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

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It"s necessary to be able to distinguish atoms of one aspect from atoms of one more element. Facets are pure substances that comprise all various other matter, so each one is given a distinctive name. The name of aspects are additionally represented by distinctive one- or two-letter symbols, such together (ceH) because that hydrogen, (ceC) because that carbon, or (ceHe) for helium. However, it would an ext powerful if these names can be used to identify the number of protons and also neutrons in the atoms. That"s whereby atomic number and also mass numberare useful.

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Figure (PageIndex1): it is challenging to find attributes that differ between each element, and to differentiate one facet from another. Each element, however, does have a unique variety of protons. Sulfur has 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Pictures used v permission (public domain for sulfur and silicon, yellow is license is granted by CC-BY-SA-NC-ND; Altasiilaq.netist-hp).

Atomic Number

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

Since one atom that one facet can be identified from an atom of one more element by the variety of protons in its nucleus, researchers are constantly 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 really important due to the fact that it is unique for atoms of a offered element. Every atoms the an aspect have the same variety of protons, and every aspect has a different variety of protons in that atoms. For example, all helium atoms have actually two protons, and no other elements have atoms through two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atoms 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 for every proton, an element"s atomic number additionally tells friend how many electrons space in a neutral atom of the element. For example, hydrogen has an atomic variety of 1. This way that an atom of hydrogen has actually one proton, and, if it"s neutral, one electron as well. Gold, on 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 as well.



Mass Number

The mass number ((A)) of one atom is the total number 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 atomic mass unit is the fixed of a proton, or around (1.67 imes 10^-27) kilograms, i beg your pardon is one extremely tiny mass. A neutron has just a small bit an ext mass than a proton, but its massive is often assumed to it is in one atom mass unit together well. Due to the fact that electrons have actually virtually no mass, just about all the massive of one atom is in its protons and neutrons. Therefore, the total variety of protons and neutrons in an atom identify its fixed in atomic mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have actually two neutron in enhancement to 2 protons. Because of this the massive of many helium atoms is 4 atom mass units ((2 : extamu) because that the proton + (2 : extamu) because that the neutrons). However, some helium atom have more or less than two neutrons. Atoms through the same number of protons but different numbers of neutron are referred to as isotopes. Because the number of neutrons deserve to vary for a given element, the mass numbers of various atoms of an aspect may likewise vary. For example, some helium atoms have three neutrons rather of two (these are dubbed isotopes and are questioned in detail later on).

Why execute you think that the "mass number" contains protons and neutrons, but not electrons? You understand that most of the fixed of an atom is concentrated in its nucleus. The massive of one atom relies on the variety of protons and also neutrons. Girlfriend have already learned that the mass of one electron is very, very little compared to the mass of one of two people a proton or a neutron (like the massive of a penny compared to the massive of a bowling ball). Count the variety of protons and neutrons tells scientists around the complete mass of an atom.

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

An atom"s mass number is an extremely easy come calculate, listed that you know the number of protons and also neutrons in an atom.


Example 4.5.1

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

Solution

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

(left( extnumber of neutrons ight) = 2)

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