Atomic structure

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Notas de aula

Big idea: using models to see the invisible

Big idea (key concept): Systems. An atom is a small system of particles that work together, and everything around you is built from atoms.
Related concept: Models. Atoms are far too small to see, so scientists use models. A model is a simplified picture or idea that explains evidence and helps us make predictions.
Global context: Scientific and technical innovation. Better equipment let scientists learn more about atoms, and each new discovery changed the model.
An atom is the smallest part of an element that still has the properties of that element. Atoms are so small that millions of them would fit across the full stop at the end of this sentence.
At the centre of an atom is the nucleus. It is tiny but contains almost all of the atom's mass. The nucleus is made of protons and neutrons.
Moving around the nucleus are electrons, arranged in layers called shells (or energy levels). Most of an atom is empty space.
The picture shows a simple model of a carbon atom: six protons and six neutrons in the nucleus, with six electrons in two shells.

A carbon atom: the nucleus and its electron shells

Protons, neutrons and electrons

Proton: found in the nucleus, relative mass 1, relative charge +1.
Neutron: found in the nucleus, relative mass 1, relative charge 0 (no charge).
Electron: found in the shells outside the nucleus, relative mass very small (about 1/2000 of a proton), relative charge -1.
The atomic number is the number of protons in an atom. Every atom of an element has the same atomic number, and this is what makes it that element. Carbon always has 6 protons, and an atom with 7 protons is always nitrogen.
The mass number is the number of protons plus the number of neutrons. It is a count of the particles in the nucleus.
To find the number of neutrons: neutrons = mass number - atomic number. Sodium has mass number 23 and atomic number 11, so it has 23 - 11 = 12 neutrons.
In the symbol, the mass number is written at the top left and the atomic number at the bottom left of the chemical symbol, as in the picture for sodium.

Atomic notation: mass number, atomic number and symbol

Electron shells and the Periodic Table

An atom has no overall charge. Its positive protons are balanced by an equal number of negative electrons, so the number of electrons = the number of protons.
Electrons are arranged in shells, and each shell can hold only a certain number. The first shell holds up to 2 electrons, the second up to 8 and the third up to 8 (for the first 20 elements).
Electrons fill the shell closest to the nucleus first. Only when it is full does the next shell start to fill.
We write the electron arrangement as numbers. Carbon (6 electrons) is 2,4. Oxygen (8) is 2,6. Sodium (11) is 2,8,1. Chlorine (17) is 2,8,7.
The outer shell is the one furthest from the nucleus. The electrons in it are the outer electrons, and they decide how an atom reacts.
The number of occupied shells gives the period (row) and the number of outer electrons gives the group (column, Groups 1 to 7). Chlorine is 2,8,7: three shells and 7 outer electrons, so Period 3, Group 7.
Elements in the same group have the same number of outer electrons, which is why they react in similar ways. Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) each have one outer electron.
Helium (2), neon (2,8) and argon (2,8,8) have full outer shells. This is why these noble gases are so unreactive.

A carbon atom with the electron arrangement 2,4

Isotopes

All atoms of one element have the same number of protons, but they do not all have the same number of neutrons.
Isotopes are atoms of the same element with the same number of protons but a different number of neutrons. They therefore have the same atomic number but different mass numbers.
Hydrogen has three isotopes. Hydrogen-1 has 1 proton and 0 neutrons. Hydrogen-2 (deuterium) has 1 proton and 1 neutron. The picture shows hydrogen-2.
Carbon-12 has 6 protons and 6 neutrons. Carbon-14 has 6 protons and 8 neutrons. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons.
Isotopes of an element have the same number of electrons, so they have the same chemical properties. They only differ in mass.
Many elements are a mixture of isotopes. This is why the mass number shown for chlorine in the Periodic Table is not a whole number: it is an average of its isotopes.

Hydrogen-2: one proton, one neutron and one electron

The history of the atom: models change with evidence

Around 1803 Dalton pictured atoms as tiny solid spheres that cannot be split.
In 1897 Thomson discovered the electron. His plum pudding model showed negative electrons scattered through a ball of positive charge.
In about 1909 to 1911, Rutherford and his team fired positive particles at thin gold foil. Most went straight through, but a few bounced back. This showed that most of an atom is empty space, with a tiny, dense, positive nucleus at the centre.
In 1913 Bohr suggested that electrons orbit the nucleus in fixed shells. This is the model used in this lesson.
In 1932 Chadwick discovered the neutron, which explained why atoms are heavier than their protons alone.
A model is not wrong forever or right forever. Scientists keep a model until new evidence does not fit it, and then they improve it. Today's electron-cloud model is more detailed still.

