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Learning Objectives

Define atomic number. Define mass number. Determine the variety of prolots, neutrons, and also electrons in an atom.

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It"s important to have the ability to identify atoms of one element from atoms of an additional facet. Elements are pure substances that make up all various other matter, so each one is provided a distinct name. The names of elements are additionally stood for by unique one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) for helium. However, it would more powerful if these names can be supplied to recognize the numbers of proloads and neutrons in the atoms. That"s wright here atomic number and also mass numberare helpful.

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Figure (PageIndex1): It is hard to find features that differ in between each aspect, and also to differentiate one facet from one more. Each element, but, does have a distinct number of prolots. Sulfur has actually 16 proloads, silicon has 14 proloads, and gold has actually 79 protons. Imeras provided through permission (public domain for sulhair and also silsymbol, gold is licensed by CC-BY-SA-NC-ND; Allutz-heilmann.infoist-hp).

Atomic Number

Scientists distinguish between different facets by counting the number of proloads in the nucleus (Table (PageIndex1)). If an atom has actually just one proton, we understand that it"s a hydrogen atom. An atom via two prolots is constantly a helium atom. If researchers count 4 prolots in an atom, they recognize it"s a beryllium atom. An atom with 3 proloads is a lithium atom, an atom through 5 protons is a boron atom, an atom through 6 prolots is a carbon atom . . . the list goes on.

Because an atom of one facet can be distinguimelted from an atom of an additional aspect by the variety of proloads in its nucleus, scientists are always interested in this number, and exactly how this number differs in between various aspects. The number of prolots in an atom is referred to as its atomic number ((Z)). This number is very essential because it is distinctive for atoms of a given aspect. All atoms of an facet have the same number of prolots, and eexceptionally aspect has a different number of protons in its atoms. For instance, all helium atoms have actually 2 prolots, and also no other elements have actually atoms with two proloads.

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, considering that neutral atoms need to have one electron for eincredibly proton, an element"s atomic number also tells you just how many kind of electrons are in a neutral atom of that aspect. For example, hydrogen has an atomic number of 1. This implies that an atom of hydrogen has one proton, and also, if it"s neutral, one electron as well. Gold, on the various other hand also, has an atomic number of 79, which means that an atom of gold has actually 79 protons, and, if it"s neutral, 79 electrons also.



Mass Number

The mass number ((A)) of an atom is the complete variety of proloads and also neutrons in its nucleus. The mass of the atom is a unit called the atomic mass unit (left( extamu ight)). One atomic mass unit is the mass of a proton, or around (1.67 imes 10^-27) kilograms, which is an extremely little mass. A neutron has just a tiny little bit even more mass than a proton, however its mass is frequently assumed to be one atomic mass unit too. Since electrons have virtually no mass, simply about all the mass of an atom is in its prolots and also neutrons. As such, the total variety of proloads and neutrons in an atom determines its mass in atomic mass units (Table (PageIndex1)).

Consider helium aacquire. Most helium atoms have actually 2 neutrons in enhancement to 2 proloads. Because of this the mass of a lot of helium atoms is 4 atomic mass units ((2 : extamu) for the protons + (2 : extamu) for the neutrons). However, some helium atoms have even more or much less than two neutrons. Atoms via the exact same variety of proloads but different numbers of neutrons are called isotopes. Because the variety of neutrons can differ for a offered aspect, the mass numbers of different atoms of an aspect might additionally vary. For instance, some helium atoms have 3 neutrons instead of two (these are called isotopes and are questioned in information later on).

Why carry out you think that the "mass number" contains protons and neutrons, yet not electrons? You know that many of the mass of an atom is concentrated in its nucleus. The mass of an atom depends on the variety of proloads and neutrons. You have actually currently learned that the mass of an electron is incredibly, incredibly little compared to the mass of either a proton or a neutron (prefer the mass of a penny compared to the mass of a bowling ball). Counting the number of proloads and neutrons tells scientists about the total mass of an atom.

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

An atom"s mass number is extremely easy to calculate, gave that you recognize the number of prolots 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 of protons ight) = 2) (Remember that an atom of helium always has 2 prolots.)

(left( extnumber of neutrons ight) = 2)

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