Why do atoms stick together? The atoms in my table "stick together" to form a rectangle. Why? What makes them stick together?
I know about ionic/covalent bonding etc., but consider a sheet of pure iron. Just atoms of one element. The atoms still stick together to form a sheet instead of being "everywhere".But if I place ten balls on the ground beside each other they are completely independent of each other.
So what "connects" the atoms?
 A: Consider two $H$ atoms $rotating^{(1)}$ about their centre of mass, now both the atoms are electrically neutral and far apart so thay neither strong nor weak nuclear force comes into consideration. If they were to stay in this state only then they would never combine and form $H_2$ but what happens is that when the electrons of each atom are moving during various instances dipoles are formed and  london dispersion forces come into play due to these interactions the 2 atoms move towards each other and at a certain distance acheive a stable equilibrium, and are therefore bonded to each other.
On the other hand lets suppose you somehow got 2 balls of $H$ in elemental form and placed them near each other, the atoms of both ball will still get induced dipoles but since all the dipoles are randomly oriented, their would not be any significant overall dipole moment which may force the two balls to come close and join.
This was highly simplified version of what happens with iron and other elements and compounds, on thd atomic scale considerable dipole moments are developed even in neutral atoms/compounds , these motivate the further bonding to form other objects such as lattices, crystals, sheets and so on. On the other hand at the macro scale, no considerable attraction forcd develops between two neutral objects which may motivate them to join/bond. But indeed if you have to oppoisitely charged objects they may join together. Also to join objects on macroscales we have different proceses such as different types of welding etc 
$(1)$ I said rotating about centre of mass to avoid considering attraction due to gravitational interaction between the two atoms.
A: The metal atoms are bound together by a chemical bond called metallic bonding. Metals are characterized by practically free electrons moving between the atomic ions forming a crystal lattice. In a simplistic view, the bonding is due to the electrostatic forces between the positive ions mediated by the negative free electrons.
A: Our world is made up of 4 fundamental forces, which are:


*

*Gravity

*Electromagnetic

*Weak Interaction

*Strong Force


Each of these forces serves a purpose, for example. Gravity is what holds the planets and stars and galaxies together.
In the Atomic world, there are two fundamental forces that bind atoms together, and they are the Electromagnetic Force & Strong Force. 
Because every atom is made up of protons and electrons, as well as Neutrons but they have a neutral charge. Protons and Electrons however have positive and negative charges. Protons have a positive charge, and Electrons have a negative charge. And as we know a positive and negative charge with both attract one another.
The centre of an atom is actually positively charged so with neutrons and protons. This means that they will exert a positive charge on electrons, which means that electrons will swarm around the Nucleus of an atom.
Strong Force and this is how protons and neutrons are bound together. The strong force is what binds the Nucleus of an atom together i.e. Protons and Neutrons. The electromagnetic force couldn't possibly keep the nucleus of an atom together, because it is much too weak, and the protons would simply repel one another and fly, because they are all positively charged, and we know that two positively charged particles won't attract each other. So there needs to be a stronger force, and that just happens to be the Strong Force.
The Strong Force is a force which attracts protons to protons and neutrons to neutrons, and protons and neutrons to each other. This force has a very short range, and this is the reason why a nucleus is very very small. This force is also responsible for binding Quarks and Gluons into Protons and Neutrons.
So from this we now know that the Electromagnetic Force is what keeps the electrons held in and orbiting the nucleus. The Strong Force is what holds the Nucleus of an atom together
