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

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It"s crucial to be able to distinguish atoms of one facet from atoms of another element. Facets are pure substances that make up all other matter, so every one is provided a unique name. The name of elements are likewise represented by distinctive one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) because that carbon, or (ceHe) because that helium. However, that would more powerful if this names can be offered to determine the number of protons and also neutrons in the atoms. That"s where atomic number and mass numberare useful.

Figure (PageIndex1): it is complicated to find characteristics that differ between each element, and to identify one element from another. Each element, however, does have a unique number of protons. Sulfur has actually 16 protons, silicon has 14 protons, and gold has actually 79 protons. Photos used v permission (public domain because that sulfur and silicon, yellow is license is granted by CC-BY-SA-NC-ND; Alcg-tower.comist-hp).

Atomic Number

Scientists distinguish between different elements by count the variety of protons in the cell core (Table (PageIndex1)). If an atom has only one proton, we understand that it"s a hydrogen atom. An atom through two proton is constantly a helium atom. If researchers count 4 protons in an atom, they know it"s a beryllium atom. One atom with three protons is a lithium atom, one atom with five protons is a boron atom, an atom with six protons is a carbon atom . . . The perform goes on.

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

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom of the an initial 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, due to the fact that neutral atoms need to have one electron for every proton, an element"s atomic number additionally tells girlfriend how plenty of electrons room in a neutral atom of that element. For example, hydrogen has an atomic variety of 1. This method that an atom the hydrogen has actually one proton, and, if it"s neutral, one electron as well. Gold, top top the various other hand, has an atomic number of 79, which means 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 the 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 m sorry is an extremely small mass. A neutron has just a small bit more mass 보다 a proton, yet its massive is often assumed to be one atom mass unit together well. Since electrons have actually virtually no mass, just about all the fixed of an atom is in the protons and also neutrons. Therefore, the total variety of protons and neutrons in one atom identify its fixed in atomic mass systems (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutron in enhancement to 2 protons. Thus the massive of many helium atoms is 4 atomic mass devices ((2 : extamu) because that the proton + (2 : extamu) because that the neutrons). However, some helium atoms have an ext or much less than two neutrons. Atoms with the same variety of protons yet different number of neutrons are referred to as isotopes. Due to the fact that the variety of neutrons have the right to vary because that a offered element, the mass number of various atoms of an element may also vary. For example, part helium atoms have three neutrons rather of 2 (these are dubbed isotopes and are discussed in detail later on).

Why execute you think the the "mass number" includes protons and neutrons, however not electrons? You understand that most of the mass of an atom is concentrated in the nucleus. The mass of an atom relies on the variety of protons and neutrons. You have already learned that the fixed of one electron is very, very tiny compared to the mass of one of two people a proton or a ghost (like the massive of a penny contrasted to the massive of a bowling ball). Counting the variety of protons and neutrons tells scientists around the full mass of one atom.

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

An atom"s fixed number is really easy to calculate, noted 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 that helium that contains 2 neutrons?


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

(left( extnumber of neutrons ight) = 2)

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