
Beryllium (Be) and Magnesium (Mg) behave similarly because they both have two electrons in their outermost shell.
Magnesium has 12 electrons: 2 in the first shell, 8 in the second shell, and 2 in the third (valence) shell.
Beryllium has 4 electrons: 2 in the first shell and 2 in the second (valence) shell.

An orbital is the three-dimensional space where an electron is found 90% of the time.
This is more specific than describing electrons as orbiting in concentric circles around the nucleus.
Each electron shell contains electrons at a particular energy level.
Within a particular shell, electrons are distributed among a specific number of orbitals, which have distinctive shapes and orientations.

The first shell has one orbital, called the 1s orbital, which is spherical.
The second shell has one 2s orbital (spherical) and three 2p orbitals (dumbbell-shaped).
The 2p orbitals are at right angles to one another (along the x, y, and z axes).
Each orbital can hold no more than two electrons.
The second shell can hold a total of eight electrons: two in the 2s orbital and two in each of the three 2p orbitals.
If all orbitals are superimposed, it shows the space where electrons are likely to be found.
Electrons do not orbit the nucleus in a predictable pattern but move unpredictably within their orbitals.

Atoms are made of subatomic particles: protons (positively charged, in the nucleus), neutrons (neutral, in the nucleus), and electrons (negatively charged, forming a negative cloud around the nucleus).
Electrons determine chemical behavior.
Helium, with a filled valence shell, does not participate in chemical bonding because it's stable.

Electrons have different energy levels, called shells, which can only fit a certain amount of electrons.
Electrons don't orbit the nucleus in a fixed path but are unpredictable.
It is impossible to know both where an electron is and where it's going at any given time.
Orbitals are specific shapes where electrons live.

Atoms with incomplete valence shells can share or transfer electrons with other atoms to complete their outermost shell.
Atoms can transfer electrons (give off or accept) or share electrons.
Sharing electrons results in chemical bonds that hold atoms together.
Sharing of electrons is called covalent bonds, which results in the formation of molecules.

A covalent bond results when atoms share a pair of valence electrons.
Hydrogen (H) has one proton and one electron, which whizzes around in the 1s orbital.
In a covalent bond, shared electrons count as part of each atom's valence shell.
Each electron is attracted to the other atom's nucleus.
When atoms get close enough, each electron is attracted to the other atom's nucleus.
When two hydrogen atoms share electrons, they form a hydrogen molecule held together by a covalent bond.
Each hydrogen atom has the benefit of the other one's electron, effectively filling its valence shell.
The electron cloud holds the two nuclei together.
The sharing of valence electrons completes the outermost shell of each participating atom.