Electrostatic forces in atoms
Now there are two types of atoms, non-metals and metals. Non-metals generally have moderate to strong electrostatic forces attracting the outer most electrons(valence electrons) to the nucleus. Metals on the other hand, have very low electrostatic attraction for their outer most electrons and often tend to lose these electrons when reacting with other atoms.
Atoms and the realtive distance between the valence elctrons(outer level electrons) and the nucleus.

Have a look at the atoms on the left, there are metal (sodium and lithium) atoms and non-metal (fluorine, helium, oxygen and hydrogen) atoms.
Lets compare lithium and fluorine. Lithium has one electron in its outer energy level while fluorine has 7. The distance the outer level (valence) electrons are from the nucleus is roughly the same for both atoms. However, the charge that attracts lithium's outer electron to the nucleus is a single positive charge. You must be thinking, how can this be when there are three protons in lithium's nucleus. You see, the two electrons between the outer level electron and the nucleus neutralises the effect of two protons.
Fluorine, on the other hand has seven electrons in its outer level and each electron is attracted into the nucleus by a charge of 7+. Even though there are nine protons in the nucleus the two electrons between the valence electrons and the nucleus neutralise the effect of two protons.
You might wonder, all this talk of attraction between protons and electrons and not one mention of repulsion between electrons and repulsion between protons. Well, there is repulsion between electrons. As electrons move around the nucleus they tend to keep away from one another due to eletrostatic repulsion. In the nucleus where protons are packed in tightly, massive repulsive forces are acting trying to push the protons apart. An even greater force, called the nuclear binding force, holds the nucleus together and prevents it from blowing apart.

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Lithium has one valence electron that is attracted to the nucleus by, effectively, one positive charge. Sodium has one valence electron attracted tot he nucleus by one positive charge. The distance between the charges is significantly greater than that in lithium and so sodium has less attractio for its valence electron than lithium. Oxygen has 6 valence elctrons (outer level electrons) attracted to the nucleus by an effective charge of 6+ Hydrogen has one valence electron(outer level electron) that is pulled into the nucleus by a charge of 1+. Hydrogen is a non-metal and has moderate attraction for its valence elctron because the distance involved is small. Fluorine has valence electrons that are about the same distance from the nucleus as lithium's valence elctron but are attracted towards the nucleus by a charge of 7+