The definition of atoms

All matters in this world are composed by atoms, small particles that can't be seen using our eyes.

If you cut a softdrink can until  it becomes the smallest part and can only be seen using an electron microscope, then that part is called aluminum atom. Such atom is the smallest particle of the aluminum can which still has the same properties as the aluminum element. what will happen if that aluminum atom is cat again into smaller parts? If you do it, than you won't find the properties of aluminum element anymore. that's why we can define atoms as the smallest part of an element which still has the initial properties of the element.

Each atom of an element has specific properties which are different from the properties of the atoms of other elements. for example, the atoms' properties of sodium element are different from the atoms' properties of oxygen element or silver element. These differences cover all of the chemical and physical properties of such elements. The differences in chemical properties are coused by the difference of electrons amount owned by the atoms. The physical properties are including the size, mass, melting point, and atomic specific gravity.