Ordinary atoms have equal numbers of electrons and protons so an ordinary atom is electrically neutral. Each element has a different atomic number which is the number of protons in the atom. Hydrogen has one proton, so its atomic number is 1, while helium has 2 so its atomic number is 2. The atomic weight is the combined number of protons and neutrons. Hydrogen has an atomic weight of 2, since its nucleus contains a proton and an electron while helium has an atomic weight of 4 since it has 2 neutrons, and 2 protons in its nucleus.
Isotopes are when an atom has varying number of neutrons.
Hydrogen and deuterium have the same number of protons and electrons, but deuterium has one more neutron, making it an isotope of hydrogen. Another example is 12 C and 13 C, where the number in front tells you the atomic weight of the atom. Phases of Matter Solid, liquid, gas, in order of increasing kinetic energy of the particles. Increasing the temperature of a substance will cause a phase change. The change from solid to liquid is called melting .