Metal atoms are more stable if they lose electrons to have a full outer shell.
Electrons lost by metal atoms in solid metal have nowhere to go and stay attracted to the positive charge of the metal ions.
Electrons are not held by the metal atom but are free to move - they are delocalised.
In metallic bonding, positive metal ions are attracted to the negative charges of the delocalised electrons and this gives metals a dense, rigid structure.
It is seems obvious, but when talking about metallic bonding, we are talking about a specific type of atomic bond that only occurs in metals.
Metals easily form positive ions. They are more stable with a full outer shell, even if it means they have to have a positive charge.
In metallic substances, metal atoms ‘lose’ electrons to become positive ions. The electrons have no-where to go, so they remain around the ions. They are attracted to the positive ions (as they themselves are negatively charged) but they are not physically held in one place, so they are described as being delocalised. This is why metals are able to conduct electricity.
Metallic bonding is very strong, which is why metals have high melting points and are used for strength in applications.