Boron Element Protons Neutrons Electrons

4.5: Elements- Defined by Their Number of Protons

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Learning Objectives

Define atomic number.
Ascertain mass number.
Determine the number of protons, neutrons, and electrons in an atom.

It's important to be able to distinguish atoms of one chemical element from atoms of another element. Elements are pure substances that brand up all other matter, so each one is given a unique name. The names of elements are also represented by unique i- or ii-alphabetic character symbols, such equally \(\ce{H}\) for hydrogen, \(\ce{C}\) for carbon, or \(\ce{He}\) for helium. However, information technology would more powerful if these names could be used to place the numbers of protons and neutrons in the atoms. That's where atomic number and mass number are useful.

Left: sulfur, center: silicon, right: gold — Figure \(\PageIndex{i}\): Information technology is difficult to observe qualities that differ between each element, and to distinguish ane element from some other. Each chemical element, all the same, does have a unique number of protons. Sulfur has 16 protons, silicon has xiv protons, and gold has 79 protons. Images used with permission (public domain for sulfur and silicon, golden is licensed by CC-By-SA-NC-ND; Alchemist-hp).

Diminutive Number

Scientists distinguish between different elements by counting the number of protons in the nucleus (Table \(\PageIndex{1}\)). If an atom has only one proton, we know that it'southward a hydrogen cantlet. An atom with 2 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 cantlet, an cantlet with v protons is a boron cantlet, an atom with six protons is a carbon atom . . . the list goes on.

Since an cantlet of one element can exist distinguished from an atom of another 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 cantlet is called its diminutive number (\(Z\)). This number is very important considering information technology is unique for atoms of a given element. All atoms of an chemical element have the same number of protons, and every element has a unlike number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons.

Table \(\PageIndex{i}\): Atoms of the Starting time Six Elements
Name	Protons	Neutrons	Electrons	Diminutive Number (Z)	Mass Number (A)
Hydrogen	1	0	1	ane	1
Helium	2	ii	2	ii	four
Lithium	3	4	iii	three	7
Glucinium	4	5	iv	4	9
Boron	5	6	5	5	11
Carbon	half-dozen	half-dozen	6	6	12

Of course, since neutral atoms have to have i electron for every proton, an chemical element's diminutive number also tells you how many electrons are in a neutral cantlet of that element. For example, hydrogen has an atomic number of i. This means that an cantlet of hydrogen has ane proton, and, if it'southward neutral, one electron likewise. Gilded, on the other hand, has an diminutive number of 79, which means that an atom of gold has 79 protons, and, if it's neutral, 79 electrons as well.

Neutral Atoms

Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons (Table \(\PageIndex{1}\)). Therefore, the diminutive number of an atom besides tells you how many electrons the atom has. This, in turn, determines many of the atom's chemical properties.

Mass Number

The mass number (\(A\)) of an atom is the total number of protons and neutrons in its nucleus. The mass of the atom is a unit called the atomic mass unit of measurement \(\left( \text{amu} \right)\). I diminutive mass unit of measurement is the mass of a proton, or about \(1.67 \times 10^{-27}\) kilograms, which is an extremely small mass. A neutron has just a tiny bit more mass than a proton, merely its mass is oftentimes causeless to be one atomic mass unit of measurement as well. Because electrons accept virtually no mass, just almost all the mass of an cantlet is in its protons and neutrons. Therefore, the full number of protons and neutrons in an atom determines its mass in atomic mass units (Table \(\PageIndex{1}\)).

Consider helium once more. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of almost helium atoms is 4 atomic mass units (\(2 \: \text{amu}\) for the protons + \(2 \: \text{amu}\) for the neutrons). Notwithstanding, some helium atoms take more or less than two neutrons. Atoms with the same number of protons just different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of dissimilar atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two (these are called isotopes and are discussed in detail later on).

Why do you think that the "mass number" includes protons and neutrons, just non electrons? You know that nearly 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 modest compared to the mass of either a proton or a neutron (like the mass of a penny compared to the mass of a bowling brawl). Counting the number of protons and neutrons tells scientists about the full mass of an cantlet.

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

An atom's mass number is very piece of cake 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 2 neutrons?

Solution

\(\left( \text{number of protons} \right) = 2\) (Remember that an cantlet of helium always has 2 protons.)

\(\left( \text{number of neutrons} \right) = 2\)

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

\(\text{mass number} = 2 + two = 4\)

A chemical symbol is a one- or 2-letter designation of an element. Some examples of chemic symbols are \(\ce{O}\) for oxygen, \(\ce{Zn}\) for zinc, and \(\ce{Fe}\) for fe. The first letter of a symbol is always capitalized. If the symbol contains two messages, the second letter is lower case. The majority of elements have symbols that are based on their English language names. However, some of the elements that have been known since ancient times have maintained symbols that are based on their Latin names, every bit shown in Tabular array \(\PageIndex{2}\).

Table \(\PageIndex{2}\)*: Symbols and Latin Names for Elements*
Chemical Symbol	Proper noun	Latin Name
\(\ce{Na}\)	Sodium	Natrium
\(\ce{K}\)	Potassium	Kalium
\(\ce{Atomic number 26}\)	Fe	Ferrum
\(\ce{Cu}\)	Copper	Cuprum
\(\ce{Ag}\)	Silver	Argentum
\(\ce{Sn}\)	Tin can	Stannum
\(\ce{Sb}\)	Antimony	Stibium
\(\ce{Au}\)	Gold	Aurum
\(\ce{Lead}\)	Lead	Plumbum

Summary

Elements are pure substances that make upward all thing, so each 1 is given a unique name.
The names of elements are besides represented past unique one- or ii-letter symbols.
Each element has a unique number of protons. An chemical element's atomic number is equal to the number of protons in the nuclei of whatsoever of its atoms.
The mass number of an cantlet is the sum of the protons and neutrons in the atom.
Isotopes are atoms of the same element (aforementioned number of protons) that have different numbers of neutrons in their atomic nuclei.