Its the mass number of an atom.
The mass of an atom is concentrated in the nucleus. Both protons and neutrons which are collectively known as nucleons are found within the nucleus.
Mass of electrons are not considered to be contributing towards the mass no. of an atom as their size is small and thus their mass is negligible.
In this case, the number 209 represents the sum of protons + neutrons.
Do you mean "Why is the number of protons in an atom called the atoms atomic number?" The defining characteristic of an atom is the number of protons in its nucleus. Hydrogen has one proton. All atoms of hydrogen have one proton. Helium has two protons. If you add a proton to a hydrogen nucleus you get a helium nucleus. Since the number of protons defines what element the atom is, it is the defining characteristic.
The three main sub-atomic particles of an atom are proton, neutron and electron. The mass of a proton is about 1800 times the mass of an electron and the masses of proton and neutron are almost similar. Hence, mass of only proton and neutron is counted towards the mass of an atom.
The whole is less than the sum of the parts. A proton or neutron (nucleon) will have less mass in a nucleus that outside it. That's because some of the mass of a nucleon is converted into binding energy to hold an atomic nucleus together. That's the so-called mass deficit. Oh, and before we go, a proton or neutron is called a nucleon only inside the nucleus of an atom. We don't apply that term to either one when they're outside the nucleus.
nucleons
The sum of the number of the protons and neutrons in the nucleus of an atom is called as atomic mass.
The sum of protons in the nucleus of an atom is called the atomic number. It determines the element's identity.
mass number = protons + nuetrons
mass number
The Mass Number.
The mass number is (by definition) the total sum of proton and neutron number in the nucleus of one particular isotope of an element.
The number of protons in the nucleus determines the element's identity, while the sum of protons and neutrons gives the atom's mass number. The nucleus contains nearly all of an atom's mass and is surrounded by a cloud of electrons.
The mass of an atom depends on the nucleus because the nucleus contains the majority of the atom's mass in the form of protons and neutrons. Electrons, which orbit the nucleus, have a much smaller mass compared to protons and neutrons. The mass of an atom is essentially the combined mass of its protons and neutrons in the nucleus.
The mass number of an atom is equal to the sum of its protons and neutrons. It is represented by the letter "A" in the chemical symbol notation, where A = number of protons + number of neutrons in the nucleus of the atom.
In this case, the number 209 represents the sum of protons + neutrons.
Do you mean "Why is the number of protons in an atom called the atoms atomic number?" The defining characteristic of an atom is the number of protons in its nucleus. Hydrogen has one proton. All atoms of hydrogen have one proton. Helium has two protons. If you add a proton to a hydrogen nucleus you get a helium nucleus. Since the number of protons defines what element the atom is, it is the defining characteristic.
The three main sub-atomic particles of an atom are proton, neutron and electron. The mass of a proton is about 1800 times the mass of an electron and the masses of proton and neutron are almost similar. Hence, mass of only proton and neutron is counted towards the mass of an atom.