Define atom number. Specify mass number. Identify the number of protons, neutrons, and electrons in an atom.

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It"s important to be able to distinguish atoms of one element from atoms of one more element. Facets are pure building material that make up all various other matter, so each one is provided a distinct name. The surname of aspects are additionally represented by unique one- or two-letter symbols, such together \(\ceH\) for hydrogen, \(\ceC\) because that carbon, or \(\ceHe\) because that helium. However, it would more powerful if this names can be supplied to recognize the number of protons and neutrons in the atoms. That"s whereby atomic number and also mass numberare useful.

Figure \(\PageIndex1\): it is daunting to find attributes that differ between each element, and to identify one element from another. Every element, however, does have actually a unique number of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and also gold has 79 protons. Images used v permission (public domain because that sulfur and silicon, yellow is licensed by CC-BY-SA-NC-ND; Aly2kcenter.orgist-hp).

Atomic Number

Scientists distinguish between different aspects by count the variety of protons in the cell core (Table \(\PageIndex1\)). If an atom has only one proton, we know that it"s a hydrogen atom. An atom through two proton is constantly a helium atom. If scientists count 4 protons in one atom, they understand it"s a beryllium atom. One atom with three protons is a lithium atom, an atom with 5 protons is a boron atom, one atom with six protons is a carbon atom . . . The perform goes on.

Since one atom that one element can be differentiated from one atom of another element through the number of protons in the nucleus, scientists are constantly interested in this number, and how this number differs between different elements. The number of protons in an atom is referred to as its atomic number (\(Z\)). This number is really important because it is distinctive for atoms of a offered element. Every atoms of an facet have the same variety of protons, and every facet has a different number of protons in that is atoms. For example, all helium atoms have actually two protons, and also no other elements have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table \(\PageIndex1\): atoms of the first Six aspects
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, since neutral atoms need to have one electron for every proton, one element"s atom number also tells girlfriend how many electrons room in a neutral atom of the element. Because that example, hydrogen has an atomic variety of 1. This method that an atom of hydrogen has one proton, and, if it"s neutral, one electron together well. Gold, on the various other hand, has an atomic variety of 79, which way that one atom of gold has 79 protons, and, if it"s neutral, 79 electrons as well.

Mass Number

The mass number (\(A\)) of an atom is the total variety of protons and neutrons in that is nucleus. The mass of the atom is a unit referred to as the atom mass unit \(\left( \textamu \right)\). One atom mass unit is the mass of a proton, or about \(1.67 \times 10^-27\) kilograms, i beg your pardon is one extremely little mass. A neutron has just a small bit more mass than a proton, yet its massive is often assumed to be one atom mass unit as well. Because electrons have actually virtually no mass, just around all the massive of one atom is in that protons and also neutrons. Therefore, the total number of protons and neutrons in one atom determines its mass in atomic mass units (Table \(\PageIndex1\)).

Consider helium again. Many helium atoms have two neutron in enhancement to 2 protons. Because of this the fixed of most helium atom is 4 atom mass systems (\(2 \: \textamu\) because that the protons + \(2 \: \textamu\) because that the neutrons). However, part helium atoms have more or less than 2 neutrons. Atoms with the same variety of protons but different numbers of neutron are dubbed isotopes. Since the variety of neutrons deserve to vary because that a offered element, the mass number of different atoms of an element may also vary. Because that example, part helium atoms have actually three neutrons rather of two (these are dubbed isotopes and are questioned in detail later on).

Why perform you think the the "mass number" has protons and neutrons, yet not electrons? You know that many of the massive of one atom is concentrated in that is nucleus. The fixed of one atom counts on the number of protons and neutrons. You have currently learned that the mass of one electron is very, very tiny compared come the massive of one of two people a proton or a ghost (like the massive of a penny compared to the fixed of a bowling ball). Count the variety of protons and also neutrons tells scientists around the full mass of an atom.

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\<\textmass number \: A = \left( \textnumber the protons \right) + \left( \textnumber of neutrons \right)\>

An atom"s massive number is very easy come calculate, provided that you understand the number of protons and also neutrons in an atom.

Example 4.5.1

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


\(\left( \textnumber of protons \right) = 2\) (Remember that an atom of helium constantly has 2 protons.)

\(\left( \textnumber that neutrons \right) = 2\)

\(\textmass number = \left( \textnumber the protons \right) + \left( \textnumber the neutrons \right)\)