Define atom number. Define mass number. Determine the variety of protons, neutrons, and electrons in one atom.

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It"s important to have the ability to distinguish atoms of one aspect from atoms of one more element. Elements are pure building materials that make up all other matter, so every one is offered a distinctive name. The surname of elements are likewise represented by distinctive one- or two-letter symbols, such together $$\ceH$$ because that hydrogen, $$\ceC$$ for carbon, or $$\ceHe$$ because that helium. However, it would an ext powerful if this names could be provided to determine the numbers of protons and also neutrons in the atoms. That"s whereby atomic number and also mass numberare useful.

Figure $$\PageIndex1$$: the is daunting to find characteristics that differ in between each element, and to distinguish one element from another. Each element, however, does have actually a unique variety of protons. Sulfur has actually 16 protons, silicon has actually 14 protons, and gold has actually 79 protons. Images used v permission (public domain because that sulfur and also silicon, yellow is licensed by CC-BY-SA-NC-ND; Albsci-ch.orgist-hp).

## Atomic Number

Scientists distinguish in between different facets by counting the variety of protons in the nucleus (Table $$\PageIndex1$$). If one atom has actually only one proton, we recognize that it"s a hydrogen atom. An atom through two proton is always 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, an atom with six protons is a carbon atom . . . The list goes on.

Since one atom the one facet can be identified from an atom of one more element by the number of protons in that nucleus, scientists are always interested in this number, and also how this number differs in between different elements. The variety of protons in an atom is dubbed its atomic number ($$Z$$). This number is really important since it is distinctive for atom of a provided element. All atoms that an element have the same variety of protons, and also every facet has a different number of protons in that atoms. Because that example, all helium atoms have two protons, and no other aspects have atoms with two protons.

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table $$\PageIndex1$$: atoms of the very first Six elements
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, because neutral atoms have to have one electron for every proton, one element"s atomic number likewise tells you how numerous electrons are in a neutral atom of the element. For example, hydrogen has an atomic variety of 1. This means that one atom of hydrogen has one proton, and, if it"s neutral, one electron as well. Gold, on the other hand, has an atomic variety of 79, which means that an atom that gold has 79 protons, and, if it"s neutral, 79 electrons together well.

## Mass Number

The mass number ($$A$$) of one atom is the total variety of protons and neutrons in its nucleus. The fixed of the atom is a unit dubbed the atom mass unit $$\left( \textamu \right)$$. One atomic mass unit is the massive of a proton, or about $$1.67 \times 10^-27$$ kilograms, i m sorry is an extremely tiny mass. A neutron has actually just a tiny bit much more mass 보다 a proton, however its fixed is frequently assumed to it is in one atomic mass unit together well. Due to the fact that electrons have virtually no mass, just about all the mass of one atom is in the protons and neutrons. Therefore, the total variety of protons and also neutrons in one atom determines its fixed in atomic mass systems (Table $$\PageIndex1$$).

Consider helium again. Many helium atoms have two neutron in addition to 2 protons. Thus the massive of most helium atoms is 4 atom mass units ($$2 \: \textamu$$ for the protons + $$2 \: \textamu$$ because that the neutrons). However, part helium atoms have an ext or less than 2 neutrons. Atoms with the same number of protons however different numbers of neutron are dubbed isotopes. Because the variety of neutrons deserve to vary because that a provided element, the mass numbers of different atoms that an element may additionally vary. Because that example, part helium atoms have three neutrons instead of two (these are referred to as isotopes and are debated in detail later on).

Why carry out you think that the "mass number" contains protons and neutrons, but not electrons? You recognize that most of the fixed of an atom is concentrated in that nucleus. The fixed of an atom relies on the variety of protons and neutrons. You have currently learned the the massive of one electron is very, very little compared to the mass of one of two people a proton or a spirit (like the massive of a penny compared to the mass of a bowling ball). Count the variety of protons and neutrons tells scientists around the total mass of an atom.

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

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

Example 4.5.1

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

Solution

$$\left( \textnumber of protons \right) = 2$$ (Remember the an atom the helium always has 2 protons.)

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

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