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

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It"s essential to be able to distinguish atom of one element from atom of an additional element. Facets are pure substances that consist of all various other matter, so each one is offered a unique name. The surname of elements are also represented by distinct one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, it would much more powerful if this names could be used to recognize the number of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): the is challenging to find attributes that differ in between each element, and also to identify one aspect from another. Each element, however, does have a unique variety of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Images used through permission (public domain for sulfur and silicon, gold is licensed by CC-BY-SA-NC-ND; Algimpppa.orgist-hp).

## Atomic Number

Scientists distinguish in between different aspects by counting the number of protons in the cell nucleus (Table (PageIndex1)). If an atom has actually only one proton, we understand that it"s a hydrogen atom. One atom with two proton is always a helium atom. If researchers count 4 protons in an atom, they recognize it"s a beryllium atom. An atom with 3 protons is a lithium atom, one atom with 5 protons is a boron atom, an atom with 6 protons is a carbon atom . . . The perform goes on.

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

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom 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, since neutral atoms have to have one electron because that every proton, one element"s atom number additionally tells friend how countless electrons room in a neutral atom of that element. For example, hydrogen has an atomic variety of 1. This way that one atom of hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, ~ above the other hand, has actually an atomic variety of 79, which means that an atom of 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 that is nucleus. The mass of the atom is a unit referred to as the atomic mass unit (left( extamu ight)). One atom mass unit is the mass of a proton, or about (1.67 imes 10^-27) kilograms, i m sorry is an extremely small mass. A neutron has actually just a tiny bit more mass 보다 a proton, but its mass is often assumed to it is in one atomic mass unit together well. Since electrons have virtually no mass, just around all the fixed of an atom is in the protons and also neutrons. Therefore, the total number of protons and also neutrons in an atom determines its massive in atom mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutrons in addition to two protons. Because of this the massive of many helium atom is 4 atom mass systems ((2 : extamu) because that the proton + (2 : extamu) for the neutrons). However, some helium atom have more or much less than two neutrons. Atoms v the same variety of protons however different number of neutrons are referred to as isotopes. Due to the fact that the number of neutrons can vary for a offered element, the mass numbers of various atoms of an aspect may likewise vary. For example, part helium atoms have three neutrons rather of two (these are dubbed isotopes and are questioned in detail later on).

Why execute you think the the "mass number" has protons and also neutrons, however not electrons? You recognize that most of the massive of an atom is concentrated in its nucleus. The mass of an atom depends on the variety of protons and also neutrons. You have currently learned the the fixed of one electron is very, very small compared come the mass of either a proton or a ghost (like the massive of a penny contrasted to the fixed of a bowling ball). Counting the variety of protons and neutrons tells scientists about the full mass of an atom.

See more: What Happens When Chlorine Reacts With Bromine ? Halogen Displacement Reactions

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

An atom"s fixed number is very easy come calculate, provided that you know the number of protons and also neutrons in one atom.

Example 4.5.1

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

Solution

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

(left( extnumber of neutrons ight) = 2)

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