### Learning Objectives

Define fixed number.Calculate the fixed number as soon as given number of protons and also neutrons.Calculate variety of neutrons when given atomic number.

You are watching: How many protons does helium have?

### How can you identify the massive of a chemical?

Often a college student will need to weigh the end a chemical because that an experiment. If the or she offers a watch glass (a small, round piece that will host the hard chemical), the weight of the clock glass have to be determined first. Then the hard is added to the glass and also the load of the glass add to the heavy is measured. The balance analysis will be the total of the glass to add the chemical.

## History the Atomic load Determinations

As a component of his research on atoms, john Dalton determined a variety of atomic weights of facets in the beforehand 1800s. Atom weights were the basis for the routine table that Mendeleev developed. Initially all atomic weights were based on a comparison to hydrogen, which has actually an atomic load of one. After ~ the discovery of the proton, researchers assumed that the load of an atom was basically that of the protons—electrons were recognized to contribute nearly nothing come the atomic load of the element.

This technique worked till we learned just how to determine the variety of protons in one element. We then observed that the atomic weight for an element was regularly twice the variety of protons (or more). The discovery of the neutron detailed the missing component of the picture. The atom mass now known to it is in the sum of the protons and also neutrons in the nucleus.

### Mass Number

Rutherford showed that the vast majority of the mass of an atom is focused in that is nucleus, i m sorry is composed of protons and neutrons. The mass number is defined as the total number of protons and neutrons in one atom. Think about Table below which reflects data from the very first six elements of the regular table.

Atoms of the an initial Six Elements
 surname symbol atom Number proton neutrons electron massive Number Hydrogen H 1 1 0 1 1 Helium He 2 2 2 2 4 Lithium Li 3 3 4 3 7 Beryllium Be 4 4 5 4 9 Boron B 5 5 6 5 11 Carbon C 6 6 6 6 12

Consider the aspect helium. Its atomic number is 2, therefore it has actually two proton in that nucleus. That nucleus also contains two neutrons. Since 2 + 2 = 4, we recognize that the mass variety of the helium atom is 4. Finally, the helium atom additionally contains 2 electrons due to the fact that the number of electrons should equal the number of protons. This instance may lead you to think that atoms have actually the same number of protons and neutrons, however further check of the Table above will show that this is not the case. Lithium, for example has three protons and also four neutrons, leaving it v a mass number of 7.

Knowing the massive number and also the atomic variety of an atom allows you to identify the number of neutrons existing in that atom through subtraction.

Number of neutron = massive number − atomic number

Atoms that the element chromium (Cr) have actually an atomic number of 24 and also a mass variety of 52. How many neutrons are in the cell core of a chromium atom? To identify this, you would subtract as shown:

52 − 24 = 28 neutron in a chromium atom

The ingredient of any type of atom deserve to be illustrated with a shorthand notation using the atomic number and the fixed number. Both room written prior to the chemistry symbol, with the fixed number written as a superscript and also the atom number written as a subscript. The chromium atom discussed over would be composed as ^52_24\textCr.

Another way to refer to a certain atom is to compose the mass number of the atom after the name, be separated by a hyphen. The above atom would be created as chromium-52.

See more: What Is 3/8 Divided By 2 /10?

### Summary

The mass number is identified as the total number of protons and neutrons in one atom.The variety of neutrons = massive number − atom number.

### Practice

Use the link listed below to price the complying with questions:

http://education.jlab.org/qa/pen_number.html

What data in the regular table tells you the variety of protons in an atom?How carry out you determine the number of neutrons in an atom?What is the fixed number because that an atom?

### Review

Who an initial determined atomic weights because that elements?What to be the initial atomic weights based on?Why were calculations based on numbers of protons not valid because that determining atomic weights?A tin atom has an atomic variety of 50 and a mass number of 118. How countless neutrons are current in this atom?What is the mass number of a cobalt atom that has 27 protons and 30 neutrons?