Rows in the periodic table are called periods. For example, all of the elements in the second row are referred to as 'second period elements'. All elements currently known fall in the first seven periods. Compare with isotope and allotrope . Solid substances that occur in several distinct forms.
Polymorphs have different chemical and physical properties.
An elementary particle found the atomic nucleus with a positive charge equal and opposite that of the electron . Protons have a mass of 1.007276 daltons . Spontaneous emission of particles or high-energy electromagnetic radiation from the nuclei of unstable atoms. Ratios of atoms in a compound. Ratios of moles of compounds in a reaction.