define atomic number. Specify mass number. Determine the variety of protons, neutrons, and electrons in one atom.

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It"s essential to be able to distinguish atoms of one facet from atom of one more element. Facets are pure building material that make up all other matter, so each one is given a distinct name. The name of elements are likewise represented by distinct one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, the would much more powerful if these names can be supplied to identify the number of protons and neutrons in the atoms. That"s wherein atomic number and mass numberare useful.

Figure (PageIndex1): the is challenging to find characteristics that differ in between each element, and also to differentiate one element from another. Every element, however, does have a unique number of protons. Sulfur has 16 protons, silicon has actually 14 protons, and also gold has actually 79 protons. Images used v permission (public domain for sulfur and also silicon, gold is license is granted by CC-BY-SA-NC-ND; Alstarrkingschool.netist-hp).

Atomic Number

Scientists distinguish between different aspects by counting the variety of protons in the nucleus (Table (PageIndex1)). If an atom has actually only one proton, we understand that it"s a hydrogen atom. An atom v two proton is always a helium atom. If researchers count four protons in one atom, they recognize it"s a beryllium atom. One atom with 3 protons is a lithium atom, one atom with five protons is a boron atom, an atom with 6 protons is a carbon atom . . . The list goes on.

Since one atom the one aspect can be identified from one atom of an additional element by the variety of protons in the nucleus, scientists are always interested in this number, and also how this number differs between different elements. The variety of protons in one atom is referred to as its atomic number ((Z)). This number is very important because it is distinct for atoms of a offered element. All atoms that an aspect have the same number of protons, and also every aspect has a different variety of protons in that atoms. For example, all helium atoms have actually two protons, and also no other elements have atoms v two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atoms of the an initial Six aspects
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, due to the fact that neutral atoms need to have one electron because that every proton, an element"s atom number additionally tells you how many electrons room in a neutral atom of that element. For example, hydrogen has actually an atomic variety of 1. This method that one atom of hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, on the other hand, has an atomic variety of 79, which way that an atom that gold has 79 protons, and, if it"s neutral, 79 electrons as well.

Mass Number

The mass number ((A)) of an atom is the total number of protons and also neutrons in its nucleus. The massive of the atom is a unit called the atom mass unit (left( extamu ight)). One atomic mass unit is the mass of a proton, or around (1.67 imes 10^-27) kilograms, i beg your pardon is an extremely little mass. A neutron has actually just a small bit more mass than a proton, but its mass is frequently assumed to be one atomic mass unit as well. Since electrons have actually virtually no mass, just around all the massive of an atom is in that protons and also neutrons. Therefore, the total variety of protons and also neutrons in an atom determines its mass in atom mass systems (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutron in addition to two protons. Thus the mass of most helium atoms is 4 atomic mass devices ((2 : extamu) because that the proton + (2 : extamu) for the neutrons). However, some helium atoms have much more or much less than two neutrons. Atoms with the same variety of protons yet different numbers of neutrons are dubbed isotopes. Since the variety of neutrons deserve to vary because that a offered element, the mass numbers of various atoms of an aspect may also vary. Because that example, some helium atoms have actually three neutrons instead of two (these are dubbed isotopes and also are debated in detail later on).

Why carry out you think the the "mass number" has protons and also neutrons, however not electrons? You understand that most of the massive of one atom is concentrated in that is nucleus. The mass of an atom counts on the number of protons and neutrons. Friend have already learned the the mass of one electron is very, very little compared come the massive of either a proton or a spirit (like the massive of a penny contrasted to the mass of a bowling ball). Count the number of protons and also neutrons tells scientists about 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 come calculate, provided that you know the variety of protons and also neutrons in one atom.

Example 4.5.1

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


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

(left( extnumber of neutrons ight) = 2)

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