How the model of the atom has changed over time

Think like a scientist: modelling the unseen

The question: how can we find out the shape of something we cannot see? Rutherford could not see inside atoms, so he used indirect evidence.
A safe classroom model: hide a shape under a board, roll marbles at the board from the same place, and record where each marble ends up. Different hidden shapes make the marbles bounce in different ways.
The independent variable is the hidden shape or its size. The dependent variable is what you record, such as how many marbles bounce back or the angle they leave at.
Control variables: the same marble, the same rolling speed and distance, the same start point and the same number of rolls each time.
Evaluate: the model shows how scientists can work out structure from patterns of results, but it is not an atom. Repeating the test and using more marbles makes the pattern clearer and the result more reliable.
Your task: plan an investigation to work out the size of a hidden object under a board. Say what you will change, measure and keep the same, draw a results table, and explain how your evidence supports your conclusion.

Slides

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CADASTRE-SE — GRÁTIS ENTRAR

Questões de prática

Prévia grátis — 8 de 50 perguntas. Cadastre-se para ver todas.

1.Which particle in an atom has a positive charge?
Easy
- AProton
- BNeutron
- CElectron
- DShell
2.Which particle in an atom has no charge?
Easy
- AProton
- BNeutron
- CElectron
- DIon
3.Electrons are found inside the nucleus of an atom.
Easy
True or false?
4.Where is almost all of the mass of an atom found?
Easy
- AIn the outer shell
- BSpread evenly through the atom
- CIn the nucleus
- DIn the electrons
5.What does the atomic number of an atom tell you?
Easy
- AThe number of neutrons
- BThe number of shells
- CThe mass of the atom in grams
- DThe number of protons
6.How is the mass number of an atom worked out?
Easy
- AProtons plus neutrons
- BProtons plus electrons
- CNeutrons plus electrons
- DProtons minus neutrons
7.A neutral atom has the same number of protons and electrons.
Easy
True or false?
8.Match each particle to its relative charge.
Easy
- Proton
- Neutron
- Electron
- Whole atom
- 0
- +1
- 0 (neutral overall)

