The atomic number is the number of protons in an element, and never varies. For example, all hydrogen atoms, and only hydrogen atoms, have 1 proton. Sodium has an atomic number of 11 because its nucleus has 11 protons, while oxygen has an atomic number of 8 because its nucleus has 8 protons. You can find the atomic number of any element on the periodic table - in nearly all standard periodic tables: it's the number above an element's 1 or 2-letter  chemical symbol. This number will always be a positive whole number.   Let's say that we're working with the carbon atom. Carbon always has 6 protons, so we know its atomic number is 6. We can also see on the periodic table that the square for carbon (C) has a "6" at the top, signifying that carbon's atomic number is 6. Note that an element's atomic number doesn't have any direct bearing on its relative atomic mass as listed on the periodic table. Though, especially among elements at the top of the periodic table, it may seem that an atoms' atomic mass is about twice its atomic number, atomic mass isn't ever calculated by doubling an element's atomic number. The number of neutrons can vary among atoms of a certain element. While 2 atoms with the same number of protons and differing numbers of neutrons are both the same element, they are different isotopes of that element. Unlike the number of protons in an element, which never changes, the number of neutrons in atoms of a certain element can vary often enough that the average atomic mass of the element must be expressed as a decimal value between two whole numbers.   The number of neutrons can be determined by the isotope designation of the element. For example, carbon-14 is a naturally occurring radioactive isotope of carbon-12. You will often see an isotope designated with the number as a superscript before the element symbol: 14C. The number of neutrons is calculated by subtracting the number of protons from the isotope number: 14 – 6 = 8 neutrons. Let's say the carbon atom we're working with has six neutrons (12C). This is by far the most common isotope of carbon, accounting for nearly 99% of all carbon atoms. However, about 1% of carbon atoms have 7 neutrons (13C). Other types of carbon atoms with more or less than 6 or 7 neutrons exist in very small amounts. This is the atomic mass of that atom. Don't worry about the number of electrons orbiting the nucleus - their combined mass is very, very small, so, in most practical cases, it won't significantly affect your answer.   Our carbon atom has 6 protons + 6 neutrons = 12. The atomic mass of this specific carbon atom is 12. If it was a carbon-13 isotope, on the other hand, we would know that it has 6 protons + 7 neutrons = an atomic weight of 13. The actual atomic weight of carbon-13 is 13.003355, and is more precise because it was determined experimentally. Atomic mass is very close to the isotope number of an element. For basic calculation purposes, isotope number is equal to atomic mass. When determined experimentally, the atomic mass is slightly higher than the isotope number due to the very small mass contribution from electrons.
جد العدد الذري للعنصر أو النظير. جد عدد نيوترونات الذرة. اجمع عدد البروتونات والنيوترونات.