# Which of the Following is Unique for Any Given Element

Which of the Following is Unique for Any Given Element.

## 4.5: Elements- Defined past Their Numbers of Protons

##### Learning Objectives
• Define atomic number.
• Define mass number.
• Determine the number of protons, neutrons, and electrons in an atom.

Information technology’s important to exist able to distinguish atoms of one element from atoms of another element. Elements are pure substances that make upwardly all other affair, so each one is given a unique name. The names of elements are also represented by unique 1- or two-letter symbols, such as $$\ce{H}$$ for hydrogen, $$\ce{C}$$ for carbon, or $$\ce{He}$$ for helium. Yet, it would more powerful if these names could be used to place the numbers of protons and neutrons in the atoms. That’s where diminutive number and mass number are useful.

## Atomic Number

Scientists distinguish between different elements by counting the number of protons in the nucleus (Table $$\PageIndex{1}$$). If an atom has but ane proton, we know that it’s a hydrogen cantlet. An atom with two protons is always a helium atom. If scientists count four protons in an atom, they know it’s a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron atom, an atom with six protons is a carbon atom . . . the list goes on.

Since an atom of ane element can be distinguished from an cantlet of some other element by the number of protons in its nucleus, scientists are always interested in this number, and how this number differs between different elements. The number of protons in an atom is called its
atomic number
($$Z$$). This number is very important because information technology is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a dissimilar number of protons in its atoms. For instance, all helium atoms have two protons, and no other elements have atoms with two protons.

Name Protons Neutrons Electrons Atomic Number (Z) Mass Number
(A)
Table $$\PageIndex{1}$$: Atoms of the First Vi Elements
Hydrogen i 0 i 1 1
Helium ii 2 2 2 four
Lithium 3 4 3 3 7
Glucinium 4 5 four 4 9
Boron 5 6 five v 11
Carbon 6 6 vi 6 12

Of course, since neutral atoms have to have ane electron for every proton, an element’s atomic number also tells you how many electrons are in a neutral atom of that element. For example, hydrogen has an atomic number of 1. This means that an atom of hydrogen has 1 proton, and, if it’south neutral, 1 electron as well. Aureate, on the other hand, has an atomic number of 79, which means that an atom of gilt has 79 protons, and, if it’southward neutral, 79 electrons as well.

##### Neutral Atoms

Atoms are neutral in electric charge considering they have the same number of negative electrons as positive protons (Table $$\PageIndex{1}$$). Therefore, the atomic number of an cantlet besides tells you how many electrons the atom has. This, in turn, determines many of the cantlet’due south chemical properties.

## Mass Number

The
mass number
($$A$$) of an cantlet is the total number of protons and neutrons in its nucleus. The mass of the atom is a unit of measurement called the atomic mass unit of measurement $$\left( \text{amu} \right)$$. 1 atomic mass unit is the mass of a proton, or virtually $$1.67 \times ten^{-27}$$ kilograms, which is an extremely minor mass. A neutron has simply a tiny bit more mass than a proton, simply its mass is often assumed to exist one diminutive mass unit of measurement also. Considering electrons take most no mass, simply about all the mass of an atom is in its protons and neutrons. Therefore, the full number of protons and neutrons in an atom determines its mass in diminutive mass units (Table $$\PageIndex{1}$$).

Consider helium again. Most helium atoms take two neutrons in add-on to two protons. Therefore the mass of most helium atoms is 4 diminutive mass units ($$2 \: \text{amu}$$ for the protons + $$two \: \text{amu}$$ for the neutrons). However, some helium atoms have more or less than 2 neutrons. Atoms with the aforementioned number of protons but different numbers of neutrons are chosen isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms accept three neutrons instead of two (these are called isotopes and are discussed in particular later on).

Why do you recollect that the “mass number” includes protons and neutrons, but not electrons? Yous know that most of the mass of an atom is concentrated in its nucleus. The mass of an atom depends on the number of protons and neutrons. Yous have already learned that the mass of an electron is very, very pocket-sized compared to the mass of either a proton or a neutron (like the mass of a penny compared to the mass of a bowling ball). Counting the number of protons and neutrons tells scientists most the total mass of an atom.

$\text{mass number} \: A = \left( \text{number of protons} \right) + \left( \text{number of neutrons} \correct) \nonumber$

An atom’south mass number is very like shooting fish in a barrel to calculate, provided that you know the number of protons and neutrons in an atom.

##### Example 4.5.1

What is the mass number of an atom of helium that contains ii neutrons?

###### Solution

$$\left( \text{number of protons} \correct) = 2$$ (Recall that an atom of helium e’er has two protons.)

$$\left( \text{number of neutrons} \right) = ii$$

$$\text{mass number} = \left( \text{number of protons} \right) + \left( \text{number of neutrons} \right)$$

$$\text{mass number} = 2 + 2 = 4$$

A
chemical symbol
is a one- or ii-letter designation of an chemical element. Some examples of chemical symbols are $$\ce{O}$$ for oxygen, $$\ce{Zn}$$ for zinc, and $$\ce{Atomic number 26}$$ for atomic number 26. The starting time letter of a symbol is always capitalized. If the symbol contains two messages, the second letter is lower case. The majority of elements accept symbols that are based on their English names. However, some of the elements that have been known since ancient times have maintained symbols that are based on their Latin names, as shown in Table $$\PageIndex{ii}$$.

Chemical Symbol Proper noun Latin Proper name
Table $$\PageIndex{ii}$$: Symbols and Latin Names for Elements
$$\ce{Na}$$ Sodium Natrium
$$\ce{K}$$ Potassium Kalium
$$\ce{Fe}$$ Fe Ferrum
$$\ce{Cu}$$ Copper Cuprum
$$\ce{Ag}$$ Silver Argentum
$$\ce{Sn}$$ Tin Stannum
$$\ce{Sb}$$ Antimony Stibium
$$\ce{Au}$$ Gold Aurum
$$\ce{Pb}$$ Lead Plumbum

## Summary

• Elements are pure substances that make up all matter, and so each one is given a unique name.
• The names of elements are besides represented by unique one- or two-alphabetic character symbols.
• Each element has a unique number of protons. An element’south diminutive number is equal to the number of protons in the nuclei of whatever of its atoms.
• The mass number of an cantlet is the sum of the protons and neutrons in the cantlet.
• Isotopes are atoms of the aforementioned element (aforementioned number of protons) that have different numbers of neutrons in their diminutive nuclei.

## Which of the Following is Unique for Any Given Element

Source: https://chem.libretexts.org/Courses/College_of_Marin/CHEM_114%3A_Introductory_Chemistry/04%3A_Atoms_and_Elements/4.05%3A_Elements-_Defined_by_Their_Numbers_of_Protons#:~:text=The%20number%20of%20protons%20in,atoms%20of%20a%20given%20element.