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.
Chlorine is an example of a metal.
Sodium, Na, is a p-block element. Isotopes are atoms of the same element with different numbers of electrons. The electron configuration for the six electrons in carbon is 1s 2 2s 2 2p 2 19. An atom with the electron configuration 1s 2 2s 2 2p 1 has one valence electron.