Mais perguntas

9.What is the largest number of electrons the first shell can hold?
Easy
- A8
- B2
- C1
- D18
10.A neutral atom has 6 protons. How many electrons does it have?
Easy
- A12
- B0
- C6
- D3
11.Fill in the missing words.
Easy
The nucleus of an atom contains protons and ____, and the atomic number is the number of ____.
12.Which model pictured an atom as a ball of positive charge with electrons scattered through it?
Easy
- AThe shell model
- BThe solid sphere model
- CThe electron-cloud model
- DThe plum pudding model
13.What is the electron arrangement of lithium, which has 3 electrons?
Easy
- A2,1
- B3
- C1,2
- D2,2
14.Every atom of the same element has the same number of protons.
Easy
True or false?
15.Sodium is written with 23 at the top and 11 at the bottom beside its symbol. Which number is the mass number?
Easy
- A11
- B23
- C12
- D34
16.What is the electron arrangement of helium, which has 2 electrons?
Easy
- A1,1
- B0,2
- C2
- D2,8
17.Electrons fill the shell closest to the nucleus before the next shell starts to fill.
Easy
True or false?
18.Which particle has by far the smallest mass?
Easy
- AProton
- BNeutron
- CA hydrogen nucleus
- DElectron
19.Which particles are found in the nucleus? (select all that apply)
Easy
- AProton
- BNeutron
- CElectron
- DShell
20.A sodium atom has mass number 23 and atomic number 11. How many neutrons does it have?
Medium
21.A chlorine atom has 17 protons and 18 neutrons. What is its mass number?
Medium
22.Sodium has 11 electrons. What is its electron arrangement?
Medium
- A2,8,1
- B2,9
- C2,1,8
- D1,8,2
23.Chlorine has atomic number 17. What is its electron arrangement?
Medium
- A2,7,8
- B2,8,7
- C2,8,8
- D2,15
24.Which element has the electron arrangement 2,8,2?
Medium
- ANeon (10 electrons)
- BAluminium (13 electrons)
- CMagnesium (12 electrons)
- DCalcium (20 electrons)
25.An atom has the electron arrangement 2,8,3. In which group and period is it?
Medium
- AGroup 2, Period 3
- BGroup 3, Period 2
- CGroup 8, Period 3
- DGroup 3, Period 3
26.What is the same for two isotopes of an element?
Medium
- AThe number of protons
- BThe number of neutrons
- CThe mass number
- DThe mass of the atoms
27.Carbon-12 and carbon-14 are isotopes of carbon. How do their neutrons compare?
Medium
- ACarbon-12 has 12 and carbon-14 has 14
- BCarbon-12 has 6 and carbon-14 has 8
- CBoth have 6
- DCarbon-12 has 8 and carbon-14 has 6
28.Why does an atom have no overall charge?
Medium
- AIt contains only neutrons inside it
- BIts electrons are all outside it
- CIts protons and electrons balance each other out
- DIts nucleus has no charge at all
29.An oxygen atom has atomic number 8 and mass number 16. Which statements are true? (select all that apply)
Medium
- AIt has 8 protons
- BIt has 8 neutrons
- CIt has 16 electrons
- DIts electron arrangement is 2,6
30.Match each element to its electron arrangement.
Medium
- Lithium (3 electrons)
- Carbon (6 electrons)
- Oxygen (8 electrons)
- Neon (10 electrons)
- 2,4
- 2,1
- 2,8
- 2,6
31.Put these models of the atom in the order they were first proposed (earliest first).
Medium
- Nuclear model (Rutherford)
- Shell model (Bohr)
- Solid sphere (Dalton)
- Plum pudding (Thomson)
32.Most particles fired at gold foil went straight through, and a few bounced back. What does this suggest?
Medium
- AAtoms are solid balls of positive charge
- BElectrons are heavier than protons
- CAtoms contain no charged particles
- DMost of the atom is empty space with a tiny dense nucleus
33.Why did scientists stop using the plum pudding model?
Medium
- ANew evidence did not fit it, so a better model replaced it
- BThomson changed his mind without evidence
- CA vote was held by the public
- DAtoms were found not to exist
34.Choose the missing words to finish the sentence.
Medium
Atoms of the same element with different numbers of neutrons are called ____.
35.Describe the structure of an atom.
Medium
36.Sodium has the electron arrangement 2,8,1. In which period is it?
Medium
- APeriod 1
- BPeriod 3
- CPeriod 2
- DPeriod 8
37.A scientist's model predicts one result, but a careful experiment gives a different one many times. What should happen next?
Medium
- AThe experiment should be ignored
- BThe model should be kept unchanged
- CThe model should be changed or improved
- DThe result should be hidden
38.A class rolls marbles at a hidden shape under a board and changes the size of the hidden shape each time. What is the independent variable?
Medium
- AThe number of marbles that bounce back
- BThe type of marble used
- CThe distance the marbles are rolled
- DThe size of the hidden shape
39.In the same investigation, which pair of things should be kept the same to make it a fair test?
Medium
- AThe marble and the rolling distance
- BThe hidden shape and its size
- CThe result and the conclusion
- DThe number of bounces and the shape
40.Chlorine-37 is an isotope of chlorine. Chlorine has atomic number 17. How many neutrons does a chlorine-37 atom have?
Hard
41.An atom has mass number 31 and 16 neutrons. What is its electron arrangement when it is neutral?
Hard
- A2,8,8,5
- B2,8,5
- C2,8,6
- D2,8,16
42.An atom has 12 protons and 12 neutrons. In which group and period is it?
Hard
- AGroup 2, Period 2
- BGroup 12, Period 3
- CGroup 2, Period 3
- DGroup 3, Period 2
43.
Use the data to answer.
Atom Protons Neutrons
A 6 6
B 6 8
C 7 7
D 8 8
Which two atoms are isotopes of the same element?
Hard
- AA and C
- BB and C
- CC and D
- DA and B
44.
Use the data to answer.
Atom Protons Neutrons
A 6 6
B 6 8
C 7 7
D 8 8
Which atoms have a mass number of 14? (select all that apply)
Hard
- AA
- BB
- CC
- DD
45.A student says: "All chlorine atoms have a mass number of exactly 35." Which comment evaluates this best?
Hard
- AWrong: chlorine has isotopes, such as chlorine-35 and chlorine-37
- BCorrect: the atomic number of chlorine is 35
- CCorrect: every atom of an element has the same mass
- DWrong: chlorine has no neutrons
46.Explain why argon (2,8,8) is very unreactive.
Hard
47.Explain how the gold foil results led scientists to the nuclear model of the atom.
Hard
48.A class models the gold foil result with marbles and a hidden board, and says: "Our model proves atoms have a nucleus." Which comment evaluates this best?
Hard
- AIt proves it, because the marbles bounced back
- BIt shows the idea, but a model alone cannot prove it
- CIt is useless because marbles are not atoms
- DIt proves atoms are made of marbles
49.Lithium, sodium and potassium all react in similar ways. What in their atomic structure explains this?
Hard
- AEach has one shell
- BEach has the same mass number
- CEach has one electron in its outer shell
- DEach has one proton
50.Why do isotopes of an element have the same chemical properties?
Hard
- AThey have the same number of neutrons
- BThey have the same mass number
- CThey have no electrons
- DThey have the same number of electrons

