How Many Protons In Boron

4.5: Elements- Defined by Their Number of Protons

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Learning Objectives

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

It's of import to exist able to distinguish atoms of one element from atoms of another element. Elements are pure substances that make upwards all other thing, and so each one is given a unique proper noun. The names of elements are also represented by unique one- or 2-letter symbols, such as \(\ce{H}\) for hydrogen, \(\ce{C}\) for carbon, or \(\ce{He}\) for helium. However, information technology would more powerful if these names could be used to identify the numbers of protons and neutrons in the atoms. That'southward where atomic number and mass number are useful.

Left: sulfur, center: silicon, right: gold — Figure \(\PageIndex{1}\): It is hard to find qualities that differ between each chemical element, and to distinguish one element from another. Each element, however, does have a unique number of protons. Sulfur has 16 protons, silicon has xiv protons, and gilded has 79 protons. Images used with permission (public domain for sulfur and silicon, gilt is licensed by CC-BY-SA-NC-ND; Alchemist-hp).

Atomic Number

Scientists distinguish betwixt different elements past counting the number of protons in the nucleus (Table \(\PageIndex{1}\)). If an cantlet has only one proton, we know that it's a hydrogen atom. An atom with 2 protons is e'er a helium cantlet. If scientists count 4 protons in an atom, they know information technology's a beryllium atom. An atom with three protons is a lithium atom, an atom with five protons is a boron cantlet, an atom with half dozen protons is a carbon atom . . . the list goes on.

Since an cantlet of 1 chemical element can be distinguished from an atom of some other element by the number of protons in its nucleus, scientists are always interested in this number, and how this number differs betwixt different elements. The number of protons in an cantlet is called its diminutive 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 different number of protons in its atoms. For instance, all helium atoms have two protons, and no other elements have atoms with two protons.

Tabular array \(\PageIndex{1}\): Atoms of the Beginning Six Elements
Name	Protons	Neutrons	Electrons	Diminutive Number (Z)	Mass Number (A)
Hydrogen	1	0	1	1	one
Helium	2	two	2	2	4
Lithium	three	4	iii	three	7
Beryllium	four	v	4	four	9
Boron	5	vi	5	five	11
Carbon	half dozen	6	6	6	12

Of form, since neutral atoms accept to accept one electron for every proton, an element's atomic number also tells you how many electrons are in a neutral cantlet of that element. For example, hydrogen has an atomic number of one. This means that an atom of hydrogen has one proton, and, if it's neutral, one electron also. Gilt, on the other paw, has an atomic number of 79, which ways that an atom of gold has 79 protons, and, if it's neutral, 79 electrons too.

Neutral Atoms

Atoms are neutral in electric charge because they have the aforementioned number of negative electrons as positive protons (Table \(\PageIndex{one}\)). Therefore, the atomic number of an atom also tells you lot how many electrons the atom has. This, in turn, determines many of the atom'south 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 \(\left( \text{amu} \right)\). 1 atomic mass unit is the mass of a proton, or about \(1.67 \times ten^{-27}\) kilograms, which is an extremely small mass. A neutron has only a tiny chip more mass than a proton, but its mass is often causeless to be 1 atomic mass unit of measurement every bit well. Considering electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units (Table \(\PageIndex{1}\)).

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

Why do yous remember that the "mass number" includes protons and neutrons, but not electrons? Y'all know that virtually of the mass of an atom is concentrated in its nucleus. The mass of an atom depends on the number of protons and neutrons. You lot take already learned that the mass of an electron is very, very small 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 about the total mass of an atom.

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

An cantlet's mass number is very easy to calculate, provided that y'all know the number of protons and neutrons in an atom.

Example four.five.1

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

Solution

\(\left( \text{number of protons} \right) = 2\) (Call up that an atom of helium e'er has 2 protons.)

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

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

\(\text{mass number} = two + two = four\)

A chemical symbol is a one- or 2-letter of the alphabet designation of an element. Some examples of chemical symbols are \(\ce{O}\) for oxygen, \(\ce{Zn}\) for zinc, and \(\ce{Fe}\) for atomic number 26. The first alphabetic character 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. All the same, some of the elements that have been known since aboriginal times have maintained symbols that are based on their Latin names, as shown in Tabular array \(\PageIndex{2}\).

Table \(\PageIndex{2}\)*: Symbols and Latin Names for Elements*
Chemic Symbol	Name	Latin Name
\(\ce{Na}\)	Sodium	Natrium
\(\ce{K}\)	Potassium	Kalium
\(\ce{Fe}\)	Iron	Ferrum
\(\ce{Cu}\)	Copper	Cuprum
\(\ce{Ag}\)	Silvery	Argentum
\(\ce{Sn}\)	Tin	Stannum
\(\ce{Sb}\)	Antimony	Stibium
\(\ce{Au}\)	Gold	Aurum
\(\ce{Pb}\)	Lead	Plumbum

Summary

Elements are pure substances that brand up all thing, so each one is given a unique name.
The names of elements are also represented by unique one- or two-letter symbols.
Each element has a unique number of protons. An element's atomic number is equal to the number of protons in the nuclei of whatever of its atoms.
The mass number of an atom is the sum of the protons and neutrons in the atom.
Isotopes are atoms of the aforementioned element (same number of protons) that have different numbers of neutrons in their atomic nuclei.