How many protons and neutrons are in an atom of Mg-25?

Every atom consists of three basic particles: protons, neutrons, and electrons. The number of protons and neutrons in an atom determines the element and isotope of that atom. For example, magnesium (Mg) atoms always contain 12 protons. However, they can contain varying numbers of neutrons, resulting in different isotopes of magnesium.

Mg-25 refers specifically to an isotope of magnesium that contains 25 nucleons – 12 protons and 13 neutrons. By understanding the basics of atomic structure and isotopes, we can determine how many protons and neutrons Mg-25 contains.

Atomic Number and Mass Number

The atomic number of an element refers to the number of protons in each atom of that element. For all magnesium atoms, the atomic number is 12. This means every magnesium atom has 12 protons in its nucleus.

The mass number refers to the total number of protons and neutrons in the nucleus of an atom. Different isotopes of an element have the same atomic number but different mass numbers.

For example, the most common isotope of magnesium has a mass number of 24 – 12 protons and 12 neutrons. However, Mg-25 has a mass number of 25 – 12 protons and 13 neutrons.

The atomic number is unique to each element, while different isotopes of an element can have varying mass numbers.


Isotopes are variants of an element that have different numbers of neutrons. As a result, isotopes have nearly identical chemical properties but differ in their atomic mass.

The various isotopes of an element are denoted by their mass number. For magnesium, there are three stable isotopes occurring in nature:

– Mg-24: 12 protons, 12 neutrons
– Mg-25: 12 protons, 13 neutrons
– Mg-26: 12 protons, 14 neutrons

Although Mg-25 and Mg-24 both have 12 protons, Mg-25 has one extra neutron, giving it a greater atomic mass. The number of neutrons can vary among isotopes while the element itself remains the same.


From the isotope notation, we know:

– Mg is the element, magnesium
– 25 is the mass number

The mass number represents the total number of nucleons (protons and neutrons) in the nucleus.

We also know:

– All Mg atoms contain 12 protons (atomic number of Mg is 12)
– Mg-25 has a mass number of 25
– Therefore, the number of neutrons must be 25 – 12 = 13

So in summary:

– Protons: 12
– Neutrons: 13
– Nucleons total: 12 protons + 13 neutrons = 25

An atom of Mg-25 contains 12 protons and 13 neutrons. This gives magnesium-25 its unique atomic mass and chemical properties.


– The atomic number (number of protons) uniquely identifies an element
– Isotopes of an element have the same atomic number but different mass numbers
– The mass number is the sum of protons and neutrons in an atom
– Mg-25 is an isotope of magnesium with 12 protons and 13 neutrons
– Knowing the atomic number of magnesium is 12, and the mass number of Mg-25 is 25, we can determine the number of protons (12) and neutrons (13)

Understanding isotopes and how to interpret element names and isotope notation allows us to extract key information about atomic structure. For Mg-25, analysis of its name and magnesium’s atomic number reveals that it contains 12 protons and 13 neutrons in its nucleus.

Practice Questions

Question 1

How many protons and neutrons are in an atom with the following notation:



– Ca is the element calcium, which has an atomic number of 20
– This means all Ca atoms have 20 protons
– The isotope notation Ca-40 indicates a mass number of 40
– With 20 protons, there must be 40 – 20 = 20 neutrons
– Therefore, Ca-40 contains 20 protons and 20 neutrons.

Question 2

The isotope K-42 contains how many protons and neutrons?


– K represents the element potassium, which has an atomic number of 19
– All K atoms contain 19 protons
– The mass number of 42 indicates total nucleons
– With 19 protons, there must be 42 – 19 = 23 neutrons
– An atom of K-42 contains 19 protons and 23 neutrons

Determining Number of Protons

The number of protons in an atom is equal to its atomic number, which is unique for each element. Some key facts:

– The atomic number identifies the number of protons in an atom of an element
– This number is always the same for all atoms of that element
– For example, all carbon (C) atoms have 6 protons
– All oxygen (O) atoms have 8 protons

To find the number of protons:

– Identify the chemical symbol of the element
– Look up its atomic number on the periodic table
– The atomic number gives the number of protons

This works for all elements. Once you have the atomic number, you have the number of protons.


Boron (B)
– Atomic number of boron is 5
– Therefore, all boron atoms contain 5 protons

Nitrogen (N)
– Atomic number of nitrogen is 7
– Therefore, nitrogen atoms contain 7 protons

Aluminum (Al)
– Atomic number is 13
– Aluminum atoms contain 13 protons

So to find protons, the atomic number of the element gives you the answer.

