specify atomic number. Define mass number. Identify the number of protons, neutrons, and electrons in one atom.

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It"s necessary to be able to distinguish atoms of one aspect from atoms of another element. Aspects are pure building material that comprise all other matter, so every one is given a distinct name. The names of aspects are likewise represented by distinct one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) for carbon, or (ceHe) because that helium. However, that would much more powerful if these names could be provided to recognize the numbers of protons and also neutrons in the atoms. That"s wherein atomic number and also mass numberare useful.

Figure (PageIndex1): that is difficult to find characteristics that differ in between each element, and also to differentiate one aspect from another. Every element, however, does have actually a unique number of protons. Sulfur has 16 protons, silicon has actually 14 protons, and also gold has actually 79 protons. Pictures used with permission (public domain because that sulfur and also silicon, gold is licensed by CC-BY-SA-NC-ND; Alsdrta.netist-hp).

Atomic Number

Scientists distinguish between different aspects by counting the variety of protons in the cell core (Table (PageIndex1)). If one atom has only one proton, we understand that it"s a hydrogen atom. One atom with two proton is always a helium atom. If scientists count 4 protons in one atom, they recognize it"s a beryllium atom. One atom with 3 protons is a lithium atom, an atom with five protons is a boron atom, one atom with six protons is a carbon atom . . . The list goes on.

Since one atom of one facet can be identified from one atom of another element by the number of protons in the nucleus, researchers are always interested in this number, and how this number differs in between different elements. The variety of protons in one atom is dubbed its atomic number ((Z)). This number is very important because it is distinct for atoms of a offered element. Every atoms of an element have the same variety of protons, and also every facet has a different number of protons in the atoms. For example, all helium atoms have actually two protons, and no other elements have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom of the first Six facets
Hydrogen 1 0 1 1 1
Helium 2 2 2 2 4
Lithium 3 4 3 3 7
Beryllium 4 5 4 4 9
Boron 5 6 5 5 11
Carbon 6 6 6 6 12

Of course, since neutral atoms need to have one electron because that every proton, an element"s atomic number likewise tells you how many electrons are in a neutral atom of the element. For example, hydrogen has actually an atomic number of 1. This way that one atom the hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, top top the other hand, has an atomic variety of 79, which method that one atom that gold has 79 protons, and, if it"s neutral, 79 electrons together well.

Mass Number

The mass number ((A)) of an atom is the total variety of protons and neutrons in its nucleus. The mass of the atom is a unit referred to as the atom mass unit (left( extamu ight)). One atomic mass unit is the fixed of a proton, or around (1.67 imes 10^-27) kilograms, i beg your pardon is an extremely tiny mass. A neutron has actually just a small bit much more mass 보다 a proton, yet its mass is frequently assumed to be one atomic mass unit as well. Due to the fact that electrons have actually virtually no mass, just about all the mass of one atom is in the protons and neutrons. Therefore, the total variety of protons and also neutrons in one atom identify its mass in atomic mass devices (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutrons in addition to two protons. As such the massive of most helium atoms is 4 atom mass units ((2 : extamu) for the protons + (2 : extamu) for the neutrons). However, part helium atom have much more or less than two neutrons. Atoms v the same variety of protons however different number of neutrons are referred to as isotopes. Because the variety of neutrons can vary for a given element, the mass numbers of various atoms of an facet may also vary. For example, some helium atoms have actually three neutrons instead of 2 (these are referred to as isotopes and are questioned in detail later on).

Why do you think the the "mass number" has protons and also neutrons, yet not electrons? You understand that most of the mass of one atom is concentrated in that is nucleus. The fixed of an atom depends on the variety of protons and also neutrons. Girlfriend have currently learned the the mass of an electron is very, very tiny compared to the fixed of either a proton or a ghost (like the mass of a penny compared to the mass of a bowling ball). Counting the variety of protons and also neutrons speak scientists around the complete mass of an atom.

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< extmass number : A = left( extnumber that protons ight) + left( extnumber of neutrons ight)>

An atom"s mass number is really easy to calculate, noted that you understand the variety of protons and also neutrons in an atom.

Example 4.5.1

What is the mass number of an atom of helium that has 2 neutrons?


(left( extnumber that protons ight) = 2) (Remember that an atom of helium constantly has 2 protons.)

(left( extnumber of neutrons ight) = 2)

( extmass number = left( extnumber that protons ight) + left( extnumber the neutrons ight))