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

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

Figure (PageIndex1): it is challenging to find features that differ in between each element, and to differentiate one facet from another. Each element, however, does have a unique number of protons. Sulfur has actually 16 protons, silicon has 14 protons, and also gold has actually 79 protons. Images used through permission (public domain because that sulfur and silicon, gold is license is granted by CC-BY-SA-NC-ND; Albeer-selection.comist-hp).

Atomic Number

Scientists distinguish between different elements by count the number of protons in the cell core (Table (PageIndex1)). If an atom has only one proton, we know that it"s a hydrogen atom. One atom with two proton is always a helium atom. If scientists count 4 protons in one atom, they understand it"s a beryllium atom. One atom with three 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 an atom of one facet can be identified from an atom of an additional element by the variety 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 an atom is referred to as its atomic number ((Z)). This number is really important because it is distinct for atom of a offered element. Every atoms that an facet have the same variety of protons, and also every facet has a different number of protons in that atoms. For example, every helium atoms have two protons, and also no other facets have atoms through two protons.

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 one electron because that every proton, an element"s atom number additionally tells girlfriend how many electrons space in a neutral atom of the element. For example, hydrogen has an atomic number of 1. This way that an atom that hydrogen has actually one proton, and, if it"s neutral, one electron as well. Gold, on the various other hand, has an atomic variety of 79, which method that an atom the 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 number of protons and neutrons in that is nucleus. The massive of the atom is a unit dubbed the atom mass unit (left( extamu ight)). One atomic mass unit is the massive of a proton, or around (1.67 imes 10^-27) kilograms, i beg your pardon is an extremely little mass. A neutron has just a small bit much more mass 보다 a proton, but its mass is frequently assumed to be one atomic mass unit as well. Since electrons have virtually no mass, just around all the mass of an atom is in its protons and also neutrons. Therefore, the total number of protons and neutrons in an atom determines its massive in atom mass units (Table (PageIndex1)).

Consider helium again. Most helium atoms have actually two neutrons in addition to two protons. As such the fixed of many helium atom is 4 atomic mass systems ((2 : extamu) for the proton + (2 : extamu) for the neutrons). However, part helium atoms have an ext or less than 2 neutrons. Atoms through the same variety of protons but different numbers of neutron are dubbed isotopes. Since the variety of neutrons can vary because that a provided element, the mass numbers of various atoms the an aspect may additionally vary. Because that example, part helium atoms have actually three neutrons rather of two (these are called isotopes and are debated in information later on).

Why carry out you think that the "mass number" includes protons and also neutrons, but not electrons? You recognize that many of the mass of an atom is focused in that nucleus. The mass of one atom depends on the variety of protons and neutrons. Girlfriend have currently learned that the massive of one electron is very, very tiny compared come the mass of one of two people a proton or a spirit (like the mass of a penny contrasted to the fixed of a bowling ball). Count the number of protons and neutrons tells scientists around the total mass of one atom.

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

An atom"s massive number is an extremely easy to calculate, detailed that you understand the number of protons and also neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber of neutrons ight) = 2)

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