Atomic Structure Flashcards

All 37 cards in this deck

Describe the structure of an atom.

A small, positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons orbiting at set distances.

What is the typical size (order of magnitude) of an atom?

About $1\times10^{-10}\ \mathrm{m}$ (0.1 nm) across. Small molecules are of a similar order, up to about $10^{-9}\ \mathrm{m}$.

Where is almost all of the mass of an atom found?

In the nucleus.

How does the radius of the nucleus compare with the radius of the whole atom?

The nucleus is at least 10 000 times smaller — about $10^{-14}\ \mathrm{m}$ compared with $10^{-10}\ \mathrm{m}$.

True or false? Most of an atom is empty space.

True. The nucleus and electrons occupy a tiny fraction of the atom's volume.

What are the steps to estimate how many atoms fit across a thin foil?
e.g. foil $4.0\times10^{-7}\ \mathrm{m}$ thick, atom diameter $0.15\ \mathrm{nm}$

1. Put the atom diameter in metres: $0.15\ \mathrm{nm}=1.5\times10^{-10}\ \mathrm{m}$
2. Divide thickness by atom diameter: $\dfrac{4.0\times10^{-7}}{1.5\times10^{-10}}$
3. Number of atoms $\approx 2700$

What are the relative mass and relative charge of a proton?

Relative mass 1, relative charge $+1$.

What are the relative mass and relative charge of a neutron?

Relative mass 1, relative charge 0.

Why is an atom electrically neutral?

It has equal numbers of protons and electrons, so the $+1$ and $-1$ charges cancel.

True or false? An electron and a proton have equal but opposite charges, and equal masses.

False. Their charges are equal and opposite ($-1$ and $+1$), but an electron has far less mass than a proton.

What is the atomic (proton) number of an atom?

The number of protons in its nucleus.

What is the mass (nucleon) number of an atom?

The total number of protons and neutrons in its nucleus.

In the nuclear symbol $^{A}_{Z}\mathrm{X}$, what do $A$ and $Z$ stand for?
e.g. $^{23}_{11}\mathrm{Na}$

$A$ is the mass (nucleon) number and $Z$ is the atomic (proton) number — so sodium has 23 nucleons and 11 protons.

How do you find the number of neutrons in a nucleus from its symbol?
e.g. $^{7}_{3}\mathrm{Li}$

1. Read the mass number (top): 7
2. Read the atomic number (bottom): 3
3. Neutrons = mass number $-$ atomic number = 4

In a neutral atom, what else does the atomic number tell you besides the number of protons?

The number of electrons.

True or false? The mass number tells you the number of neutrons in a nucleus.

False. It is the number of protons plus neutrons.

What are isotopes?

Atoms of the same element with the same number of protons but different numbers of neutrons.

How do the atomic number and mass number compare for two isotopes of an element?

Same atomic number, different mass number.

How do you decide whether two nuclei are isotopes of the same element?
e.g. $^{12}_{6}\mathrm{C}$ and $^{14}_{6}\mathrm{C}$

1. Compare atomic numbers — they must be equal: both 6
2. Compare mass numbers — they must differ: 12 and 14
3. So they are isotopes of carbon (6 and 8 neutrons)

What gives the nucleus of an element its characteristic positive charge?

Its number of protons, which is the same for every atom of that element.

True or false? Isotopes of an element have nuclei with different positive charges.

False. The nuclear charge is the same; only the number of neutrons (and so the mass) differs.

True or false? Neutral atoms of two isotopes of the same element have the same number of electrons.

True. They have the same number of protons, so the same number of electrons.

Where are the electrons in an atom?

Orbiting the nucleus at different set distances (energy levels/shells).

What happens to an electron when it absorbs electromagnetic radiation?

It moves to a higher orbit, further from the nucleus.

What happens when an electron moves to an orbit closer to the nucleus?

It emits electromagnetic radiation.

True or false? Absorbing electromagnetic radiation makes an electron move closer to the nucleus.

False. Absorbing radiation moves it to a higher orbit, further from the nucleus.

What is a positive ion?

An atom that has lost one or more outer electrons, so it has more protons than electrons.

Which electrons does an atom lose when it forms a positive ion?

Outer electrons — they are furthest from the nucleus and least strongly held.

What are the steps to work out the charge on a positive ion?
e.g. a lithium atom (3 protons) loses 1 outer electron

1. Count the electrons lost: 1
2. Compare protons with remaining electrons: 3 protons, 2 electrons
3. Charge is $+1$ for each electron lost: charge $=+1$

True or false? An atom becomes a positive ion by gaining protons.

False. It loses outer electrons; the nucleus is unchanged.

Does forming a positive ion change an atom's mass number?

No. Only electrons are lost, and the nucleus (protons and neutrons) is unchanged.

Describe the plum pudding model of the atom.

A ball of positive charge with negative electrons embedded in it, like plums in a pudding.

What were the three observations in Rutherford's alpha particle scattering experiment?

Most alpha particles passed straight through the gold foil, a few were deflected, and a very small number bounced back.

What did most alpha particles passing straight through the gold foil show about the atom?

That the atom is mostly empty space.

What did the very small number of alpha particles deflected back through large angles show?

That the positive charge and nearly all the mass are concentrated in a very small, dense nucleus.

What did Bohr add to Rutherford's nuclear model?

That electrons orbit the nucleus at set distances (fixed energy levels), not at any distance.

Give one limitation of modelling alpha scattering by rolling marbles down a slope past a weight.

The marble and weight have no charge (also: the model is 2D, not to scale, and marbles only deflect on contact).