Determining Number of Neutrons

While all atoms of an element contain the same number of protons, the number of neutrons can vary among isotopes. To find neutrons:

– Identify the isotope notation (e.g. Mg-25)
– Subtract the atomic number from the mass number

This gives you the number of neutrons. For example:

– Mass number is 25
– Atomic number of Mg is 12
– Therefore, number of neutrons is 25 – 12 = 13

Cu-63 (copper):
– Mass number is 63
– Atomic number of Cu is 29
– So Cu-63 has 63 – 29 = 34 neutrons

With the isotope notation and atomic number, you can easily calculate the number of neutrons.

Practice Problems

How many protons and neutrons are in each of the following isotopes?

1) Fe-56

2) Sr-87

3) At-210


1) Fe (iron) has an atomic number of 26, so Fe-56 has 26 protons. With a mass number of 56, Fe-56 has 56 – 26 = 30 neutrons.

2) Sr (strontium) has an atomic number of 38, so Sr-87 has 38 protons. It has 87 – 38 = 49 neutrons.

3) At (astatine) has an atomic number of 85, so At-210 has 85 protons. It has 210 – 85 = 125 neutrons.

Determining Protons and Neutrons from Atomic Mass

In addition to mass number, the atomic mass of an element is another way to find protons and neutrons. Atomic mass takes into account the relative abundance and mass of all naturally occurring isotopes.

– The proton has a mass of about 1 atomic mass unit (amu)
– The neutron has a mass of about 1 amu
– The atomic mass of an element is approximately equal to its total number of protons and neutrons

For example:

Magnesium (Mg) has an atomic mass of 24.3 amu

– Atomic number of Mg is 12 (this is the number of protons)
– Subtracting the number of protons (12) from the atomic mass (24.3) gives an estimate of the neutrons
– 24.3 – 12 = 12.3 neutrons

So magnesium has approximately 12 protons and 12 neutrons, which is consistent with its most abundant isotope Mg-24 (12 protons and 12 neutrons).

From the atomic mass, we can derive protons and neutrons. This method provides an average number of neutrons, since an element’s atomic mass reflects abundances of all isotopes.

Practice Problems

Use atomic masses to determine the number of protons and approximate number of neutrons:

1) Boron (B), atomic mass = 10.8 amu

2) Silver (Ag), atomic mass = 107.9 amu


1) B has an atomic number of 5, so it has 5 protons. 10.8 – 5 = 5.8 neutrons.

2) Ag has an atomic number of 47, so 47 protons. 107.9 – 47 = 60.9 neutrons.

Use in Nuclear Reactions and Decay

The number of protons and neutrons determines an isotope’s stability and its decay characteristics. Isotopes with too few or too many neutrons tend to be unstable and undergo radioactive decay to become more stable.

Some examples:

– U-235 is an unstable uranium isotope prone to fission
– U-238 is a more stable uranium isotope
– C-14 is a radioactive carbon isotope that decays to N-14 by beta decay

Nuclear reactions also depend on the number of protons and neutrons:

Alpha decay:
– An alpha particle contains 2 protons and 2 neutrons
– When an atom undergoes alpha decay, its mass number decreases by 4 (loss of 2 protons and 2 neutrons)
– Example: Ra-226 decays to Rn-222 by alpha emission

Positron emission:
– A positron is an anti-electron emitted in beta plus decay
– This converts a proton into a neutron, decreasing atomic number by 1
– Example: K-40 decaying to Ar-40

Having the proton and neutron counts allows prediction of nuclear stability, decay modes, and reaction products. This information is critical for applications like nuclear power and medical radiation.

Abundance of Isotopes

Most elements exist as mixtures of several stable isotopes. The relative abundance of an isotope refers to the fraction of atoms of that element that have the specific isotope’s nucleon number.

For example, the three isotopes of magnesium have the following approximate terrestrial abundances:

– Mg-24: 79% abundance
– Mg-25: 10% abundance
– Mg-26: 11% abundance

Knowing the natural abundances of isotopes helps understand the average atomic mass of an element. Mg-24 is the most abundant, so its protons and neutrons have the greatest influence on magnesium’s average atomic mass.

Isotope fractions can be measured using a mass spectrometer. By detecting the different mass numbers and their intensities, the isotope distribution can be determined.

Abundances may vary on other planetary bodies due to different nucleosynthesis environments. The isotope ratios can reveal details about stellar processes and the age of materials.

Isotope Geochemistry

Variations in isotope abundances have many applications in geochemistry and geology:

– Radioactive dating uses decay of unstable isotopes to date rocks and artifacts
– Stable isotope analysis helps determine temperature histories of rock samples
– Tracing isotopic signals helps identify sources of contaminants

An isotope fingerprint provides clues about the source and history of a material. For example, changing rainwater isotope ratios reveal climate fluctuations.

