Define atomic number. Define mass number. Determine the variety of protons, neutrons, and also electrons in an atom.

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It"s crucial to have the ability to distinguish atoms of one element from atoms of one more aspect. Elements are pure substances that make up all other matter, so each one is offered a distinct name. The names of facets are additionally represented by unique one- or two-letter icons, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) for helium. However before, it would certainly even more effective if these names might be used to determine the numbers of proloads and also neutrons in the atoms. That"s wbelow atomic number and also mass numberare useful.

Figure (PageIndex1): It is hard to find attributes that differ between each aspect, and to identify one aspect from an additional. Each aspect, yet, does have actually a distinct variety of proloads. Sulfur has 16 prolots, silicon has 14 proloads, and also gold has actually 79 proloads. Images supplied with permission (public domajor for sulhair and silicon, gold is licensed by CC-BY-SA-NC-ND; Aljiyuushikan.orgist-hp).

Atomic Number

Scientists distinguish between various aspects by counting the number of prolots in the nucleus (Table (PageIndex1)). If an atom has actually only one proton, we know that it"s a hydrogen atom. An atom through two protons is always a helium atom. If researchers count four protons in an atom, they know it"s a beryllium atom. An atom through three proloads is a lithium atom, an atom via 5 prolots is a boron atom, an atom with 6 protons is a carbon atom . . . the list goes on.

Since an atom of one element have the right to be distinguimelted from an atom of one more aspect by the variety of protons in its nucleus, researchers are always interested in this number, and also just how this number differs between various elements. The variety of proloads in an atom is referred to as its atomic number ((Z)). This number is exceptionally necessary bereason it is distinctive for atoms of a offered aspect. All atoms of an facet have actually the very same variety of proloads, and eexceptionally element has actually a different number of prolots in its atoms. For example, all helium atoms have actually two prolots, and no other elements have actually atoms via 2 prolots.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): Atoms of the First Six Elements
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, because neutral atoms have to have actually one electron for eexceptionally proton, an element"s atomic number additionally tells you exactly how many type of electrons are in a neutral atom of that element. For instance, hydrogen has actually an atomic number of 1. This indicates that an atom of hydrogen has actually one proton, and also, if it"s neutral, one electron as well. Gold, on the various other hand also, has an atomic number of 79, which indicates that an atom of gold has actually 79 prolots, and, if it"s neutral, 79 electrons also.

Mass Number

The mass number ((A)) of an atom is the total number of protons and also neutrons in its nucleus. The mass of the atom is a unit dubbed the atomic mass unit (left( extamu ight)). One atomic mass unit is the mass of a proton, or about (1.67 imes 10^-27) kilograms, which is a really tiny mass. A neutron has simply a tiny bit even more mass than a proton, but its mass is regularly assumed to be one atomic mass unit too. Since electrons have actually basically no mass, just around all the mass of an atom is in its protons and also neutrons. As such, the full variety of protons and also neutrons in an atom determines its mass in atomic mass units (Table (PageIndex1)).

Consider helium aobtain. Many helium atoms have actually two neutrons in addition to two proloads. As such the mass of most helium atoms is 4 atomic mass devices ((2 : extamu) for the proloads + (2 : extamu) for the neutrons). However before, some helium atoms have more or much less than 2 neutrons. Atoms via the very same variety of protons but different numbers of neutrons are referred to as isotopes. Due to the fact that the variety of neutrons can vary for a provided element, the mass numbers of various atoms of an element might additionally differ. For instance, some helium atoms have actually three neutrons rather of 2 (these are called isotopes and also are discussed in information later on).

Why perform you think that the "mass number" contains protons and neutrons, however not electrons? You know that the majority of of the mass of an atom is concentrated in its nucleus. The mass of an atom depends on the number of protons and neutrons. You have actually already learned that the mass of an electron is exceptionally, exceptionally small compared to the mass of either a proton or a neutron (favor the mass of a penny compared to the mass of a bowling ball). Counting the variety of prolots and neutrons tells scientists about the complete mass of an atom.

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

An atom"s mass number is incredibly easy to calculate, gave that you know the variety of proloads and neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber of neutrons ight) = 2)

( extmass number = left( extvariety of protons ight) + left( extvariety of neutrons ight))