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Learning Objectives

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

You are watching: Why is each element unique and different from other elements in chemical properties?

It"s necessary to have the ability to distinguish atoms of one aspect from atoms of one more facet. Elements are pure substances that make up all other matter, so each one is offered a unique name. The names of elements are additionally represented by distinctive one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) for helium. However before, it would certainly even more effective if these names might be used to identify the numbers of protons and neutrons in the atoms. That"s where atomic number and mass numberare valuable.

Figure (PageIndex1): It is hard to find features that differ between each element, and to identify one facet from one more. Each element, but, does have actually a unique number of prolots. Sulhair has 16 protons, silicon has 14 protons, and also gold has actually 79 prolots. Imperiods offered through permission (public doprimary for sulfur and silicon, gold is licensed by CC-BY-SA-NC-ND; Aljiyuushikan.orgist-hp).

Atomic Number

Scientists differentiate in between various aspects by counting the variety of prolots in the nucleus (Table (PageIndex1)). If an atom has actually just one proton, we know that it"s a hydrogen atom. An atom with 2 protons is always a helium atom. If scientists count 4 prolots in an atom, they understand it"s a beryllium atom. An atom via 3 protons is a lithium atom, an atom via 5 proloads is a boron atom, an atom with 6 protons is a carbon atom . . . the list goes on.

Since an atom of one facet deserve to be distinguished from an atom of another element by the variety of protons in its nucleus, scientists are always interested in this number, and also exactly how this number differs between various elements. The variety of prolots in an atom is referred to as its atomic number ((Z)). This number is extremely vital bereason it is unique for atoms of a given facet. All atoms of an aspect have actually the very same variety of proloads, and also eextremely aspect has actually a different number of proloads in its atoms. For instance, all helium atoms have actually two prolots, and also no various other aspects have atoms via two prolots.

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 how many type of electrons are in a neutral atom of that element. For instance, hydrogen has an atomic variety of 1. This indicates that an atom of hydrogen has actually one proton, and, if it"s neutral, one electron also. Gold, on the various other hand also, has actually an atomic variety of 79, which suggests that an atom of gold has 79 protons, and also, if it"s neutral, 79 electrons too.

Mass Number

The mass number ((A)) of an atom is the total variety of proloads and also neutrons in its 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 mass of a proton, or about (1.67 imes 10^-27) kilograms, which is an extremely tiny mass. A neutron has simply a tiny little even more mass than a proton, however its mass is often assumed to be one atomic mass unit too. Because electrons have actually practically no mass, just around all the mass of an atom is in its proloads and neutrons. As such, the total variety of proloads and neutrons in an atom determines its mass in atomic mass devices (Table (PageIndex1)).

Consider helium aget. Most helium atoms have actually 2 neutrons in addition to 2 protons. As such the mass of many helium atoms is 4 atomic mass devices ((2 : extamu) for the prolots + (2 : extamu) for the neutrons). However before, some helium atoms have even more or less than two neutrons. Atoms via the exact same number of protons however different numbers of neutrons are referred to as isotopes. Since the number of neutrons deserve to vary for a given aspect, the mass numbers of various atoms of an aspect may additionally differ. For instance, some helium atoms have actually 3 neutrons rather of 2 (these are dubbed isotopes and also are discussed in information later on).

Why do you think that the "mass number" includes protons and also neutrons, but not electrons? You recognize that a lot of of the mass of an atom is focused in its nucleus. The mass of an atom counts on the number of prolots and also neutrons. You have currently learned that the mass of an electron is extremely, very 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 protons and also neutrons tells researchers about the complete mass of an atom.

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

An atom"s mass number is exceptionally straightforward to calculate, provided that you know the number of proloads and neutrons in an atom.

Example 4.5.1

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


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

(left( extnumber of neutrons ight) = 2)

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