Resumos de revisão

## The big idea - An atom is a **system** of particles, and scientists use **models** to explain what we cannot see. - The **nucleus** holds **protons (+1)** and **neutrons (0)**. **Electrons (-1)** move in shells around it. ## Numbers - **Atomic number** = protons. **Mass number** = protons + neutrons. - **Neutrons = mass number - atomic number.** A neutral atom has equal protons and electrons. ## Shells and the Periodic Table - Shells hold **2, 8, 8**. Examples: carbon 2,4; sodium 2,8,1; chlorine 2,8,7. - **Shells = period**; **outer electrons = group**. Full outer shells make the noble gases unreactive. ## Isotopes - Same protons, different neutrons, same chemical properties. ## History - Dalton, Thomson, Rutherford, Bohr and Chadwick: the model changes when new evidence does not fit.

Atom: The smallest part of an element that still has the properties of that element.
Nucleus: The tiny, dense centre of an atom that contains the protons and neutrons.
Proton: A particle in the nucleus with a relative charge of +1 and a relative mass of 1.
Neutron: A particle in the nucleus with no charge and a relative mass of 1.
Electron: A tiny negative particle (-1) that moves in a shell outside the nucleus.
Atomic number: The number of protons in an atom; it decides which element it is.
Mass number: The number of protons plus neutrons in an atom.
Electron shell: A region around the nucleus where electrons are found; the first holds 2 electrons, then 8, then 8.
Electron arrangement: The number of electrons in each shell, written as numbers such as 2,8,1.
Outer electrons: The electrons in the outermost shell, which decide the group and how the atom reacts.
Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.
Model: A simplified picture or idea that explains evidence and helps make predictions.
Plum pudding model: Thomson's model of electrons scattered through a ball of positive charge.
Noble gases: Group 0 elements with full outer shells, which makes them very unreactive.

Flashcards

What is the atomic number?

The number of protons in an atom.

What is the mass number?

The number of protons plus neutrons.

How do you find the number of neutrons?

Mass number minus atomic number.

What are the relative charge and mass of a proton?

Charge +1 and mass 1.

What are the relative charge and mass of a neutron?

Charge 0 and mass 1.

What are the relative charge and mass of an electron?

Charge -1 and a very small mass (about 1/2000).

Where are the protons, neutrons and electrons in an atom?

Protons and neutrons are in the nucleus; electrons are in shells around it.

Why is an atom neutral?

It has equal numbers of protons (+) and electrons (-).

How many electrons fit in the first three shells?

2, then 8, then 8 (for the first 20 elements).

What is the electron arrangement of sodium?

2,8,1.

What is the electron arrangement of chlorine?

2,8,7.

What does the number of shells tell you?

The period of the element in the Periodic Table.

What do the outer electrons tell you?

The group of the element (for Groups 1 to 7).

Why are the noble gases unreactive?

They have full outer shells, which are stable.

What are isotopes?

Atoms of the same element with different numbers of neutrons.

How many neutrons are in carbon-14?

8, because 14 minus 6 protons is 8.

What did the gold foil experiment show?

Most of an atom is empty space, with a tiny dense positive nucleus.

Why do scientists change a model?

Because new evidence does not fit the old model.

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Em breve

Atom	Protons	Neutrons
A	6	6
B	6	8
C	7	7
D	8	8

Atom	Protons	Neutrons
A	6	6
B	6	8
C	7	7
D	8	8