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Learning Objectives

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

You are watching: Atoms with the same number of protons

It"s essential to have the ability to distinguish atom of one aspect from atoms of an additional element. Facets are pure building material that comprise all other matter, so every one is provided a distinctive name. The name of elements are also represented by distinctive one- or two-letter symbols, such as $$\ceH$$ because that hydrogen, $$\ceC$$ for carbon, or $$\ceHe$$ because that helium. However, that would an ext powerful if these names might be provided to recognize the number of protons and also neutrons in the atoms. That"s whereby atomic number and mass numberare useful. Figure $$\PageIndex1$$: that is an overwhelming to find characteristics that differ between each element, and also to differentiate one facet from another. Every element, however, does have a unique variety of protons. Sulfur has 16 protons, silicon has 14 protons, and gold has actually 79 protons. Pictures used with permission (public domain because that sulfur and also silicon, gold is license is granted by CC-BY-SA-NC-ND; Alptcouncil.netist-hp).

## Atomic Number

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

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

NameProtonsNeutronsElectronsAtomic Number (Z)Mass Number(A) Table $$\PageIndex1$$: atoms of the an initial 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, because neutral atoms have to have one electron because that every proton, an element"s atom number additionally tells friend how countless electrons are in a neutral atom of that element. For example, hydrogen has an atomic variety of 1. This way that one atom of hydrogen has actually one proton, and, if it"s neutral, one electron together well. Gold, ~ above the various other hand, has an atomic variety of 79, which way that an atom of gold has 79 protons, and, if it"s neutral, 79 electrons as well.

## Mass Number

The mass number ($$A$$) of one atom is the total number of protons and also neutrons in its nucleus. The massive 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 tiny mass. A neutron has actually just a tiny bit much more mass 보다 a proton, however its mass is often assumed to be one atom mass unit as well. Due to the fact that electrons have virtually no mass, just around all the fixed of one atom is in its protons and neutrons. Therefore, the total variety of protons and neutrons in an atom identify its massive in atom mass systems (Table $$\PageIndex1$$).

Consider helium again. Many helium atoms have actually two neutrons in addition to two protons. Because of this the massive of many helium atoms is 4 atomic mass systems ($$2 \: \textamu$$ because that the proton + $$2 \: \textamu$$ for the neutrons). However, some helium atoms have more or much less than 2 neutrons. Atoms v the same variety of protons but different number of neutrons are dubbed isotopes. Since the variety of neutrons deserve to vary because that a given element, the mass numbers of various atoms of an aspect may likewise vary. For example, part helium atoms have three neutrons rather of two (these are dubbed isotopes and are questioned in detail later on).

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

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

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

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

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

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