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Learning Objectives

specify atomic number. Define mass number. Identify the number of protons, neutrons, and also electrons in an atom.

You are watching: All atoms have the same number of protons

It"s vital to have the ability to distinguish atoms of one facet from atom of one more element. Aspects are pure building materials that make up all other matter, so each one is provided a distinctive name. The name of elements are additionally represented by distinct one- or two-letter symbols, such as (ceH) for hydrogen, (ceC) for carbon, or (ceHe) because that helium. However, the would much more powerful if these names might be used to recognize the number of protons and also neutrons in the atoms. That"s whereby atomic number and also mass numberare useful.

Figure (PageIndex1): the is an overwhelming to find features that differ in between each element, and also to differentiate one element from another. Each element, however, does have actually a unique variety of protons. Sulfur has 16 protons, silicon has actually 14 protons, and also gold has 79 protons. Photos used v permission (public domain for sulfur and also silicon, gold is licensed by CC-BY-SA-NC-ND; Altennis2007.orgist-hp).

## Atomic Number

Scientists distinguish between different aspects by counting the variety of protons in the cell nucleus (Table (PageIndex1)). If an atom has only one proton, we recognize that it"s a hydrogen atom. An atom through two protons is constantly a helium atom. If scientists count 4 protons in an atom, they recognize it"s a beryllium atom. An atom with 3 protons is a lithium atom, one atom with five protons is a boron atom, one atom with six protons is a carbon atom . . . The list goes on.

Since one atom the one facet can be differentiated from an atom of another element through the number of protons in its nucleus, researchers are constantly interested in this number, and also how this number differs in between different elements. The number of protons in an atom is referred to as its atomic number ((Z)). This number is very important since it is distinct for atoms of a given element. Every atoms that an element have the same variety of protons, and every element has a different variety of protons in that is atoms. Because that example, all helium atoms have two protons, and no other aspects have atoms v two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table (PageIndex1): atom of the an initial Six facets
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, due to the fact that neutral atoms need to have one electron for every proton, one element"s atom number also tells friend how many electrons are in a neutral atom of that element. For example, hydrogen has actually an atomic number of 1. This way that one atom the hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, ~ above the other hand, has an atomic variety of 79, which way that one atom that gold has actually 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 also neutrons in that is nucleus. The mass of the atom is a unit referred to as the atom mass unit (left( extamu ight)). One atomic mass unit is the fixed of a proton, or about (1.67 imes 10^-27) kilograms, which is an extremely little mass. A neutron has actually just a tiny bit an ext mass than a proton, however its massive is regularly assumed to be one atom mass unit as well. Because electrons have virtually no mass, just around all the massive of an atom is in that is protons and also neutrons. Therefore, the total number of protons and neutrons in an atom determines its massive in atom mass systems (Table (PageIndex1)).

Consider helium again. Many helium atoms have two neutron in addition to two protons. Therefore the fixed of many helium atoms is 4 atomic mass devices ((2 : extamu) for the proton + (2 : extamu) for the neutrons). However, some helium atom have much more or much less than 2 neutrons. Atoms v the same number of protons but different number of neutron are dubbed isotopes. Because the number of neutrons can vary because that a offered element, the mass numbers of various atoms of an element may additionally vary. For example, some helium atoms have three neutrons instead of 2 (these are called isotopes and also are debated in detail later on).

Why execute you think the the "mass number" has protons and neutrons, but not electrons? You understand that most of the fixed of one atom is concentrated in that is nucleus. The fixed of an atom depends on the number of protons and neutrons. Friend have already learned that the mass of an electron is very, very little compared to the fixed of either a proton or a neutron (like the mass of a penny contrasted to the mass of a bowling ball). Counting the variety of protons and neutrons speak scientists about the total mass of one atom.

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

An atom"s fixed number is very easy to calculate, noted that you understand the number of protons and neutrons in one atom.

Example 4.5.1

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

Solution

(left( extnumber that protons ight) = 2) (Remember the an atom the helium always has 2 protons.)

(left( extnumber that neutrons ight) = 2)

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