The atomic nucleus contains electrons and neutrons. All atoms of the same element have the same number of protons. Protons are more massive than neutrons. Protons repel electrons because they have the same charge. All electrons have a relative minus one charge.
Electrons repel electrons because they all have the same charge.
Mendeleev proposed the forerunner of the modern periodic table 10. Elements in the same vertical group show similar properties. Elements in the periodic table are arranged in order of increasing atomic number. Electrons show magnetic character. Valence electrons are electrons close to the nucleus of the atom.