Generating detailed isotope data requires specialized mass spectrometry equipment. But an understanding of the fundamentals of isotopes, such as the number of protons and neutrons in different variants, is essential to interpret this data.

Importance in Chemistry

The number of protons and neutrons influences chemical properties of an element. Patterns in the periodic table relate to numbers of protons and electrons:

– Groups share common valence electron configurations
– Periods represent filling of electron shells

Isotopes exhibit nearly identical chemical reactivity. For example, different isotopes of hydrogen combine with oxygen in the same 2:1 ratio to form water.

However, isotopes do have small differences in reaction rates and equilibria. Deuterium forms stronger bonds than protium. Enriching uranium in U-235 concentrates the fissile isotope.

The electron configuration resulting from the number of protons defines an element’s chemical identity. Adding or removing electrons creates charged ions. The number of neutrons fine-tunes the atomic mass and nuclear stability.

Medical and Biological Applications

Proton and neutron numbers provide vital data for nuclear medicine:

– PET scanners detect gamma rays from proton-rich isotopes administered to patients
– Isotopes including Co-57 and I-131 are used for diagnostic imaging
– Neutron-rich isotopes such as Cf-252 and Ra-226 produce radiation for targeted cancer treatments

Biological molecules also have characteristic numbers of protons and neutrons:

– DNA base pairs contain set numbers of nitrogens and oxygens
– Proteins comprise amino acids with defined protons and atomic masses
– Bone mineral density depends on proper calcium levels, identified by standard atomic number and mass

Metabolic pathways are tracked and abnormalities detected by monitoring radioactive isotopes during uptake by tissues. The proton and neutron content encodes chemical information for life processes.

Technological Uses

Semiconductors rely on controlled addition of atoms with defined numbers of protons and electrons:

– Doping silicon with boron adds acceptor sites
– Doping with phosphorus provides extra electrons
– GaN and other compounds have tunable electronic properties

Alloy properties depend on the atomic mass and proton numbers of components:

– Strong aluminum alloys include Cu and Mg dopants
– Lightweight titanium alloys contain Ni and Al
– Proton number determines location in galvanic series for corrosion prediction

Photovoltaics harness electrons ejected by photons interacting with semiconductor atoms. Materials with the optimal band gaps and electron configurations are identified by proton and neutron content.

Quantum Properties

On the microscopic scale, many quantum effects arise from interactions between protons, neutrons, and electrons:

– Spin values for nucleons and electrons dictate atomic magnetic dipole moments
– Nuclear shell structure follows from quantum mechanical neutron and proton orbitals
– Hyperfine transitions result from coupling between nuclear spin and electron orbitals

Proton and neutron numbers determine nuclear moments and angular momentum:

– Nuclear magnetic resonance (NMR) spectroscopy probes spin states
– Nuclear gyroscopic ratios arise from mass and charge distributions
– Isotope effects in vibrational spectroscopy reflect reduced mass

Quantum tunneling of nucleons underlies nuclear fusion in the Sun and thermonuclear weapons. Quantum chromodynamics governs the forces binding quarks into protons and neutrons within nuclei.

Particle Physics

Fundamental particles called quarks make up protons and neutrons:

– Protons contain two up quarks and one down quark (uud)
– Neutrons consist of one up quark and two down quarks (udd)

Forces between the quarks arise from gluon exchange. Radiation and scattering experiments probe the quark and gluon structure inside nucleons.

Beyond stable protons and neutrons, particle accelerators produce exotic hadrons by combining quarks:

– Pions contain up/down quarks with antiquarks
– Kaons consist of a strange quark with up/down quarks
– Hadron properties depend on content of up, down, strange quarks

Nucleon particle interactions reveal the strong nuclear force at small scales inside nuclei. Determining proton and neutron numbers assists calculations of binding energies and nuclear stability.


The proton and neutron content distinguishes elements and isotopes. Various analytical techniques can identify numbers of protons and neutrons in atoms, providing insight into stability, abundance, decay modes and applications.

Key knowledge includes:

– Atomic number gives the number of protons for an element
– Mass number – atomic number = number of neutrons
– Isotopes have the same atomic number but different mass numbers
– Proton number dictates electronic configuration and chemical properties
– Neutron number influences nuclear stability and decay characteristics

From new isotopes produced in accelerators to isotopic signals studied across the universe, proton and neutron totals reveal fundamental properties of matter. A quantitative understanding of atomic and nuclear structure begins with counting the constituent particles.

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